Phys 128 Lab Manual. U of Iowa

Copyright © 2011, John A. Goree Edited by John Goree 6 Jan 2011
29:128
Electronics

Laboratory
Manual






John A. Goree

Department of Physics and Astronomy
The University of Iowa


Updated for Spring 2011


ii
TABLE OF CONTENTS



page

Preface iii
Lab Notebooks iv
Lab Reports v
Symbols Used in this Manual vi
Resistor Color Code vi
Treatment of Experimental Errors vii
To the Instructor ix


Lab: Weeks (typical)
*


1. DC Measurements 1

2. AC Measurements 1.5

3. Diode, Power Supplies, Zener, SCR 1.2

4. Junction Transistor, Part I 1.2

5. Junction Transistor, Part II 0.5

6. Optoelectronics (can be scheduled any time after Lab 4) 0.4

7. Operational Amplifier 1.5

8. Digital Gates 1

9. Flip-Flops, Shift Register, Decade Counter, Decoder, LED Display 1.3

10. Digital Meets Analog 1



Special project 4

*
An average student will require approximately 3 hours to complete a lab that is
indicated as 1 week. Some students will require more than 3 hours. Lab grades will be
weighted by the number of weeks shown, when computing the lab component of the
course grade.

iii
PREFACE



GOAL

The purpose of the experiments described here is to acquaint the student with:

(1) analog & digital devices
(2) design of circuits
(3) instruments & procedures for electronic test & measurement.


The aim is to teach a practical skill that the student can use in the course of his or
her own experimental research projects in physics, astronomy, or another
science.


At the end of this course, the student should be able to:

(1) design and build simple circuits of his or her own design.
(2) use electronic test & measurement instruments such as oscilloscopes,
timers, function generators, etc. in experimental research.



REFERENCE

This manual is intended for use with the following textbook.


Horowitz and Hill
The Art of Electronics
2nd Edition, 1989/1990
Cambridge University Press


DATA BOOKS

Data books, such as the Texas Instruments or National Instruments references
for TTL, CMOS and LINEAR circuits, should be used by the student to check
pin designations, outputs, and other data not given in this lab manual. These
books are on a shelf in the lab.

iv
LAB NOTEBOOKS

As a part of training to be a scientist, students should maintain a personal
notebook just as a research scientist does. This lab notebook will not be graded,
but the student must have one and use it. It must be bound, with no loose pages.

Here is a guideline for lab notebooks: a notebook should contain sufficient
detail so that a year later the experiment could be duplicated exactly.

In the notebook, the student should:

draw a schematic diagram for every circuit that is built
label this diagram with
• part numbers
• pin designations
• output/input designations
show the major connections to external power
supplies, etc.

list the instruments used by type and model
include in this list: oscilloscope, multimeters,
function generators, etc.

record a table of all measurements
include units (e.g. mV) for inputs and outputs
record the scale (e.g. 200 mV) of the meter or
oscilloscope
for digital circuits, this table may be in the
form of a truth table

record error values for measurements that have an uncertainty:
list more than one measurement as an error check
estimate the error bar


PRELAB


In each lab, you are given prelab questions. These are intended to help you
prepare for the lab. You should write your response in this manual. These
questions are not handed in, and they are not graded. If you do not understand a
prelab question, be sure to ask your TA.

A spiral notebook is
ok for this course.
Researchers use
notebooks with
sewn-in pages

v
LAB REPORTS


For each Lab, students will individually prepare a lab report for grading. This report
is distinct from the notebook; the notebook is not a substitute.

Reports should be organized as a brief introduction, and then an experimental
section that is organized according to the section number.

Preface: brief introductory paragraph, ≈ 30 words, describing the report’s theme

Experiments

REPEAT THE FOLLOWING FOR EACH SECTION

Apparatus
schematic diagram, labeled with:
part numbers (e.g. 1N914 diode)
pin designations (on IC’s)
output / input terminals
list of instruments used (e.g., Tektronix 2235 oscilloscope)

Procedure
A maximum of three sentences to explain:
what was measured (e.g., amplifier gain)
what was varied (e.g., oscillator frequency)
how errors were estimated*

Results
indicates a response is required

where it is appropriate, response should include:
table and/or graph of results
label each curve
draw smooth curves through data points
label axes and indicate units (e.g., Hz, mV)
*estimate of errors, for analog measurements
as a column in a table
or as a representative error bar on a graph
sketch or print of the oscilloscope display, if one was used,
and discuss briefly in a few sentences the features
of the waveform to demonstrate that you understand
the significance of the waveform.
timing diagram or truth table, for digital circuits
explanation of any problems you encountered


vi

Handwritten lab reports are adequate. Typewritten reports are unnecessary. Be
brief, but write in complete sentences. For graphs, you may use “Graphical
Analysis”
†
or other software. If preparing your report consumes significantly
more than two hours, talk to your TA to ask if you are doing something that is
unnecessarily time consuming. Example lab reports are provided in the lab room
for you to inspect.

* Some instructors may not want error analysis included in the report. Ask to be
sure.


SYMBOLS USED IN THIS MANUAL

For your information
Instructions for you to follow
Requires an answer or comment in your lab report


RESISTOR COLOR CODE


MULTIPLIERS TOLERANCES

Silver -2
Gold -1
Black 0 none 20%
Brown 1 silver 10%
Red 2 gold 5%
Orange 3
Yellow 4
Green 5
Blue 6
Violet 7
Gray 8
White 9

†
as of 2001, Graphical Analysis was installed on PCs at this location:
Start\Programs\Vernier Software\GA
a b c d
a b × 10 Ω ± d%
c

vii
TREATMENT OF EXPERIMENTAL ERRORS

“Error” does not mean a mistake. It means an uncertainty.

In measuring any value, the result is not just one number, such as 5.3 Volts. It is two
numbers, 5.3 ± 0.1 Volts. The second number is the experimental uncertainty, or error
bar. It usually represents one standard deviation (one sigma) from the first value.

In making an analog measurement with a meter or an oscilloscope, you should record
not just the first number, but the error as well.

To decide what value to assign to the error, you must use some judgment. Here are
some tips:

Multimeters:
The manufacturer gives specifications for the precision for the multimeter.
The precision of a BK model 388-HD hand-held multimeter is:

DCV 0.50 % rdg + 1 LSD
ACV 1.25 % rdg + 4 LSD
F 2.00 % rdg + 4 LSD
ACA 1.50 % rdg + 3 LSD
DCA 1.00 % rdg + 1 LSD
Ω 0.75 % rdg + 1 LSD
Hz 1.00 % rdg + 3 LSD

where rdg is the reading on the display, and LSD is the least significant digit

Example:
1. You measure a DC voltage and get 5.30 Volt.
2. The uncertainty is given as 0.5 % of the reading, or 0.005 × 5.30, plus 1 ×
LSD, where the LSD is 0.01 here.
3. So the uncertainty is ± (0.005 × 5.30 + 1 × 0.01) = ± 0.0365 Volt.
4. Rounding to ± 0.04, your result is 5.30 ± 0.04 Volt.


viii
Agilent 34410A Multimeter:


ix
Analog oscilloscope:
The display has about 8 boxes (divisions) in the vertical direction and 10 boxes
horizontally. It also has 5 small tick marks within each division. You can usually
measure a value to about ± 0.25 of a tick mark, i.e. ± 0.05 divisions.

Example: You use the oscilloscope on the 2 Volt/div scale, and your result is 5.3
Volt. The error bar is ± 0.05 divisions, so your final result is 5.3 ± 0.1 Volt.

Propagation of Errors:

When two or more experimental measurements are combined into one by some
mathematical operation (such as a product P = I V), the two errors add in a non-trivial
way.

Example:
You measure I = 1.2 ± 0.1 mA and V = 2.5 ± 0.3 V. The error of P is computed using
partial derivatives, as:
dP = [ (∂P/∂ I)
2
dI
2
+ (∂ P/∂V)
2
dV
2
]
1/2

= [ V
2
dI
2
+ I
2
dV
2
]
1/2

= [ 2.5
2
0.1
2
+ 1.2
2
0.3
2
]
1/2

= 0.44 mW
so the result you report for the measurement is P = 3.00 ± 0.44 mW.

In general:
For F = F(A, B, C,...), the uncertainty δF in terms of the uncertainties dA, dB etc. is
given by:

dF
2
= Σ [(∂ F/∂ A)
2
dA
2
+ (∂ F/∂ B)
2
dB
2
+ ... ].

Note that errors add through their squares.

In this lab course, you are asked to use propagation of errors only twice, both
times in Lab 1, in order to develop this skill. These two instances are indicated in
Lab 1 with a footnote. Otherwise, to save time, do not perform propagation of
errors analysis.

x
TO THE INSTRUCTOR


HOW ONE CAN USE THIS MANUAL

This manual is intended for a one-semester course. The instructor probably
will skip some experiments or parts of experiments due to lack of time.

The first few experiments, analog circuits, will challenge the student the most.

One option is to skip some of the Labs and then finish with a project. This
project is a circuit invented by the student; it requires about four weeks. The
student must begin planning a special project several weeks before this four-
week period begins.


TO THE TA:

Before the students do their experiments, you must:
• do the experiment yourself
• inventory parts to be sure they are all available
• set up each lab table
• check batteries (in multimeters, for example)

If enough parts are available, leave your setup in working order for students
to examine when they have difficulty.

You will need to schedule additional time in the lab to accommodate students
who are unable to complete their work in 3 hours.

Students need their graded lab reports within a week of handing them to you.
If you take longer to grade them, students will not know whether they were
writing their report as expected, or whether they are omitting required
information or spending too much time on unnecessary efforts in writing the
report.

Mark your copy of the lab manual with the word “EDIT” to indicate changes
that need to be made for next year.

Copyright © 2011, John A. Goree Edited by John Goree 6 Jan 2011



REFERENCE: Horowitz and Hill: Sections 1.01 - 1.05
Appendix C (Resistor color code)
Appendix E (How to draw schematic diagrams)


INTRODUCTION

This experiment has the following objectives:


Become familiar with:
multimeter
prototyping board
resistor color code
reading a schematic diagram
wiring a circuit

Study a current source.

A voltage source (Figure 1-1) such as a battery or power supply is inherently a
device with a low internal resistance. It should provide a constant voltage for a
wide range of currents.

A current source (Figure 1-2) on the other hand should provide a constant current
to a wide range of load resistances.


Lab 1

DC Measurements

2
I
I
V
Figure 1-1
Voltage source
Figure 1-2
Current source
V
+
I
V

Study a voltage divider

A voltage divider is two resistors connected in series.


V
out
V
in
R
R
1
2
voltage
divider
V
out
/ V
in
= R
2
/ (R
1
+ R
2
)



R
series
= R
1
+ R
2

series
parallel
Series and parallel circuits.

3
EQUIPMENT

Digital multimeter (BK Model 388-HD)
Battery 1.5 V (2)
DC power supply
Resistors 100, 560, 1 k, 1.8 k, 3.2 k, 50k
Box of assorted of resistors
Switches (with wires soldered on): SPST, SPDT
Potentiometer (with wires soldered on, preferably > 1k)
Decade box (with 1000 Ω/decade or higher for top decade)
~8 V transformer (TA note: borrow these from lab coordinator if necessary)
Prototyping board
Two short solid-core wires (to stick into
prototyping board)
Alligator clips for multimeter test leads
Boards with three terminals (2)
Wire kit
2 A fuses, may be needed by TA

TA note: Check batteries and fuses
in all multimeters before use. Check
prototyping boards to verify that their
power supplies work.


Multimeter
alligator clip
banana plug

4
PRELAB


1. Identify all the formulas you will need in this Lab.

2. Describe one mistake that can damage a multimeter for each of the
following measurements:
A. voltage
B. resistance
C. current

3. State whether propagation of errors can be used to find the uncertainty
of:
A. a quantity measured directly using a multimeter
B. a calculated quantity based on more than one measurement
C. both of the above

4. Use the resistor color code chart to determine the value of a four-band
resistor with the colors brown, black, brown, gold.




5
PROCEDURE

Familiarization with equipment

The digital multimeter

The hand-held digital multimeter is used widely to make electrical measurements
of

Voltage dc & ac
current dc & ac
resistance
continuity
other quantities, such as frequency, depending on your meter’s features

Input impedance:
In the voltage mode, the input impedance of a digital multimeter is usually high
enough (several MΩ) that it has negligible effect on the circuit being measured.

Continuity check:
Many models allow you to check continuity, emitting an audible beep so
that you don’t need to look at the meter while making the test.

Hooking up the meter:
Note that the meter is connected:
• in parallel to measure voltage or resistance
• in series to measure current.
See the figures to see how this is done.

6




(a) Voltage V (b) Resistance Ω (c ) Current A
Multimeters. The meter can measure voltage, resistance and current. Note two things:
• which connectors at the bottom are used
• where the dial points (your dial might look different from the one shown here, but somewhere it will have V,
Ω, and A for measuring voltage, resistance, and current).
Input jacks on a multimeter
Figure 1-3. Using a multimeter (shown by the
circle with a R) to measure resistance.
Figure 1-4. Using a multimeter (shown by the
circle with a V) to measure voltage.
Figure 1-5. Using a multimeter (shown by the
circle with an I) to measure current.
R

7
1. DC voltage

Set the function switch of the multimeter to DC volts, with a scale commensurate
with the voltages expected in the circuit.

To protect the meter from damage:
ALWAYS SELECT THE MAXIMUM VOLTAGE SCALE TO START WITH.
Then adjust the scale downward until a meaningful reading is obtained.

(a) Batteries in series

Measure the individual voltages V1 and V2 of two 1.5 V batteries.

Then connect the batteries in series, with forward polarity, as shown in Fig. 1-7
and measure the total voltage V
f
meas
. Provide an uncertainty for this measurement
based on the multimeter’s specifications.

Reverse the polarity of one of the batteries and measure the total voltage V
r
meas
.
Provide an uncertainty for this measurement based on the multimeter’s specifications.

Calculate the expected results V
f
calc
= V1 + V2 and V
r
calc
=V1 – V2 for forward
and reverse polarity of the two batteries. Note that these are calculated quantities, not
measured quantities, so that their uncertainty must be calculated using propagation of
errors. Perform this uncertainty calculation, yielding δV
f
calc
and δV
r
calc


Compare these expected results V
f
calc
± δV
f
calc
and V
r
calc
± δV
fr
calc
to the
corresponding measured values V
f
meas
± δ

V
f
meas
and

V
f
meas
± δ

V
f
meas

In both your notebook and lab report, you must always draw a schematic of
whatever you connect, including components and any external instruments. For this
step, the schematics should look like Fig. 1-7. (As you gain experience, you will learn
how to draw schematics without being shown an example to copy.)

1.5 V
battery
1.5 V
battery
multi-
meter
1.5 V
battery
1.5 V
battery
multi-
meter
Figure 1-6.
Board with two batteries
Figure 1-7.
Two batteries connected in series, with forward
polarity (left) and reverse (right).

8
(b) Power supply

A power supply is a voltage source powered by 110VAC. You will use a “bench”
power supply, which has supplies an adjustable voltage.

A benchtop power supply typically has two knobs: voltage and current. The way it
works is that only one knob will have an effect, depending on two things: the setting
of the other knob and the load resistance. For example, if you turn the current knob up
to its maximum value and if you use a large resistance for a load across the power
supply outputs, the voltage knob will be the one that has an effect, while the current
knob merely provides an “upper limit” to how high the current is allowed to go. This
will be how you will usually operate the power supply in this course.

An analog meter, with a needle, will have a measurement error due to your ability
to read it. Use your own judgment of what you think is a reasonable value for the
error, based on factors such as the width of the needle, the spacing between tick
marks, and parallax due to viewing the meter with your head positioned at various
angles.

Set the power supply to two different voltages and measure each of these.

Does the value on the power supply's meter agree with the value measured on the
multimeter, within the error bars for the two measurements?

As a rule, remember that meters on a power supply are usually less reliable than
multimeters. Always make measurements of voltage and current with an external
meter.

As always, in both your notebook and report, draw the schematic for whatever
you connect, including external instruments. For this measurement, it looks like the
example below.

Power supply
DC
power
supply
V
+
-
+
-
Schematic

9
2. AC voltage and frequency

No error measurements are required for this section.

Set the AC/DC switch of the multimeter to AC and reset the scale to maximum
voltage.

Plug the primary of the transformer (it’s labeled 8 V rms, but you will measure
this yourself) into the AC outlet and measure the AC voltage of the transformer
secondary.

The wiring in a building in the USA is nominally rated at 110 V
rms, but will vary from this value. Since this is connected to the
primary of the transformer, and the voltage on the secondary is a
fixed fraction of the primary voltage, you may find that your output
voltage is not exactly the level printed on the transformer.

The electrical generators used by utility companies produce a 60
Hz voltage

If your multimeter has a frequency (Hz) feature, use it to
measure the frequency of the transformer output.

Return the multimeter’s ac/dc switch, if there is one, to dc.

3. Resistance
‡


Set the function switch to Ohms.

Check the meter by shorting the test leads. The meter should read zero ohms.

(a) Tolerances -- note: if you are color blind, skip this step and inform the TA.

For this step only, use the box of assorted resistors. Measure the resistance of six
of your resistors. (Use only resistors with four bands; resistors with five bands are
uncommon – sometimes the fifth band is used to indicate temperature sensitivity or to
provide an additional significant digit for the resistance value.)

Determine the fractional error from the nominal value (from the color code).


‡
If you are color blind, you will be unable to identify the colors on the resistors. Inform
the TA so that you can be excused from this step. Otherwise in this course, you will need
to rely on your multimeter to determine your resistor values, if you are unable to read the
color code.
Transformer

10
Make a table with seven columns: color code, nominal value, tolerance, measured
value, multimeter scale, multimeter error, multimeter error as a fraction of measured
value. How many of the values fall within the specified tolerance?

As always, draw the schematic. For this measurement, see Fig. 1-3.

(b) Series & parallel

Choose two resistors that are within a factor of ten of the same value. Wire them
up in series and then in parallel, using the board with three terminals. Measure the
series and parallel resistances.

Calculate the expected resistances (use measured values from part a) and
compare with your measurements, including error values calculated using
propagation of errors.
§


4. Potentiometer [no response needed for lab report]

The potentiometer has three terminals: two are the ends of a fixed resistor, and
one is the “wiper.” Using the two leads of the multimeter to test different pairs of the
potentiometer terminals, and by turning the potentiometer while observing the
resistatance: (a) determine which terminal is the wiper, and (b) verify that the
resistance between the other two terminals does not vary as the potentiometer is
turned.

Determine which pair of terminals has a resistance that increases when you turn
the potentiometer clockwise, as viewed from the top.


5. Continuity [no response needed for lab report]

With the function switch set to Ohms, touch the two test leads together while
watching the display.

Now set the function switch to continuity. Depending on your multimeter, this
might be indicated with a symbol like this: . Touch the two test leads together and
listen for the beep.


§
This is one of two instances in this lab where you are asked to use propagation of errors.

11

6. Current

(a) Measured values

Measure the actual value of the resistor shown in
Fig. 1-8. (Throughout this course, always measure your
resistor values before assembling the circuit.)

(b) Comparison to predicted values

Wire up the circuit in Figure 1-8. Set the function
switch to the maximum current scale (2 A) and connect the meter into the circuit.

Note that for current measurements the meter is in series with the other elements
of the circuit. This is different from voltage measurements, where it is in parallel.

Compare the measured value with the value of current you would calculate
using the measured values of voltage and resistance. Do they agree within the error
value range? Explain any discrepancy.


7. Current Source

Connect a 1.5 V battery and a large-valued resistor (10 kΩ or higher) to make a
current source. For a load resistor, use the decade box (choosing values ranging from
0.1 to 2.0 times the value of the large resistor), as shown in Figure 1-7.

Caution:

Avoid damaging your multimeter:

In the current function:

Never connect a current meter directly to a
voltage source like 110 VAC or a battery.
Without a resistor to limit the current, this
would destroy the meter, or at least blow a fuse
inside the meter.

In the resistance function:

Never connect the multimeter to a resistor that
is part of a circuit. Use it only to measure the
value of a loose resistor.
Figure 1-8.
1.5 V
+
I
560

12
This combination of a battery in series with a large resistor will act as a current
source, provided the load resistance connected to the terminals is small. The current
is:
I = V /(R
1
+ R
L
)
= (V / R
1
) / (1 + R
L
/ R
1
)
≈ V / R
1
for R
L
<< R
1
.

This is a crude current source, because the current depends on the load resistance,
especially when R
L
is not much less than R
1
. You will build a better current source
using a transistor in Lab 4.
Connect several different load resistances across the terminals and measure the
current through the load resistor and the voltage across it in each case.

(a) What is the most accurate way for you to use a multimeter to measure
current: passing the current directly through the meter, or using the meter to measure
the voltage drop across a known resistance and computing the current from Ohm’s
Law?

(b) Make a table and/or graph of current vs. load resistance.

(c) Compliance: Determine the range of load resistance over which the current
remains constant to 10%.
[Note: In Lab 4 (Junction Transistors) you will need this result again.]

8. Learn how to use the prototyping board [no response needed for lab report]

The prototyping board

Prototyping boards are rows of connectors wired together behind a plastic face.

Things you can stick into the little holes of prototyping boards include:

wire (22 gauge solid-core is typical)
resistor leads (1/4 or 1/8 Watt is typical)
leads for transistors, capacitors, diodes, etc.
ICs (the hole spacing is made for DIP [dual-inline package]chips)
Figure 1-7
Testing a current
source
1.5 V
load
resistor
large value
resistor

13

Put an alligator clip connector on each test lead of the multimeter.

Insert two wires into various holes in the prototyping board. Checking for
continuity, convince yourself that the board is connected as shown below.


what it looks like from outside connections under the surface

Prototyping Board

14


9. Voltage Divider - note: no error measurements required for this part

A voltage divider reduces a voltage to a desired level.

Measure the values of the resistors shown in Figure 1-8, then wire up the circuit.
Use the prototyping board. Use the 5-Volt power supply built into the
prototyping board to supply V
in
.
(a) without load

Measure V
in
and V
out
for R
1
= 3.2 k and R
2
= 1.6 k. Then repeat, with R
1
=
1.6 k and R
2
= 3.2 k. Compare V
out
/ V
in
to the predicted values.

(b) with load

Connect a 1 kΩ load resistor across the output. Determine how much V
out
is
reduced (loaded), reporting the reduction of V
out
as % of the unloaded measurement
of V
out
. Using the rules for parallel resistances, compare to theory.

Note that while a voltage divider is easy to build, it is a poor voltage source. Its
output is not “stiff.” It is easily “loaded.”

10. Potentiometer as a Voltage Divider [no response needed for lab report]

Examining Figure 1-8, use the potentiometer, instead of fixed resistors, to hook
up the circuit. Be sure that you apply the input voltage across the two terminals that
have a fixed resistance. The wiper of the potentiometer should be the output terminal
in the middle-right of Figure 1-8. Observe the output voltage while turning the
potentiometer. If the output voltage increases as you turn the knob clockwise, you
are done; otherwise change one of the output connections.


V
out
V
in
R
R
1
2
Figure 1-8
Voltage Divider

15
11. Familiarity with switches [no response needed for lab report]

An SPST switch has two terminals, while an SPDT switch has three. Use a
multimeter’s continuity-check function to test these two switches. Become familiar
with which terminals become connected or disconnected when the switch is in a
certain position.


SPST
SPDT

Copyright © 2011, John A. Goree Edited by John Goree 6 Jan 2011
BNC – banana
adapter
BNC - TEE
BNC cable


REFERENCE: Horowitz and Hill Sections 1.13, 1.18,1.19
Appendix A (Oscilloscope)


INTRODUCTION

The object of this lab is to learn measurement skills. You will become familiar
with the oscilloscope, function generator, and pulse generator, in measuring time-
varying electrical signals. You will measure:
• DC and AC voltages
• Frequency
• Phase
• Time constant of an RC circuit
• Amplitude and phase-shift responses of low-pass and high-pass RC filters.


EQUIPMENT

Analog Oscilloscope (Tektronix 2235 or equivalent)
Function generator (BK 4017)
Pulse generator (HP 8013B)
DC power supply
Multimeter – handheld
Multimeter – Agilent 34410A
~8 V transformer
Resistors 5 k
Capacitors 0.003 µF
BNC cables
BNC TEE
BNC-banana adapter
wooden board with binding posts
use computer to plot graphs
use graph paper at end of this manual
to draw waveforms
Lab 2

AC Measurements

17
ABOUT ANALOG OSCILLOSCOPES

The heart of an analog oscilloscope is its cathode ray tube (see Figure 2-1a). An
electron gun produces a beam of electrons that strikes the screen, making a visible
spot. The beam is deflected by two pairs of deflection plates. A sawtooth time-base
voltage is supplied internally to the horizontal deflection plates to sweep the beam
linearly with time across the screen horizontally. The voltage to be observed is
supplied to the vertical deflection plates. The result is a picture of the signal voltage
versus time.


Electron
Gun
Time Base Sawtooth
Vertical
Signal
Horizontal Input
Input
Figure 2-1

18
ABOUT TRIGGERING

Triggering of an oscilloscope is often confusing for beginners. A trigger is an event
that starts the horizontal sweep or trace (see Figure 2-1b). This trigger event is
defined by comparing two voltages: the moment when the “trigger source” voltage
exceeds the “trigger level” voltage is the trigger event.

The “trigger source” can be:

• an external voltage EXT that you connect to the oscilloscope,
• or it can be one of the vertical inputs, CH1 or CH2.

Usually you must choose between AUTO and NORM triggering:

• NORM triggering will allow a trigger only if there is a trigger event.
• AUTO triggering will cause a trigger even if there is never a trigger event
(the beginner will usually use this setting since it always yields a display).



trigger event
with positive
slope
trigger event
with negative
slope
time
voltage
trigger level
trigger source
waveform
Figure 2-1 b Oscilloscope triggering


19
ABOUT PHASE SHIFTS

20

One use of the oscilloscope is to measure relative phases between two
waveforms. This is done by adjusting both waveforms so that they are symmetric
about the horizontal line in the middle of the screen. Then note the time delay Δt
between the zero crossings, as shown in Figure 2-2. This corresponds to the phase
shift, which is φ = 360° Δt / T.






Figure 2-2
Ref Sig
Δt
T







Tip: Sometimes it is hard to get anything to show on the scope. In that case, follow
this procedure:

• set trigger to: AUTO, DC COUPLING,CH1
• set horizontal mode to: A
• set vertical mode to: BOTH
• set sweep speed to: 1 ms/div, CAL, MAGNIFIER OFF
• set vertical coupling to: ground
• turn up intensity
• wiggle vertical position until a horizontal line appears
• set vertical coupling to: AC or DC



21
Signal output
SYNC output
(labeled TTL/CMOS)
BK4017 Function Generator.

22
Pulse
Generator
Signal output
SYNC output
(labeled TRIGGER)

23
TA Instructions:

Before the students begin this lab, demonstrate the following tricks for using an
analog oscilloscope:


Checking trace rotation:
o Adjust the intensity as low as practical so that the trace is a thin curve
o Use GND coupling to produce what should be a horizontal line.
o Use the vertical position to move the curve to the middle of the display
o Verify that the trace is aligned with a horizontal gridline that is shown
permanently on the display
o If not aligned, the scope requires using a small screwdriver to adjust the trace
rotation; this is required for approximately one out of ten oscilloscopes each
year.


Measuring amplitude:
o Adjust the intensity as low as practical so that the trace is a thin curve
o Adjust the vertical scale so that the waveform fills most of the display area
o Use the GND coupling to adjust the zero, then the AC or DC coupling to
make the measurement.
o Adjust the horizontal position so that the peak or trough of the waveform
coincides with the vertical line that has fine tick marks
o Adjust the time scale so that the peak has an appropriate width, so that you
can accurately find its maximum; typically this will require that you see
about two complete oscillations in the horizontal direction.
o Use the second trace (other input channel) with a GND input to make a
cursor, as an optional aide in measuring the waveform’s height.


Measuring time:
o Adjust the vertical scale to overfill it by a significant amount, so that the
waveform looks like almost straight vertical lines; then look for zero
crossings
o Use the lowest practical intensity.
o Adjust the horizontal position so that one of the zero crossings occurs on a
major division of the horizontal scale.
o Choose an appropriate time scale for best resolution in measuring the time;
typically about 1.5 or 2 complete oscillations displayed

Demonstrate the use of the dual time base.

24
PRELAB


1. Recall the relationship of angular frequency ω (units s
-1
) and the
usual frequency f (Hz).

2. Find the formula for the frequency response curve for a high-pass
filter. This formula will be an expression with the ratio Vout / Vin
on the left-hand side of the equation, and the right-hand side will
depend on f, R, and C.

3. Find the formula for the frequency response curve for a low-pass
filter.

25
4. PROCEDURE

Read Appendix A in the text. You will need to know oscilloscope terminology
before you begin.

In this lab you will record a lot of data. It is recommended that you record it in
columns in your lab notebook, with separate columns for:
the reading (e.g. in mV)
the scale on the oscilloscope (e.g. 100 mV per division)
the estimated error for each measurement.


1. Measurement of Voltages

(a) DC Voltages

To start, adjust the oscilloscope settings to the following:
**


vertical mode CH1

CH1 Volts/Div 1 Volt (use 1× indicator on
dial)
input coupling GND
CAL knob fully clockwise to click

CH2 same as CH1
set INVERT switch to the out position



**
These instructions are for Tektronix 2235; other analog oscilloscopes are similar.

26
horizontal mode A

time base A sec/div 1 ms

CAL knob fully clockwise
to click

var holdoff NORM

A trigger P-P AUTO

level turn to approximately middle of knob’s
range

slope rising slope

A & B INT CH1

A source INT

A ext coupling DC







Once you have found a trace that looks like a horizontal line, use the vertical
position knob on CH1 to position the trace in the center of the display. Then
change the CH1 coupling to DC.

Connect the output of an adjustable power supply to the oscilloscope input
CH1. You should see a trace at a non-zero voltage. Change the CH1 coupling
back and forth, from GND to DC, to see the difference.

Set your power supply to three different DC voltages and measure each
voltage with both the oscilloscope and the digital multimeter.

Adjust the display intensity as low as practical so that the trace is a thin line.

Compare the DC voltage measurements of the oscilloscope and the digital
multimeter. Report the measurement uncertainty (error) values, based on the
specifications for the multimeter and the oscilloscope, and your impression of
how precisely you can read the oscilloscope display. Which is more precise, the
meter or the oscilloscope?

(b) AC Voltages


27
Connect the function generator to signal input CH1 of the oscilloscope.

Set the function generator to produce a sine wave of about 1 to 2 Volt
amplitude, a frequency of about 100 Hz, and no DC offset. Verify that the
sweep INT/EXT switch is in the EXT position, to disable the “sweep” feature of
this instrument.

Set a multimeter to the AC voltage function. Connect it to the function
generator’s output.

Adjust the display intensity as low as practical so that the trace is a thin line.

Measure the peak-to-peak AC voltage using the oscilloscope. Calculate the
RMS value of the voltage.

Compare the AC voltage oscilloscope measurements to those on the digital
multimeter. Report the measurement uncertainty (error) values. Which is more
precise, the meter or the oscilloscope?

(c) Sweeping the frequency [no response necessary for lab report]

Learn to use the frequency sweep feature of your function generator.
“Sweeping” means that the frequency is ramped up with time in a way that
repeats itself. Observe this by depressing the INT button to enable the sweep.
Observe the waveform. Try different adjustments of the TIME and WIDTH
knobs for sweep to see their effect.

When you are done, return the generator to its normal (non-sweep) operation
by pressing the INT button so that it is out (not in).


(d) AC and DC coupling

Make sure the oscilloscope coupling is set to DC.

Set the function generator frequency to about 10 kHz.

Turn on the offset voltage on the function generator, and twiddle the offset
up and down. You should see a vertical deflection of the trace.

Now change the oscilloscope coupling to AC and twiddle the offset voltage
slowly. You should see no change.

Explain the difference between AC and DC coupling. Through what
additional component inside the oscilloscope does the signal pass when using
AC coupling?

28




2. Measurement of Frequency

Use the same set-up as above. Use a digital multimeter to measure
the frequency, connecting the multimeter to the function generator’s
SYNC output (which might be labeled TTL, depending on the model of
the function generator). Determine the frequency from the measured
time per cycle. Repeat for five frequencies in total, over the entire
range of the function generator.

Make a table to compare the frequency measurements made with the
oscilloscope to those with the digital multimeter. Include columns for the
measurement uncertainty (error) values. Which is more accurate, the meter or
the oscilloscope?

As a practice, never trust the frequency and voltage readings shown on a
function generator. Always make external measurements of the frequency and
voltage.

For the remainder of this experiment, use the multimeter for your frequency
measurements.

3. Time Constant of an R-C Circuit

The output voltage of an RC filter is

V(t) =V
max
[1 - exp (-t/RC)] for charging
V(t) = V
max
exp (-t/RC) for discharging




Sync
output of
f nction

29





Rules of thumb:

RC The product RC is called the “RC time constant” or simply the “RC
time”.

When the input voltage of an R-C circuit changes from one level to
another, the output voltage will approach its final value asymptotically.
RC is the time required for the output to swing by 63% toward its final
value. [Because 1 – exp (-1) = 0.63.]

5RC is the time required to swing within 1 % of the final value. [Because
exp (-5 ) = 0.007 ± 1% ]




Measure the actual values of a 5 k resistor and an 0.003 µF capacitor.

Connect the series R-C circuit to a function generator and an oscilloscope, as
shown in Figure2-3 a and 2-3 b. (This circuit is shown two ways to help you
figure out how to wire it up.) Use a wooden board with binding posts.

Set the multimeter to measure frequency, and connect it to the function
generator.

Set the oscilloscope for external triggering (EXT), and connect the
trigger input to the SYNC output of the function generator (BNC connector
labeled TTL/CMOS, see photo).

Set the function generator to produce square waves with a peak-to-peak
amplitude of about 5 Volts. Set the frequency so that it is appropriate for
RC
time
V
max
0.63 V
max
RC
time
V
max
0.36 V
max
discharging charging
Sync
output of
f nction

30
measuring the time response of the R-C circuit -- the period τ = 1/f should be ≥
10 RC.

Set the oscilloscope vertical voltage scales to be the same, choosing a scale
so that the trace fills a large portion of the screen. (If it fills only a small portion
of the screen, your measurements will not be very precise.) Use GND input
coupling to find where zero volts is, and use the vertical position to locate this
on a gridline.

If your function generator has a DC offset, adjust it so that the bottom of the
waveform is at zero volts. Draw the oscilloscope display for the square wave,
and indicate the voltage at both the bottom and top of the waveform.

Adjust the oscilloscope horizontal time base so that the discharge time takes
a considerable portion of the display. Use the horizontal position to locate the
waveform so that the triggering time is at a convenient gridline.

(Your display should look like the figure above, labeled “charging.” Change the
oscilloscope trigger slope between + and - to see the difference it makes.)

(a) Charging

Determine the charging time constant from the oscilloscope display.
Estimate your errors. Calculate the ratio of your charging time to RC.

(b) Discharging

Change the scope triggering slope to see the discharge portion of the trace.
(Your display should look like the figure above, labeled “discharging.”)

Determine the discharging time constant from the oscilloscope display.
Estimate your errors. Calculate the ratio of your discharging time to RC.




31




V
out
OUTPUT
SYNC OUT
(labeled TTL)
5 k
0.003 µ
Figure 2-3 b
CH1 CH2
EXT
FUNCTION
GENERATOR
black
wire
red wire
BNC cable
banana plug cables
red wire
black
wire
oscilloscope CH1 displays the
input waveform, CH2 displays
the output waveform
R
C
input
output
Figure 2-3 a

32
4. R-C Low-Pass Filter

(a) Amplitude Response

Use the same circuit as above, in Figure 2-3 b.

Set the function generator to produce a sinusoidal output with an amplitude
of about 4 or 5 V peak-to-peak. Use a 1, 2, 5 frequency sequence from 500 Hz
to 10
5
Hz (i.e., 500, 1000, 2000, 5000 Hz, ...). Also include a frequency of
f =1 / (2πRC) in this sequence, as calculated from measured values of R and C.

(i) Record a table of your data with columns for:

f (Hz), error bar for f
V
in
, oscilloscope scale for V
in
, error bar for V
in

V
out
, oscilloscope scale for V
out
, error bar for V
out
.
V
out
/ V
in


(ii) Plot the voltage ratio V
out
/ V
in
with log-log axes, with frequency
on the horizontal scale. Make a theoretical plot of V
out
/ V
in
on the
same graph. You may use Graphical Analysis or other software to
create the graph using the instructions below.

Compare the frequency at V
out
/ V
in
= 0.707 to f = 1 / (2πRC)

(b) Phase Response

Use the same circuit as above.

Measure the phase shift of the voltage across the capacitor relative to the
input voltage from the oscillator, for the same frequency sequence as for the
amplitude response above.

(i) Record your data with columns for:
f (Hz), error bar for f
delay in msec, error bar for delay in msec
phase in degrees, error bar for phase in degrees.

(ii) Plot the phase curve on a graph with a semi-log scale (θ linear, f log).
Compare with a theoretical plot of the phase shift on the same graph.

(iii) What is the phase angle, measured and theoretical, at f =1/(2πRC)?


33

5. R-C High-Pass Filter

Use the same 5 kΩ resistor and 0.003 µF capacitor as in the R-C low-pass
filter, above, but swap them to make a high-pass filter.

High Pass Filter
V
out
5 k
0.003 µ



Instead of the analog oscilloscope, use the Agilent 34410A multimeter to
measure the input voltage (rms) and output voltage (rms).

Make the amplitude response measurements and plots (omit the error
values). Use the following frequencies: 100 kHz, 50, 20, 10, 3, 1 0.3 0.1 kHz;
also include a frequency of
f =1 / (2πRC) in this sequence, as calculated from measured values of R and C.


6. Measurement skills [no response necessary for lab report]

Use your remaining time in the lab session to familiarize yourself with the
oscilloscope and pulse generator.

Connect the function generator to the oscilloscope as shown in Figure 2-4.



OUTPUT
SYNC OUT
FUNCTION GENERATOR
or PULSE GENERATOR
Figure 2-4
CH1 CH2 EXT
SCOPE


34
Adjust the oscilloscope settings to the following settings (hereafter referred to as
the original settings).

Vertical mode BOTH and ALT

CH1 Volts/Div 1 Volt
(use 1X indicator on dial)
input coupling DC

CH2 same as CH1
INVERT switch set to the out position


horizontal mode A

time base A sec/div 0.2 ms
VAR HOLDOFF NORM
A TRIGGER P-P AUTO
LEVEL turn to about the middle
SLOPE out
A & B INT CH1
A SOURCE EXT
A EXT COUPLING DC

Adjust the function generator settings to produce a sine wave with a frequency
of about 100 Hz, with DC offset turned OFF.

Learn the various vertical modes

Try various settings of the vertical mode switches and see the display.

Use the “BOTH” mode on the vertical mode. Compare ALT and CHOP. (You
may need to adjust the beam intensity on the scope.)

Change time base to 5 µs, observing the display. Again compare ALT and
CHOP. You may be able to see the chopping of the signals in the chop mode.

Do you understand the difference between ALT and CHOP?


Learn how to use X-Y mode.

First return the oscilloscope to the original settings.

Then set the time base to X-Y.

Vary the CH1 and CH2 volts/div and see how the display changes.


35
Make a Lissajous figure. Connect a ~8 V transformer to CH1 and a sine wave
from the function generator to CH2. Adjust the frequency to multiples of 60 Hz
while watching the display.


Learn about trigger levels

First return the oscilloscope to the original settings, and set the time scale to 0.2
ms.

Change the cables to the function generator as shown in Figure 2-4.
Adjust the function generator to produce a sine wave of about1 kHz.

Vary the trigger level and the trigger slope. Observe the results. Do you
understand why the display depends as it does on these settings?

Change the trigger mode from AUTO to NORM. Adjust the trigger level and/or
the function generator amplitude until you see that the oscilloscope no longer
triggers. Then adjust them back so that they do trigger. Do you understand why
the display depends on these parameters in the NORM trigger mode?

Return the oscilloscope triggering to AUTO before proceeding.


Learn how to use dual time bases
††

Adjust the B Delay Time to zero (or as near as possible).

Set the HORIZONTAL MODE to ALT.

Turn B Trigger level all the way to +

You should see an additional set of traces, corresponding to A and B. If not,
adjust A and B intensity or other settings until you do.

Turn A/B SWP SEP to separate the two sets of traces.

Vary B Delay Time and observe the result.

Pull the “DLY’D SWEEP” knob and
adjust delayed sweep time scale to 20 µs. Observe the effect on the display.

Note the de-intensified display of A; this shows the portion of the A trace that is

††
Some models, such as Tektronix 2235, have a “delayed sweep” feature required for this
step, while others do not. If your scope doesn’t have this feature, and if the lab is
equipped with more than one type of scope, ask your TA.

36
represented by the B trace.

Change the function generator to produce a square wave at 1 MHz. Set the
oscilloscope to A sec/div = .5 µs, B sec/div = .05 µs.

Adjust the B DELAY ten-turn knob until you find the trailing edge of the
square-wave waveform. You may have to adjust the B intensity knob to see the
trace; start off with the B intensity knob in a central position.

Measure the decay time of the trailing edge. Do you see ringing, i.e., unwanted
oscillations? Does the function generator produce this ringing at much lower
frequencies as well?

Repeat with the leading edge.

37
Learn to use the pulse generator

Repeat the steps above, using the delayed sweep, but this time use
the Pulse Generator instead of the Function Generator. The purpose
of this exercise is to learn about pulse generators, and to further
develop oscilloscope skills.

For the output, use the pulse generator’s right-most BNC connector.

Adjust the HP pulse generator so that it has the following settings:

Sliding switches:
Pulse period: 20 n - 1 μ
Pulse delay: adjust to minimum
Pulse width: 10 n - 1 μ
Amplitude: adjust to maximum

Other switches:
Pulse: Norm
Offset Off
Output Norm
Int Load IN

Verniers:
Pulse period 3 O'Clock position
Pulse width 12 O'Clock position
Amplitude 12 O'Clock position

When examining the delayed traces, notice whether the pulse
generator produces a more ideal square pulse than the square wave from the
function generator. Look for "ringing", i.e., unwanted oscillations.

After you've finished with the delayed trace:
• Set the scope to Horizontal Mode A, so that you are no longer
viewing the delayed sweep.
• Pull the time base knob out so that the mark on the inner knob is
aligned with the two black marks on the clear outer knob; now they
will move together when you adjust them. Be sure that the vertical
coupling of CH1 is set to be DC.
• On the pulse generator, vary the following, and observe how the
waveform changes:
o Vary the vernier for pulse period.
o Vary the vernier for the pulse width (up to a maximum of 50%
of the pulse period).
o Switch between NORM and COMPL output.

38
Use these blank “oscilloscope screens” to sketch waveforms shown on an
analog scope.





39
Use these blank “oscilloscope screens” to sketch waveforms shown on an
analog scope.



40


REFERENCE Horowitz and Hill Sections 1.25- 1.30. rectification
filtering, regulation

Sections 6.11 - 6.14 power supply parts

Sections 6.16 3-terminal regulator

INTRODUCTION

In this lab we examine the properties of diodes and their applications for power supplies
and signals:

• Diode rectification in half-wave and full-wave bridge circuits
• Filtering in power-supply circuits
• 3-terminal voltage regulator
• Zener diode and its use as a voltage regulator
• Diode clamp circuit
• Silicon controlled rectifier (SCR)

Note: the instructor probably will choose not to do all of the above. Professor Goree
usually skips the SCR.

We will also build our measurement skills, learning to use a digital oscilloscope.
Lab 3

Diodes, Power Supplies,
Zeners, and SCR’s

41
EQUIPMENT

Digital Oscilloscope
Function generator
Multimeter (handheld model only; to protect the more expensive
Agilent 34410A, do not use it for this lab.)
Prototyping board
Transformer, center tapped ~8 V
Power diodes (4 & spares)
Signal diodes (2) 1N914 (or similar)
Zener diode (1 & spares) 5 V or 10 V
Resistors 91 (2 W), 1 k, 2 k, 10 k
56 k, 110 k [for SCR circuit]
Capacitors 0.47 µF, 100 µF (2),
1000 µF [for SCR circuit]
0.01 µF, 4.7 µF [for 3-term. regulator]
Variac [for 3-term.regulator]
Decade Box
Potentiometer 50 k [for SCR circuit]
SCR [for SCR circuit]
0 - 30 V dc power supply
6 V lamp [for SCR circuit]
3-terminal regulator 78L05 in TO-92 package [for 3-term. regulator]



1N914 signal diode Power diode
TO-92 package

42
PRELAB


1. Identify all the formulas you will need in this Lab.

2. Identify a type of connector you should not connect to a power diode, to
avoid melting it.


43
PROCEDURE

0.0 Ohmmeter Check of Diode

You will need a multimeter to confirm the polarity of a diode. You don’t need
to report your results here in your lab report.

Use a magnifying glass to read the part numbers of your diodes, and to
distinguish the signal, zener and power diodes.

Use your oscilloscope (or a second multimeter) to determine which of the
multimeter leads has a + voltage and which is - .

Look at your multimeter to see what special features it has. It might have a
diode-check feature, as shown by the symbol . If it has one, depending on
the model, the display might indicate the diode-drop voltage if the diode is not
damaged.

Confirm the PN polarity of a signal diode (these are smaller than the power
diodes)
.
Look at the markings on the diode to see how they show the polarity.

Repeat the diode tests above, using a power diode.

Note that for a multimeter check to work, the multimeter must apply a voltage
of at least 0.5 Volt between its two leads to bring the “diode” into conduction
when it is forward biased.




CAUTION:

In this lab it is possible to burn up components if you are not careful to make sure
everything is correct before turning on the power.

• Please note all caution statements in these instructions.

• Double-check before turning power on!

• When using the decade box, do not set the load resistance below 50 Ω.

44
Learn to use the digital oscilloscope & a scope probe – no written response
required

Connect a scope probe to the digital oscilloscope as shown in the photos
below.

A scope probe has two settings: 1X and 10X (photo shows 10X setting). This
notation might be confusing: the 10X setting has the effect of dividing, not
multiplying, the signal by a factor of ten. An advantage of a 10X probe is that
it has a higher frequency response, so that if you need to view signals >
1MHz, you should use a 10X probe. In most of this course you will view
slower signals, and a 1X probe is adequate.



45
Press the AUTOSET button to see the display of a square wave


Push the CH2 MENU button twice, to see how this turns the display for that
channel off and on. The two channels are distinguished by their color.

To see their effect, adjust these knobs: VOLTS/DIV,
SEC/DIV, VERTICAL POSITION, HORIZONTAL
POSITION. Look for the arrow at the top that indicates the
trigger time, and the arrow at the right that indicates zero
voltage.

Push the CH1 MENU button to see the display. Toggle the
“Probe” setting so that it indicates the same setting as on your
probe (1X, not 10X or 100X).You must always check this,
before using the scope, to avoid X10 errors in your voltage
measurements.

Push the TRIG MENU button and view the menu. Sometimes
in this course you will be asked to use EXT TRIG, which you
can accomplish with the digital oscilloscope by toggling
“Source” to the “Ext” setting. At other times, you might wish
to use CH1 as the “Source.”

Indicates
trigger
level
“Trig’d” Indicates the scope is
triggering & updating its display
Time per division (horizontal) Volt per division
(vertical)
Arrow
indicates
“zero voltage
level” for
Channel 1

46
Build your triggering skills:

First, choose “AUTO” trigger, with the source specified as the same channel
(1 or 2) as your probe. While viewing the square wave from the probe, adjust
the trigger level knob up and down while watching the display. Observe that
when the trigger level is adjusted too high or too low, there is a loss of
triggering, and this can casuse the displayed waveform to be unstable in the
horizontal direction.

Next, repeat with “NORM” rather than “AUTO” trigger. In this mode the
scope will not update the display unless there is a valid trigger event, unlike
the “AUTO” mode which will trigger occasionally even when there’s no
valid trigger event, just so that you can see at least something on the display.

Push the MEASURE button and view the menu. Toggle the “Type” setting to
measure the peak-to-peak voltage and the frequency of the waveform. Then
adjust the TIME/DIV knob so that less than one full oscillation is shown, and
notice how the scope is unable to measure frequency.




0.5 Digital Oscilloscope Skills


Use the function generator to apply a sine wave of approximately 1.2 – 1.8
volts amplitude and approximately 1 kHz frequency to one of the oscilloscope
inputs. Using the MEASURE button, measure the amplitude and period of the
waveform.

Compute the uncertainty of the amplitude, and the uncertainty of the period.
Do this using specifications from the oscilloscope’s manufacturer’s user manual. For
the TEDS 1000 or 2000 seies oscilloscope, see pp. 155-156. To compute the
uncertainty of the period, you will require the sampling period, which is the
reciprocal of the sampling rate; this parameter depends on the tim/div setting – see
the table on pp. 22-23. (Note: this exercise is intended to train you to look up
manufacturer’s specifications for an instrument, which is a routine all physics
experimenters should follow in their research.)


47

1. Diode Limiter

Diode limiters could be used at the inputs of small-signal instruments to protect
against accidental application of large input signals.

This is a low-current application, so you use signal diodes, not current diodes.

Using signal diodes, connect the input of the circuit shown below to the function
generator, set to about 1 kHz. Check that the –20 dB attenuator switch on the
function generator is not activated. Try sines, triangles, and square waves of various
amplitudes. Try a low amplitude for the oscillation, and adjust the function
generator’s dc offset to observe the effect of clipping.

Print the output of the clamp circuit for sine wave inputs two ways:
o With an output waveform that is severely clipped.
o With an output waveform that is only slightly clipped.

Comment quantitatively on the output amplitude where clipping is strongly
observed and explaining what parameter for a diode determines this clipping
amplitude.



2. Diode Clamp


Connect the clamp circuit in the figure. Use the +5V power supply built into your
prototyping board.

This is a low-current application, so you use signal diodes, not current diodes.

Connect the input of this circuit to the function generator with a sine wave at 10
kHz. Check that the –20 dB attenuator switch on the function generator is not
activated.

1 k in out
Diode
Limiter

48
Set up the oscilloscope to show the input and output waveforms. Use DC input
coupling on the oscilloscope,

Connect three grounds together: the ground of the 5 V power supply, the ground
of the function generator, and the ground of the oscilloscope.

Adjust the sine wave amplitude so that you can see a “clamped” output.

Print the waveforms for the input and output of the clamp circuit. (If you can
see that the clamped voltage is not quite flat, then you can see the effect of the
diode’s non-zero impedance in conduction.) From your observation explain in
one or two sentences what the clamp circuit is useful for.


+ 5 V
1 k in out
Diode Clamp

49
3. Power Supply Rectifying Circuits

(a) Half Wave Rectification

Set up the circuit as sketched here. (For this part, ignore the center-tap (CT)
terminal on the transformer, if any.) You are building a power supply, so use power
diodes rated at 1 W, not the little signal diodes. Use the decade box, set at 1 kΩ, for
the load resistor.

Caution: do not use the insulated “mini grabber” connectors
to connect the transformer to power diodes; they can melt and
they are costly. Use alligator clips, and arrange the components
on your prototyping board so that the alligator clips will be
separate by > 2 inches so that they don’t short.

If using an analog scope, set the oscilloscope as follows:
input coupling DC (CH1 and CH2)
vertical mode BOTH and either ALT or CHOP (this is the
dual trace feature)

In the figure below, the waveforms are shown as they would look if the input
voltage had a 1 V amplitude. Notice the 0.5 V diode drop. Also notice the time
interval shown between the dashed lines, when the input voltage is positive but not
large enough for the diode to conduct.



110 VAC
Scope
Input 2
Scope
Ground
~8 VAC
PRI SEC
R
L
Scope
Input 1
variac transformer
Mini-grabber
connector

50

-1
0
1
time
0.5 V diode drop
0.5 V diode drop
Half-wave rectification
shown for input
of 1 V amplitude
output
input



Adjust the variac to indicate 10 VAC output.

Measure the P-P amplitude of:
o the input voltage V
i
from the transformer
o the rectified sine wave output voltage V
o
across the 1 kΩ "load resistor"
R
L
.

Print the input and output waveforms and compare the peak output voltage with
half of the PP input voltage. Explain the difference.

Repeat without the variac, i.e., with the transformer powered directly from a
110 VAC outlet, so that the waveform from the transformer has a larger output.


(b) Full Wave Rectification

CAUTION: In this part, carefully check the polarity of your diodes before
turning on the 110 VAC power-- otherwise you may burn up the diodes.

Measure the output waveform of the full-wave bridge circuit in Figure 3-3. Do
not attempt to measure the input waveform. Use a 1 kΩ load resistor.
Print the output waveform. Compare to the half-wave rectifier.

51
4. Power Supply Filtering

CAUTION: In this part, carefully check the polarity of your capacitor before
turning on the 110 VAC power.

Starting with the full wave rectifier circuits in
Figure 3-3, add a capacitor C = 100 µF across the
load resistor as shown in Figure 3-4. Note that
capacitors of such a large value are polarized: one
of the capacitor’s two leads is marked – or +.

Measure the DC output voltage.

Measure the PP AC ripple using the
oscilloscope.
[Try this two ways:
• first with DC input coupling
• repeat with AC input coupling.
You should find that AC coupling allows you to use a smaller scale in Volts/div
and thereby make a more accurate measurement.]

Print the output waveform. Discuss in one sentence how the filter capacitor
improves the performance of a power supply.

Repeat with a capacitance of 1000 µF.

Using the capacitance of 100 µF, try a smaller load resistance (CAUTION: do not
set the load resistance below 50 Ω).

List your results. Compare with the calculated values of the DC output and ripple
voltages. (See text p. 46.)

Discuss (in two or three sentences) two factors that cause ripple to become worse.

R
L
Figure 3-4 Filter Capacitor
100 µF
Figure 3.3. Full-wave rectifier bridge circuit. The input is 110 VAC line voltage.
Scope
Input
P RI SEC
Scope
Gnd
1 1 0 V A C
R
L
+
-

52

Note that in a power supply, a bigger capacitance gives better filtering, but with
the tradeoff that the component is costlier, larger and heavier.

Note that the smaller the load resistance, i.e. the larger the current that the power
supply must deliver, the worse the ripple.

Do not disassemble the full-wave rectifier yet.

5. Voltage regulation with 3-terminal regulator

3-terminal voltage regulators are easy to use.
From the outside it looks like a transistor, but on
the inside there is a good regulator that makes use
of negative feedback. It features thermal
protection so that it is hard to burn up.


(a) Simple test

Connect the 78L05 regulator on your prototyping board, as shown below. For
an input, use the +12 Volt power supply that is built into your prototyping board, or
an external power supply set to about +10 V.

Confirm that the output is +5 Volts.


An ideal voltage regulator supplies the same output voltage,
• regardless of the input voltage, as in test (c), below
• regardless of the output load, as in test (d), below

(b) Use as a power supply regulator

Now connect the input of the 78L05 regulator, as shown below, to the output of
the power supply you built in step 3. Include a 1 kΩ load resistor.
78L05
input +5 V output
4.7 µF
(to guard
against
oscillations)
0.01 µF
(to provide
short
term smoothing)
out
in
GND
78L05

53

Turn on the power supply, and observe the output voltage.

Compare to the filtered output without regulation, as measured in step 2.


(c) Regulation as the input voltage is varied

Now plug the transformer of your power supply
into a variac instead of into 110 VAC. Connect a
multimeter to measure the Volts ac from the variac

Set R
L
to 1 kΩ.

Adjust the variac output voltage, beginning at110
VAC and going downward, using the printed scale on
the top of the variac. (Caution: Do not operate the
variac at voltages above 110 V)

Confirm that the regulator maintains the same +5
Volt output over a wide range of AC voltage (typically from 80 to 110 VAC).

(d) Regulation as the load is varied

Disconnect the variac. Connect your multimeter to measure current through the
load.

CAUTION: In this step, to protect the decade box, always keep 50 or
100 Ω switched in while you adjust the other scales. This precaution will keep you
from accidentally setting the decade box to zero resistance.

CAUTION: As always, measure current beginning with the meter set to the highest
scale.

Set R
L
to about 10 kΩ, and vary it downward.

Variac


78L05
+5

V

output
4.7

µF

0 . 01

µF

100 µF

filter

capacitor
3-termina l
regulator
PR I SE C
R
L

54
Note the load resistance at which ripple begins to appear. What current value
does this correspond to? (This is the maximum regulated current.)

(e) Thermal protection

Continue to decrease the load resistance. Does the output of the regulator shut
down when the current exceeds a certain threshold? This is the current limit of your
regulator.

[To work, this shut-down test requires using a regulator in the TO-92 package; don’t
use a larger package like TO-220 (LM78M05CT) for this lab -- it won’t shut down
under these conditions.]


An advantage of these three-terminal regulators is the shutdown feature. Another
alternative for voltage regulation is the zener diode in the next experiment, but
zeners do not have thermal protection, so you must be careful to select the right one
and use it within its design parameters.


6. Voltage Regulation with Zener Diodes

[TA note: usually there is enough time to do all of this lab except that some
instructors omit the SCR circuit to save time. If it is necessary to reduce the time
further, this zener diode section is the next choice for omission.]

Zener diodes can be used as a simple voltage regulator to establish a reference
voltage source for non-critical applications.

CAUTION:
Zener diodes are very easy to burn up if you make a goof in wiring them up.

Remove the 3-terminal regulator and its accompanying two capacitors.

TO-92 package TO-220 package

55
Select an appropriate Zener, depending on your transformer (5V for a 6.3 V
transformer or 12 V for a 12.6 V transformer). Assume the Zener has a power rating
of 0.4 W in either case.

Add a Zener diode across the output of the PI filtered power supply you built in
part 3b, as shown in Figure 3-6.


91 Ω
100 µF 100 µF
R
L
Figure 3-6 Pi filter with Zener diode voltage regulator




(a) Calculations

Before you power up your circuit, do the following:

(i) Determine the maximum Zener current I
Z max
from the power rating.
(ii) Determine the total current through the 91 Ω resistor:

I
tot
= (V
in
- V
Z
) / 91.0 Ω

This current is the sum of the Zener current I
Z
and the load current I
L
. Check
whether it exceeds I
Z max
because there must then be a minimum load
current to prevent overloading the Zener. This will imply a maximum load
resistance.

(b) Measurements

(i) Output voltage

Power up your circuit.

Measure the voltage across the load resistance.


56
CAUTION: in the next step, to protect the decade box, always keep 100 Ω switched
in while you adjust the other scales. This precaution will keep you from accidentally
setting the decade box to zero resistance.

(ii) Regulation

Vary R
L
beginning at 10.1 kΩ and stepping downward to 500 Ω.

Note the load resistance above which the voltage stays approximately constant.
To what current does this correspond?



7. SCR Circuits [optional depending on whether TA finds there is enough time]

(a) DC Control – RC Timer

The circuit in Figure 3-8 switches on a voltage across the load after a delay. The
SCR switches on when the gate voltage (G) exceeds the cathode voltage (C). The
gate voltage is developed across the capacitor C
1
by the R-C combination.

Short C
1
to remove all charge.

(i) Switching time

Turn on the supply and determine the time for the voltage to be switched across
the load.

(ii) Gate voltage

Short C
1
again and repeat with each of the resistors in the time constant portion
of the circuit.

Determine the gate voltage necessary for firing.



57


56 k, or
110 k
1 K
SCR
A
G
C
+
6 V
C1
100 µF
scope in
CH1
scope in
CH2
scope
GND


Figure 3-8 RC Timer


(b) AC Control -- Lamp Dimmer

In the circuit in Figure 3-9, the R-C combination acts as a phase shifter for the 60
Hz AC voltage. As R is increased, the phase is shifted from 0° to -90 °. With this
circuit it is possible to control the output half wave from completely on to
completely off.

Use the input from the transformer secondary as a reference signal for the
oscilloscope.

Observe the phase shifted wave across C and the output wave across R
L
.
Explain why the lamp dims.



R
L
A C
G
91 Ω
50 K pot
0.5 µf
110 VAC
SCR
5 V
Lamps
(1 or 2)




Figure 3-9

Copyright © 2011, John A. Goree Edited by John Goree 6 Jan 2011

REFERENCE: Horowitz and Hill Section 2.01 - 2.12
Appendix G
Appendix K data sheet for 2N4400

INTRODUCTION

Junction transistors are either NPN or PNP, with the symbols in Figure 4-1.

Small signal-type transistors come in various pin configurations. An NPN in a TO-92
package is shown in Figure 4-2.

In the junction transistor, a small base current (≈ few µA) controls a much larger
Emitter-Collector current (≈ 1.0 mA).

You will demonstrate the transistor in three common applications:

• Emitter Follower (Common Collector Amplifier)
• Transistor Switch
• Current Source (Better performing than the one in Lab 1)

This experiment will also improve your skills at wiring circuits and using test &
measurement instruments.


EQUIPMENT

Prototyping board
Digital Oscilloscope
Function Generator - Agilent 33220A
Pulse Generator - HP 8013B
Digital Multimeter - Agilent 34410A
NPN Silicon Transistor 2N4400 for switching – you may substitute 2N2222
2N3904 for amplification
Light bulb (typically # 47, 0.15 A @ 6.3 V, with wires)
Resistors 1 kΩ, 3.3 kΩ, 10kΩ, 22 kΩ, 33 kΩ, 1 MΩ
Decade Box
Capacitors 0.1 µF, 10 uF
Lab 4

Junction Transistor, Part I

59

60
A BASIC FACT TO KNOW


In electronics:
• It is USUALLY OK to connect an output of a circuit to the inputs of two
circuits.
• It is NEVER OK to connect two outputs together


Circuit 1
NEVER
OK
out
Circuit 3
in
Circuit 2 out
Circuit 1
Usually
OK
out
Circuit 2
in
Circuit 3 in
Circuit 1 Always OK
out
Circuit 2
in

61
PRELAB


1. Identify all the formulas you will need in this Lab.

2. In this lab we will use a method of measuring the input impedance Z
in
of
a circuit. This impedance will be purely resistive because the circuit will
have no significant capacitance or inductance. The scheme is to connect a
large input resistor R
A
in series with the circuit’s input, and measure the
resulting decrease in the circuit’s output. You can think of R
A
and the
circuit’s input impedance as being like two resistors in series, i.e., a
voltage divider. See the diagram below; the circuit, which has a gain A
v
,
is shown as a box with an input and an output.


(a) Derive a formula for Z
in
as a function of R
A
, V
osc
, V
out
and the
gain A
v


(b)Write a simplified version of this formula valid for A
v
= 1

(c) Write an even more simplified version valid if A
v
= 1 and V
out
=
V
osc
/ 2.
R
A
measure
V
out
here
V
osc
circuit
in
out
measure
V
out
here
V
osc
R
A
Z
in
Circuit
setup
Conceptual
equivalent
circuit

62
PROCEDURE

C
B
E
C
B
E
Figure 4-1
NPN PNP





The 2N3904 and 2N4400 are similar, except that:
• 2N4400 withstands a higher collector current, and is therefore better suited for
switching circuits (you may you may substitute 2N2222 for 2N4400).
• 2N3904 is better suited for signal amplifier circuits.


1. Multimeter Check of Transistor

This test is similar to the one you did in Lab 3 with the diode. If you are unsure of
how to do this test, first go back to Lab 3 and repeat the diode test.

TO-92 case TO-18 case
Figure 4-2:
NPN Transistor pinouts

63

base
collector
emitter
B
C
E
­


Figure 4-3


(a) Diode equivalent

Use your multimeter’s diode-check feature. It will display the forward bias
voltage. Verify that the transistor looks like two diodes, as shown in Figure 4-3.

(b) Beta measurement

If your multimeter has special connectors labeled C B E, then it has a transistor
check function. For some multimeters, this will display the beta, or h
FE
. Try it. For
the 2N3904, beta is typically h
FE
≈ 210.


2. Emitter Follower

(a) Emitter-Follower Operation

Using the ± 12 Volt power supply that is built into your prototyping board, wire
up an NPN transistor as an emitter follower, as shown in Figure 4-4.


+ 12 V
2N3904
10 k
R
E
= 3.3 k
- 12 V
in
out
cc
V


Figure 4-4


64

Hint: On your prototyping board, use wires to connect the outputs of the ± 12 V
power supply and ground (three wires in all) as shown in the left of Figure 4-5 to a
couple of strips that look like those shown on the right of Figure 4-5:

Set up the Agilent 33220A
function generator to produce a
sine wave that is symmetric
about zero volts (turn off the
“offset”). Initially, adjust it for f
≈ 1 kHz and P-P amplitude ≈ 2
V. (Do not use the “Burst” or
“Sweep” modes here.)

Set up the oscilloscope to
show a dual-trace, using DC
input coupling, with one channel
showing in, and the other
showing out.

Be sure the grounds of the oscilloscope, function generator, and prototyping
board are connected.

(i) input and output waveforms

Print the oscilloscope display, and number your printout so that you can
identify it in your report. Repeat for an input amplitude that is much larger and
much smaller. Comment on any significant differences.

(ii) operation with non-symmetric power supply

Now connect the emitter return (the resistor on the bottom of the figure) to
ground instead of - 12 V. Observe the display for several amplitudes of input.
Explain how the circuit functions more poorly.
+12 V
GND
-12 V
Figure 4-5. Power supply on prototyping board (left). Connect these using wires to the
strips sketched on the right for convenient use.
Agilent 33220A Function Generator
If output button is not lit, there will be no waveform
applied to the instrument’s output.

65

(iii) breakdown

Look for bumps appearing at reverse bias. This is called breakdown. Measure
the breakdown voltage, i.e., the voltage at which breakdown first occurs. Compare
your result to the specification in a data sheet for the transistor. [Note, the
breakdown voltage specifications for the 2N3904 and the 2N4400 are identical, so
for this purpose you may use either data sheet.]

(iv) voltage gain

Connect the emitter return back to -12 V. Add a blocking capacitor to the output
as shown in Figure 4-6.

Why don’t we use a blocking capacitor on the input? It’s not necessary. When
the function generator feeds the base directly, the DC bias of the base is held to that
of the function generator, which is near the middle of the transistor’s operating
region. That’s where we want it.



Figure 4-6


Adjust the input amplitude of the sine wave from the function generator so that
the output looks like a good sine wave. Measure the ac voltage gain.

(v) blocking capacitor test

+ 12 V
2N3904
10 k
3.3 k
- 12
in
out
cc
V
0.1 µF
R
E

66
Use your oscilloscope with dc coupling to measure the dc bias of the output on
each side of the output capacitor. Confirm that the output capacitor is blocking a dc
bias.


Note: the following procedures for measuring input and output
impedances will be used again in later experiments.


67

(b) Input Impedance

You will measure the input impedance Z
in
. Because there is no significant
capacitance or inductance at the input of this circuit, we can write that Z
in
= R
in
.
See the discussion and diagram in the PRELAB.



Insert a large-value resistor R
A
(typically 1 MΩ) in series with the function
generator on the input of the amplifier, as shown in Figure 4-7.



1 MΩ
to amplifier
input


Figure 4-7


Use the oscilloscope with AC input coupling to measure the output signal with
and without the large input resistor R
A
.

Calculate the input impedance R
in
. If you measured h
fe
earlier, compare your
result for R
in
to the formula given in the text, p. 66: R
in
= (h
fe
+ 1 )R
E
.

(c) Output Impedance
– TA note 2011 – test this procedure, then check with Prof. Goree; we might need to
redesign the circuit, perhaps with the load resistor between the output and ground.

To measure an amplifier’s output impedance, you will connect a load resistor
across the output. Recall that when two resistances R
1
R
2
are connected in parallel,
the parallel resistance is R
eff
= R
1
R
2
/(R
1
+ R
2
). When the two resistances are
identical, R
1
= R
2
= R, then R
eff
= R/2. Now consider that a load resistance
connected across the output of an amplifier is effectively a resistance in parallel with
the output impedance of the amplifier. If a fixed current i passes through the
effective resistance, the voltage drop across it as given by v = i R will be reduced by
half, if R is reduced by half.

Set the decade box resistance (R
L
) to the largest possible value. Then connect
the decade box across the output of the amplifier, to serve as a load resistance, as
shown in Figure 4-8.

68



+ 12 V
2N3904
10 k
3.3 k
- 12
in
out
cc
V
0.1 µF
R
L


Figure 4-8


Adjust the input signal to obtain a good sine wave on the output. Then reduce
R
L
until the peak-to-peak output signal falls by ½. Then, R
L
equals the output
impedance of the circuit.

(d) Comparison

Compare the measured input and output impedances by calculating their ratio
R
out
/ R
in
. Usually it is desirable for an amplifier to have a high input impedance
and a low output impedance. To what extent is that true here?


3. Transistor switch

Transistors are used often as switches. One application is “cold switching”,
where a remotely located mechanical switch, which carries little current, controls a
larger current someplace else.

A transistor switch operates in the saturated mode when the switch is “on”, and
in the cutoff mode when the switch is “off”. It should not be operated in the normal
or linear mode, where I
C
= h
fe
I
B
. See Appendix G in the text for more on
saturation.

Use a wire on your prototyping board instead of a mechanical switch.

Using the + 5 Volt power supply that is built into your prototyping board, wire up
an NPN transistor as a “cold switch”, as shown in Figure 4-9. The lamp is the “load”

69
here. (The 10 k resistor is not essential-- it makes sure that the base is near ground
potential when the switch is open.)

(a) Cold switching

Verify that the lamp is turned on as you close the mechanical switch and off as
you open it.

(b) Current through the switch

Setting your multimeter to operate in its highest current range to begin with,
use it to measure the current flowing through the lamp.

Setting your multimeter to measure voltage, measure the voltage-drop across
each of the two resistors. Compute the current flowing into the base.

Comparing these two currents, is it true that the “cold switch” allows you to
switch a larger current through the load (lamp) than passes through the actual
switch?

(c) Saturation mode

With the switch closed, use your multimeter to measure the DC voltage drops
V
CE
and

V
BE
.

Figure 4-9. Cold switching


+ 5V
1 k
cc
V
# 47 lamp
10 k

70
Is V
C
<

V
B
as expected for saturation?

(d) Cold switching from a pulse generator’s output

Replace the manual switch with the pulse generator’s output, as shown in the
diagram, so that a high output from the pulse generator will cause the lamp to light.
Adjust the pulse generator to produce its maximum output voltage and maximum
period, choosing a square-wave output. Observe the collector voltage waveform on
an oscilloscope, with DC input coupling and the time/division adjusted to a long
time.

Verify that the lamp is turned on and off periodically. Print the oscilloscope
display, and mark the waveform indicating when the lamp is on and off. Measure the
period using the oscilloscope.

4. Current Source

A current source (Figure 4-10) will supply a constant current to a range of load
resistances. The current that is "sourced" or "sinked" is determined by the emitter
potential and the emitter resistor R
E
.

A transistor current source is a far better current source than the crude voltage-
resistor combination you tested in Lab1.


+5V
1 k
cc
V
#47 lamp
10 k
pulse
generator
out
gnd

71

1 k
load
+ 12 V V
cc
R
E
A
R
1
R
2
current
meter


Figure 4-10

Use R
1
= 22 k and R
2
= 33 k for the biasing resistors.


(a) Calculation

Measure the component values. Then calculate:
• the bias on the base.
• the potential at the emitter (assuming an 0.6 V B-E drop).

(b) Experiment

CAUTION:
In using the current measurement function of the multimeter,
start at the highest possible range.

Using the +12 V power supply built into your prototyping board, connect the
circuit shown in Figure 4-10. For the load resistor, use a decade box, set to 2 kΩ to
begin.

(i) comparison to calculated values

Measure and compare to the values calculated above:
the bias on the base.
the potential at the emitter.

(ii) current vs. load resistance


72
Vary R
L
from 100 Ω to 2 kΩ, in 100 Ω increments.

Make a table of current vs. load resistance.

(iii) compliance

Determine the range of load over which the current remains constant to 10%.
Compare this result to your result in Lab 1 with the crude current source.

Copyright © 2011, John A. Goree Edited by John Goree 6 Jan 2011


REFERENCE: Horowitz and Hill Sections 2.07, 2.12 (common-emitter amp)





INTRODUCTION

We continue the bi-polar transistor experiments begun in the preceding experiment.

In the common emitter amplifier experiment, you will learn techniques and terminology
that are valuable not only in building circuits, but also in the use of commercially-made
amplifiers.


EQUIPMENT

Prototyping board
Oscilloscope
Function generator - Agilent 33220A
Digital multimeter - Agilent 34410A
NPN Silicon Transistor (1) 2N3904
Resistors 220, 1 k, 6.8 k, 12k, 82 k [for CE amp]
Decade Box
Capacitors 0.1 µF, 10 µF electrolytic [for CE amp]




Lab 5

Junction Transistor, Part II

74
PRELAB


1. Identify all the formulas you will need in this Lab.
2. Compute the intrinsic emitter resistance r
e
for a transistor with a collector
current of I
c
= 2 mA. Your answer should have units of Ohms.
3. Referring to your text or lecture notes, describe the undesirable circuit
behavior that can result if the dc bias of the common-emitter amplifier’s
output is not centered between the two power supply voltages.

75

PROCEDURE


1. Common Emitter Amplifier

The common-emitter amplifier shown below is based on Figure 2.37 in the text,
with changes to accommodate V
cc
= 12 V instead of 15 V in the power supply. The
purpose of resistor R
3
is to reduce the gain to avoid excessive high frequency noise.


R
E
+ 12 V V
CC
R
2
R
1
R
c
+
0.1 µF
in
10 µF
out
common-
emitter
amplifier
R
3



Use the following resistor values:

R
1
= 82 k R
C
= 6.8 k
R
2
= 12 k R
E
= 1 k
R
3
= 220 Ω


(a) Calculations

Measure your component values. If your multimeter has a beta-measurement
feature, use it to measure h
fe
.

Compute the following quantities:

(i) Predicted value of the dc base bias and the emitter potential, assuming an
0.6V B–E drop.

76

(ii) Predicted value of the dc collector current and the dc bias on the output.

(ii) Compare the predicted value of the dc bias calculated in (ii) to 0.5 V
cc
.
Explain, in terms of an undesirable circuit phenomenon that you wish to
avoid, why the value of the dc bias you predicted is desirable.

(iii) the impedance Z
eb
of the emitter bypass, i.e., the parallel sub-circuit
consisting of the emitter resistor R
e
& the capacitor in series with resistor R
3
,
for the following frequencies:
0, 100 Hz, 5 kHz, ∞ .

Recall that:
a resistor R
3
and capacitor C that are connected in series have a
combined impedance Z
series =
R
3
+ Z
c
where Z
c
= j / ωC is the
impedance of a capacitor at frequency ω = 2πf

a resistor R
e
in parallel with impedance Z
series
has a total impedance
1 / Z
eb
= 1 / R
e
+ 1 / Z
series


Also recall that the magnitude of the resulting impedance is computed
from its complex value as |Z| = [Z Z
*
]
1/2
.

(iv) the input impedance at 5 kHz (if you didn’t measure h
fe
, assume it is 100).
Note that resistors R
1
and R
2
appear to be in parallel with h
fe
times the
emitter bypass. Use the 5 kHz result (see above) for the emitter bypass
impedance.

(v) the theoretical gain A
v
= - R
c
/ (r
e
+ Z
eb
) at f = 5 kHz, assuming
r
e
= 25 / I
C
(mA).

(b) Operating point

Connect the circuit shown above using a prototyping board. In wiring it up, it
may be easiest if you use three of the long thin rails on the prototyping board for +12
V, GND, and -12 V.

Measure and compare to the calculations above:
(i) the dc base bias and the emitter potential.
(ii) the collector current (use Ohm’s Law and a measurement of the
potential
across the collector resistor to determine this).

(c) Voltage gain
Sync output of
function
generator

77

(i) with emitter bypass capacitor

Set up the function generator to produce a 5 kHz sine wave with a low output
amplitude. Set up the oscilloscope to make a dual-trace measurement of the input
and output of the amplifier.

Connect the SYNC output of the function generator to the EXT TRIGGER input
on the scope, and use external triggering. Make sure that the grounds of the
oscilloscope, the prototyping board, and the function generator are all somehow
connected.

Adjust the function generator amplitude to produce a good sine wave on the
output of your amplifier. If the output waveform is saturated or clipped, reduce the
input amplitude from your function generator. If it is necessary to reduce the input
amplitude further, wire up a 1 k potentiometer as an adjustable voltage divider and
connect it between the output of the function generator and the input of the amplifier.

Measure the input and output amplitudes using the oscilloscope.

Compute the gain, and compare to the theoretical value, using measured
component values, from above. Is this an inverting amplifier?

(ii) without emitter bypass capacitor

Repeat these steps with the bypass capacitor disconnected. (Note: if clipping
occurs, check that you have not shorted the emitter C
E
.


(d) Input impedance

Measure the ac input impedance of the circuit with the emitter bypass. Use the
procedure from Lab 4. Compare to the theoretical value from above.

(e) Output impedance

Measure the ac output impedance using the procedure from Lab 4. (** TA
note 2011 – test this procedure and then check with Prof. Goree to see if it should be
changed.)


Copyright © 2011, John A. Goree Edited by John Goree 6 Jan 2011

REFERENCE: Horowitz and Hill: rise time: p. 268; optoelectronics: pp.
590 - 598



INTRODUCTION

The term “optoelectronics” refers to semiconductor light sources and
light sensors.

Light sources include light-emitting diodes (LED) and laser diodes. In
this lab, you will use a visible LED. LED’s are cheaper, and are usually
used as visual indicators rather than for high-speed communication.
Laser diodes are more expensive, and are used for high-speed fiber-optic
communication. In addition to their high speed in turning on and off,
laser diodes have the advantage of producing a narrower spectrum of
wavelengths helps overcome the dispersive effects of glass, where
different wavelengths travel at different speeds

There are many kinds of light sensors. In this lab, you will test a photo
transistor.
Other light sensors include:
• photodiodes (faster time response but less sensitive),
• photo-darlingtons (these are like a phototransistor but include an
additional transistor to provide additional gain at the expense of a
slower time response)
• photomultiplier tubes (a vacuum tube with a very fast time
response).

Measurement skills you will learn in this lab include:
• using a digital oscilloscope to measure a high-speed pulse.
• using a 10X probe, which is useful because it has a higher
bandwidth (and faster time response) than a 1X probe.


Lab 6

Optoelectronics

79

Overshoot

A pulsed waveform does not transition instantly between two levels. Instead, it
follows a curve, which asymptotically approaches the final level.

In some cases, a waveform will overshoot” the final level.

Rise time and fall time

The speed of switching between the two levels is characterized by a “rise time”
and “fall time.” These are often measured as the time interval between the 10%
and 90% voltage levels, as shown below. (Sometimes, however, a criterion
different from the 10% - 90% rule might be adopted.)

Voltage
overshoot
Square wave with overshoot
10%
90%
voltage
risetime
time
V
f
V
0
V
r
falltime
(V
r
– V
0
) / (V
f
– V
0
) = 90%

80
EQUIPMENT

Digital Oscilloscope
Pulse generator - HP8013B
DC power supply
Prototyping board
10X probes (2) and a manual for this probe model
Resistor 270, 1k

Light emitting diodes:
HLMP-C115 (obsolescent -- this LED chosen for its
30 nsec response)
Red diffused LED

NPN Phototransistor: Panasonic PNA 1801L

Small screwdriver to adjust the 10X probe.
LED HLMP-C115
Note: the flat edge of rim
indicates cathode
NPN Phototransistor PNA1801L
Note: emitter pin is longer

81
PRELAB


1. Identify all the formulas you will need in this Lab.

82
PROCEDURE

1. Adjust 10X probe compensation

As you previously learned in Lab 3, a 10X passive prove attenuates a
voltage by a factor of 10, so that an oscilloscope measures one-tenth the
original voltage. It is preferred over a 1X probe for high frequencies (>1
MHz) and therefore it is used whenever one needs to measure fast rise times
(< a few microseconds).

Due to variations in oscilloscope input capacitances it is necessary to adjust
a compensation “trimmer” on a 210X probe after moving it from one
oscilloscope to another.

Adjust the compensation on both of your 10X probes. This requires a small
screwdriver. Verify that the probe’s switch is in the 10X position. Follow
the steps of the 10X probe manual (p. 3 of the manual for the P2200 200
MHz Tektronix probe). This procedure requires a square-wave input; use the
two metal loop connectors labeled “Probe Comp” on the oscilloscope. One
connector is for the probe tip and the other for the probe’s ground
connector.

Set up the digital oscilloscope so that both the CH1 and CH2 menus are set
for:
• Probe 10X
• DC coupling
• BW Limit off
• Invert off.



83
Identify the phototransistor and LED (they look alike but the LED is
much larger and the bumps on its leads are nearer its lens.). Identify
the two leads of the phototransistor (collector and emitter) and the
LED (cathode and anode).

Discuss in one or two sentences why the phototransistor has no base
lead.

Set up the pulser as shown in the photo, except that you should use
the following switch settings: OUTPUT: norm, INT LOAD: out,
PULSES DELAY: doesn’t matter, PULSE: norm. Produce a square
wave with an amplitude in the range 5.0 – 5.5 V peak-to-peak, and a
period of about 1 – 3 sec (slow enough that you can see the LED
blink, when it is connected later). Set Apply the TRIGGER
OUTPUT of the pulser to the external trigger input of the scope. Use
the scope’s trigger menu to choose “normal” and “external”
triggering. Apply the OUTPUT (+) across the LED and ground.


Hook up the circuit shown in the diagram:
• Use a 270 Ohm resistor in series with the LED.
• Use an emitter resistor R
L
= 1k on the phototransistor.
• Use the bench power supply to provide +5 VDC for V
CC
.
• Connect the CH1 10X probe to the output of the pulser, and the CH2
10X probe to the emitter of the phototransistor to observe its output.

Align the parts on your prototyping board so that the lens of the LED faces
directly toward the lens of the phototransistor, with a separation of about 1
cm.

Perform the following test to verify that the beam of light from the LED is
pointed at the phototransistor. Hold a piece of paper immediately in front of
the phototransistor so that it blocks half the phototransistor’s lens, as shown
in the photo (which is best seen in color). If the LED is aligned properly,
you will see half of a big red dot on the paper, with the center of the red dot
located at the center of the phototransistor’s lens.

84
Obtain an oscilloscope display similar to what is shown in the screenshot,
adjusting the horizontal position as necessary.

Check that that the signal disappears when you insert a piece of paper
between the LED and the phototransistor to fully block the light.

Press the “measure” button on the scope and set it to measure Pk-Pk and rise
time, as shown in the screen shot below.

Without disturbing the LED or phototransistor, determine the criterion used
by the oscilloscope to measure the rise time. Do this by measuring the
waveform yourself. Measure three voltages: V
0
before the rise, V
f
after the
rise is complete, and V
r
at the time indicated by the scope as the measured
rise time.

Hint: Remember the skills you learned when using the analog scope: manual
measurements are easiest if you make use of the major division lines on the
scope display. Adjust the horizontal position or trigger delay so that the
pulse begins on a major division on the time scale. Adjust the vertical
position so that the baseline for the pulse is similarly on a major division
line. Adjust the pulse amplitude so that the phototransistor output varies an
integer number of divisions. Choose appropriate scales for the vertical and
horizontal axes so that the interesting part of the waveform fills a large
fraction of the display.

Print out your oscilloscope display, resembling the screenshot shown here.

Record the value of rise time indicated by the oscilloscope.

LED, facing the
phototransistor at a distance
of about 1 cm
Phototransistor, facing the
LED
Paper, blocking half the
phototransistor’s lens, so
that you can see where the
light is
LED illuminates a spot of
this size on the paper,
shown here with dashed
lines

85
Compute the ratio (V
r
– V
0
) / (V
f
– V
0
), expressing your answer as a
percentage (i.e., XX%). (This result should be the same regardless of the
amplitude of the waveform or the student performing the test; it is a criterion
determined by the oscilloscope’s manufacturer.)

Things to know about measuring speed:

The rise time that you measure for the output of the phototransistor is tens of
microseconds. This is the result of the rise times of several components and
instruments:

• Oscilloscope: The oscilloscope has a bandwidth that is probably printed
on top of its front panel. If it has a bandwidth of 100 MHz, for example,
it will have a rise time of about 0.35 / 100 MHz = 3.5 nsec. Faster
oscilloscopes cost more.
• Probe: The 10X probe also has a bandwidth that limits how fast the
measurement of the waveform can be made; look in the manual for the
10X probe to find the specification for rise time. Record this value
• Pulse generator: The rise time for the pulse generator is finite, but it is
much better than for an inexpensive function generator. Pulse generators
are optimized to produce excellent pulses with fast rise times; function
generators are not.
• The phototransistor and LED have finite response times; here are the
data from the manufacturer’s specifications:
PNA1801L phototransistor, 4 µsec, typical
‡‡

HLMP-C115 LED: 30 nsec, typical
• The HLMP-C115 LED, which costs about $0.60, is one of the fastest
visible LEDs that I was able to find. Usually, in applications where the
response time of a light source is important, you would not choose an
LED; instead you would choose a laser diode, which is much faster and
costs ~ $10.

Replace the HLMP-C1115 with any common LED (for example a red
diffused LED like the one used in Experiment 8) and repeat the
measurements of the time response for that LED.

For both measurements (HLMP-C115 and the other LED), identify the
component or instrument you believe is most responsible for the finite rise
time that you measured, and discuss briefly how you came to that
conclusion.


‡‡
Note 2008: manufacturer’s specification at V
cc
= 10 V, I
CE
= 1 mA, R
L
= 100. Your
result might be slower at parameters different from these.

86
When finished, reset the CH1 and CH2 menus so that it has Probe 1X, not Probe
10X, so that this does not confuse measurements made with this scope in
following labs.

87
Use switch
settings
indicated in
instructions

88

REFERENCE: Horowitz and Hill Sections 4.01-4.09,4.11
Appendix K: `411 datasheet

INTRODUCTION

The operational amplifier (op amp):
• is an integrated circuit, i.e., “chip.”
• is a multistage transistor amplifier with a differential input.
• has a high input impedance (10
12
Ω for J-FET input op-amps).
• is usually used with an external feedback network.
• can amplify or perform mathematical and logical operations.
• can operate from DC to MHz frequencies, depending on the op-amp model.
• requires bi-polar power supply, typically ± 15 V.

The comparator is similar to an op-amp, but it is used for a different application: its
output swings from one rail to the other, depending on which of the two inputs is
more positive.

Note: The student will need software to make graphs

EQUIPMENT

Prototyping board
Digital Oscilloscope
Function generator
Digital Multimeter
Op-amp (2) LF411CN (or equivalent 8-pin DIP)
Comparator LM311N
Resistors: 1 k (2), 10 k (4), 22 k, 100 k, 5 M
4.7 k
Decade box
Capacitor: 0.01 µF, 50 pF
Potentiometer with wires soldered on (preferably ~ 50 k).
Photocell NSL-4522
Lab 7

Operational Amplifiers (Op-amps)

89

PRELAB


1. Identify all the formulas you will need in this Lab.
2. In this lab you will use “decibels”, or dB. This is a dimensionless ratio, in
logarithmic form. The formula is X
dB
= 20 log
10
(|X|), where X is any
dimensionless ratio. For example, X might be the gain A of an amplifier.
If the gain A of an amplifier is 100, you can also say that the amplifier
has a gain of 40 dB. Note that negative values correspond to a ratio of
less than unity, for example an amplifier with a gain of 0.01 has a gain of
-40 dB. You can compute a voltage ratio by taking the exponent of 10, for
example the voltage ratio corresponding to a gain of 15 dB is 10(15/20) =
5.623. Calculate the following:
a. The gain in dB of an amplifier with a gain of 10,000.
b. The gain in dB of an amplifier with a gain of 0.1
c. The voltage ratio that corresponds to – 3 dB.

90
PROCEDURE

0. Things to know



+
-
out
non-inverting input
inverting input


Figure 7-1 Symbolic representation of an operational amplifier


An op-amp ideally obeys the rules:

(1) The output voltage does whatever necessary to make the two inputs at the
same voltage.
(2) The current drawn by the two inputs is zero.

Also, an external feedback network must be provided that connects the output to the
inverting input; this provides negative feedback.

The pin configuration for an 8-pin DIP op-amp is shown in Figure 7-2, as viewed
from the top.


1
2
3
4
8
7
6
5
+
-
out
non-inverting
input
inverting input V
+
V
-
op-amp mini-DIP package

in most applications,
the offset null is not connected
offset null
offset null
not
connected


Figure 7.2 Op-amp pin configuration


91
Note that an op-amp is an active component that must be powered to work. Most
models need a bi-polar power supply. On a schematic diagram, the power-supply
connections to an op-amp are often not shown.

Connect the two terminals V
+
and V
-
to a ±15 V power supply (or ± 12 V on your
prototyping board).


The prototyping board

A prototyping board is made especially for DIP (dual-inline package) ICs. Many
models have built in power supplies that you may use. Use wires to connect the
outputs of the ± 12 V power supply and ground (three wires in all) to a couple of
narrow strips that look like:


+12 V
GND
-12 V



Insert the ICs (carefully to avoid bending their leads) into the wider strips, like this:




1. Inverting amplifier

To measure the amplitude of a sine wave, measure peak-to-peak and divide by
two.

Set up the oscilloscope to show a dual trace, one for the input of the op-amp and
one for the output. Use DC input coupling. Set up the function generator to produce
a sine wave with an amplitude ≈ 0.2 V and frequency 1 kHz. (Use the –20 dB button

92
on your function generator, if necessary, to reduce your function generator output to
such a low amplitude.) Make sure that the grounds of the function generator,
oscilloscope, and prototyping board are somehow connected.

Measure the values of your resistors, then using the prototyping board, wire up
the inverting amplifier shown in Figure 7-3.


+
-
out
in
R1 1 k
R2 1k, 10 k, 22k



Figure 7-3 Inverting amplifier

(a) Gain

(i) Measure the gain A
V
for R
2
= 1 k, 10 k, and 22 k.
(ii) Using measured values of R
2
and R
1
, compare to A
V
= - R
2
/ R
1
.
(iii) Verify that the amplifier inverts.

(b) Saturation

For the circuit with R
2
= 22 k , increase the input signal amplitude to observe
clipping.

Calculate the ratio Vout / Vin that corresponds to a – 3 dB drop in gain.

Measure the input amplitude that results in clipping with a 3 dB reduction of
the output as compared to the ideal output amplitude. What is the output amplitude
as a ratio of V
cc
, when clipping occurs at this level?


93
(c) Frequency response

(i) roll-off frequency

For the circuit with R
2
= 22 k, vary the frequency from a low value (≈ 100 Hz)
upward to find f
3dB
, the frequency at which the gain drops by 3 dB. Compare to the
value given by Figure 4.31 in the text.

(ii) graph of frequency response

For the circuit with R
2
= 100 k, measure the gain A
V
at the logarithmically-
spaced frequencies of 100 Hz, 300 Hz, 1 KHz, 3 kHz,..., 1 MHz, 3 MHz. Repeat for
R
2
= 10 k.

Plot your results on a log-log graph. Also show on this graph the theoretical
gain |A
V
| = R
2
/ R
1
.

Note that high gain is achieved at the expense of frequency bandwidth.


2. Voltage Follower (Unity Gain Non-Inverting Amp)

The circuit in Figure 7-4 has a voltage gain of unity but a very high current gain.

(a) Input impedance

Use a 5 MΩ resistor in series with the function generator at the input to
measure the input impedance. If it is too high to measure, report it as Z
in
>> 5 MΩ.


+
-
out
in
voltage follower
source for
measuring Z
in
5 MΩ




Figure 7-4 Unity gain non-inverting amplifier (Voltage Follower)

94


3. Summing Amp

Build the circuit shown in Figure 7-5. Choose R
1
= 10 k.


+
-
in 1
10 k
10 k
out
in 2
R1


Figure 7-5 Summing Amplifier


Use a 4 Volt P-P sine wave for input 1. Use a + 5 V dc voltage from a power
supply (your prototyping board is ok) for input 2. Connect the oscilloscope, using
DC input coupling, to monitor input 1 and the output.

(a) Summing

Confirm that the output is (minus) the sum of the inputs. Sketch or print the
waveforms, and mark values at the following phases of the sine wave: peak, trough
and zero. Also present calculations showing that the output is approximately equal to
the expected value of the sum of the inputs; do this for three cases, corresponding to
the three phases peak, trough and zero.

(b) Clipping

Use the DC offset on your function generator to raise the dc input voltage until
you see a pronounced clipping of the waveform. Sketch or print the waveforms, and
mark values at particularly significant times


4. Difference Amp

The circuit in Figure 7-6 is a differential amplifier.

Recall that the common mode rejection ratio is defined as:


95
CMRR = differential mode gain / common mode gain, i.e.,


CMRR = A
D
/ A
C


Using Op-amp rules 1 and 2 as described in the text, one can derive that the
output is



V
o u t
= −
R
2
R
1
V
i n

1
+ V
in

2

R
4
R
1

⎝
⎛
⎜
R
1
+R
2
R
3
+ R
4
⎠
⎞
⎟
(4.1)

Note that if

R
1
= R
2
= R
3
= R
4
, then the output is a simple difference

V
out
= V
in2
- V
in1
.
(4.2)


More generally, we can find expressions that are valid even if the resistor values are
not all exactly identical, which of course they won’t be in practice, due to resistor
tolerances. Substituting V
in1
= - V
in2
, we find that the “differential mode gain” is

A
D
= - R
2
/ R
1
(4.3)


Substituting V
in1
= V
in2
, we find that the “common mode gain” is



A
C
= −
R
2
R
1
+
R
4
R
1

⎝
⎛
⎜
R
1
+R
2
R
3
+ R
4
⎠
⎞
⎟
(4.4)

which is zero if R
3
= R
1
and R
4
= R
2
. So, the CMRR = A
D
/ A
C
is perfect (infinity)
if R
3
= R
1
and R
4
= R
2
. However, resistors actually have a tolerance of typically
5%, and therefore the common mode gain will not actually be zero, and the CMRR
will not be infinity.



96

+
-
R3
R2
out
R1
R4
in 1
in 2


Figure 7-6 Differential Amplifier


Connect the circuit in Figure 7-6, using resistor values
R
1
= R
2
= R
3
= R
4
= 10 k.

(a) Differencing

Use the setup with a sine wave and a dc signal set-up that you used for the
summing amp. Verify that the output is the difference of the two inputs, as predicted
by Eq. (4.2), by marking values at particularly significant times on the printout of
the scope display.


(b) Differential mode gain

Apply the sine wave to input 1 and connect input 2 to ground. Measure A
D
=
V
out
/ V
in1
. Compare to the value predicted by Eq. (4.3), using actual measured
values for the resistors.

(c) Common-mode gain

Connect both inputs to the sine wave. Measure A
C
= V
out
/ V
in
.

Compare to
the value predicted by Eq. (4.4), using actual measured values for the resistors.

(d) CMRR

- Compute CMRR = A
D
/

A
C
.


97
5. Integrator (low-pass filter)

The circuit in Figure 7-7 is an integrator, which is also a low-pass filter with a
time constant = R
1
C. Resistor R
2
provides DC feedback for stable biasing, since
without it there would be feedback through the capacitor only, and that would not
provide any DC feedback. Note: if you observe a severe dc offset on the output of
the circuit, you may replace R2 with a 100k resistor.

The output is V
out
= −
1
R
1
C

∫
V
in
dt + const


Adjust your function generator to produce a square wave, which will be applied
to the integrator circuit’s input.

(a) Integration

Print waveforms for V
in
and V
out
at 1 kHz.

(b) DC stability

Remove the resistor R2, and see if anything unfortunate happens to the output
waveform (the effect might not occur, or it might not be extreme). Be sure to use
DC coupling on your oscilloscope input.

(c) Frequency response
Vary the frequency while observing the input and output waveforms on the
oscilloscope. It is not necessary to record data for this step. Discuss in one sentence
how your observations are consistent with the description “low-pass filter.”


+
-
in
out
R1 10 k
C 0.01 µF
R2 5 MΩ


Figure 7-7 Op-Amp Integrator

98


6. Differentiator (high-pass filter)

The configuration in Figure 7-8, with the main components C
1
and R
2
, is a
differentiator. Differentiators always have difficulty with high frequency noise, so a
low-pass filter combination C
2
and R
1
is added to reduce the noise.

Connect the circuit shown in Figure 7-8, and apply a sine wave to the input. If
the output of the circuit exhibits clipping, reduce the input amplitude; if turning the
function generator’s amplitude knob isn’t sufficient, see if your function generator
has an attenuation button, which might be labeled -20 dB for example.

(a) Differentiation
Print the outputs of the differentiator circuit for a sine wave and a ramp wave
inputs at a frequency 100 Hz. Explain how your waveforms are consistent with
the circuit taking the “derivative” of the input.

(b) Frequency response
For a sine wave input, vary the frequency while observing the input and output
waveforms on the oscilloscope. It is not necessary to record data for this step.
Discuss in one sentence how your observations are consistent with the
description “high-pass filter.”


+
-
in
out
R2
100 k
0.01 µF
C2
50 pF
R1
1 k
C1


Figure 7-8 Op-Amp Differentiator

99
7. Comparator

You will observe how a comparator will output a (relatively) clean digital output
(+5V or 0V) depending on whether an analog input voltage is > or < a threshold
voltage. You will establish the threshold voltage using a potentiometer.

You will also observe an undesired oscillation in the comparator’s output, a d you
will diminish this oscillation by adding positive feedback to the comparator.

Connect the comparator chip, as shown below (pinout is shown for ‘311; the ‘411
it is different). Use the power supplies of the prototyping boarding:
+/- 12 V power supply for analog
+5 V power supply for digital.



Adjust the function generator to provide an 8 V peak-peak sine wave at 1.5 kHz.

Observe the (digital) output of the comparator on CH1 of your oscilloscope, and
the sine wave (analog) input on CH2. Trigger the scope on CH1 using the falling
edge, with 100 microsec / division for the horizontal scale.

100


Record the oscilloscope display when the threshold is set to the following two
values:
(i) + 3 V
(ii) 0 V.


Discuss qualitatively how the output waveform “changes states” in response to:

• Upgoing analog input voltage
• Downgoing analog input voltage.


Now zoom in to the downgoing change in digital output voltage using a 250 nsec/div
on the oscilloscope’s horizontal scale.


  Record the oscilloscope display showing the “ringing” or “oscillations” present in
the digital output (with a typical characteristic time of ~ 200 nsec).

Now connect a 100k resistor between output pin 7 and the non-inverting input pin 2 of
the comparator. This provides positive feedback.

Record the waveform. Discuss whether the undesired oscillations are diminished
by using positive feedback, and if so, whether the effect is a strong one.



8. Using sensors


There are several ways to detect light. Here you will use a photocell, which is a
simple two-terminal sensor that acts like a resistor, with a resistance that diminishes
as the light level is increased. It is not a linear sensor, so its use if often limited to
“electric eye” applications that require detecting whether conditions are light are
dark, for example to turn street lights on and off. (If you needed to measure light
intensity over a continuous scale, you would choose a different sensor, one that has
a linear response to light intensity).

NSL-4522 CdS Photocell specifications (from Silonex datasheet):

R
dar k
( mi n) 1 M ( mi ni mum)
R @1 f oot - candl e 18. 6 k ( +/ - 40%)
R @100 f oot - candl e 400 ( t ypi cal )


101


Design a simple voltage divider to replace the function generator in the
comparator circuit above, so that the comparator circuit’s output voltage will change
when you cover the sensor with your hand to block the room lights.

• The voltage divider should consist of the photocell and a suitable fixed
resistor, and it should be connected to a suitable power supply.

• It may be helpful to know that indoor room lighting is typically in the range of
10 – 100 foot candles. Whether this circuit works will depend on your choice
for the voltage divider resistance, power supply, and threshold voltage for the
comparator. (You must decide on your own what values to use for the
resistance, and what to use for the voltage-divider’s power supply.)

• After designing the circuit:

Verify that the output of the comparator changes when you
block/unblock room lights from hitting the photocell.

Report the values you chose for the resistor and the power supply
voltage for the voltage divider.

Copyright © 2011, John A. Goree Edited by John Goree 6 Jan 2011


REFERENCE: Horowitz and Hill Sections 8.01 - 8.13


INTRODUCTION

Digital devices are basically electronic switches that use two states, or logic
levels.

TTL and CMOS are two families of digital circuits. Most commonly, they
are powered by a +5 V power supply, although CMOS is sometimes used
with +12 V.

These are integrated circuits, i.e., IC’s, or “chips.”

A digital signal is either ON or OFF:

state other names for the state TTL CMOS

ON (HI, "1", TRUE) 3.0 -5.5 V 3.5 - 5.5 V
or 8.5 - 12.5 V

OFF (LO, "0", FALSE) -0.5 - 1.5 V -0.5
- 1.5 V


Digital ICs called “gates” perform arithmetic and logical operations:
one input: NOT (inverter)
two or more inputs: OR, AND, NOR, NAND, XOR (Exclusive OR).

On 14-pin logic chips, usually pin 14 is V
cc
= + 5V, and pin 7 is ground.

We will try out these devices and their truth tables, then combine them to
perform more complicated logical operations. We will also test a half adder,
which perform the arithmetic operation of addition, and a multiplexer.

Lab 8

Digital Gates

103
EQUIPMENT

Prototyping board
Power supply
Digital multimeter
Resistors: 220 Ω (1)
LEDs (4)
Digital ICs: 74HC00 and 74LS00 Quad NAND
74LS02 (or 74HC02) Quad NOR
74LS04 (or 74HC04) Hex inverter
74LS86 (or 75HC86) XOR
74LS151 8 Input Multiplexer





104
A BASIC FACT TO KNOW


In electronics:
• It is USUALLY OK to connect an output of a circuit to the inputs of two
circuits.
• It is NEVER OK to connect two outputs together


Circuit 1
NEVER
OK
out
Circuit 3
in
Circuit 2 out
Circuit 1
Usually
OK
out
Circuit 2
in
Circuit 3 in
Circuit 1
Always OK
out
Circuit 2
in

105
PRELAB



1. Look at all the diagrams in this lab, and verify that you are never asked to connect
two outputs together, although you are often asked to connect an output to multiple
inputs.

2. The two-input exclusive or (XOR) truth table (with inputs A and B) is


A B Y
0 0
0
0 1 1
1 0 1
1 1 0

Find the truth table for a 3-input XOR, with three inputs A B and C.

Note: the “exclusive or” function is usually defined for only two inputs.
Sometimes the term is used for a three-input device – when this is done, it is
usually a “parity generator” which outputs a true (1) signal if an odd number
of inputs are true, and a false (0) signal if an even number of inputs are true.
The reason this device is sometimes called a “three-input exclusive or” is that
it can be built from combining a pair of two-input XOR gates, applying their
inputs to an OR gate, which then generates the final output.


3. Refer to a textbook (e.g. Horowitz Hill p. 495) for a description of a multiplexer.
Learn how the address bits are used to select which of several inputs is sent to the
multiplexer’s output.


106
PROCEDURE


0. Things to know

About digital inputs

In making tests, you will need to connect some inputs to your digital circuits.

For a LO input, use a wire between the digital input and ground.
For a HI input, use a 1 k resistor between a digital input and +5 V.

An alternative method (more convenient than using
resistors as described above) requires using a prototyping
board that has built-in switches for logical HI and LO
inputs. These provide suitable inputs for logic circuits and
don’t require resistors.

On your prototyping board:

Use two wires to connect the +5 V and GND outputs of
the power supply to a strip that looks like:



Your prototyping board has built-in LED “Logic
Indicators” that you can use for displaying a signal. As an
alternative, you may use a discrete LED as an indicator,
but be sure to use a series resistor, as described below.


How to read part numbers:

The part number consists of numbers and letters.

Example: SM74LS00 -- the SM indicates that the manufacturer is Motorola
(and this is not important), 7400 indicates that the chip is a quad 2-input
NAND, and the LS indicates that it is a variety of TTL that is “low-power
Schottky-input.” Other chips that perform the same logical function are
7400 (plain old-fashioned TTL) and 74HC00 (“High-speed” variety of
CMOS). All of these chips are logically identical, and their pin
configurations are the same.

About CMOS:


107
CMOS is more common nowadays than TTL because it has a lower power
dissipation. Nevertheless, your instructor may prefer TTL for this course
because TTL chips, unlike CMOS, are immune to damage by static
electricity.

Static electricity can destroy CMOS chips. To avoid damage:
• store unused chips properly, in special plastic tubes or special plastic
bags.
• before handling a CMOS chip, ground yourself by touching a ground,
such as the metal cover plate on an electrical power outlet on wall.

About the light-emitting diode (LED)

In building digital circuits, you will often use an LED as an indicator of the
state, HI or LO.

ALWAYS USE A RESISTOR IN SERIES WITH THE LED.

The series resistor (220 Ω typical) is needed to avoid destroying the LED.
The LED is a diode, and when it is on, it has very little resistance. If you
connected an LED directly between a source and either ground or V
cc
, it
would try to pass an infinite amount of current, which will burn up the LED.


220 Ω



About buffers for the LED

A buffer is often needed to drive an LED. The outputs of many gates and
circuits cannot source enough current to drive an LED directly; this is when
you must provide a “buffer”. The buffer is a digital version of a follower; its
output is at the same voltage as the input, but is capable of driving more
current while sinking very little at its input.

Your prototyping board has LEDs that are
already wired so that you can use them as
digital indicators.


108


220 Ω
out in
one part
of digital
circuit
next part
of digital
circuit
220 Ω
out in
one part
of digital
circuit
next part
of digital
circuit
buffer
without a buffer
with a buffer

109


1. The `04 Hex inverter


Use the 5V power supply for V
cc
.

Install the `04 hex inverter on the
prototyping board. Examining the
pin diagram, choose any of the six
inverters. Connect its input to a
“data switch” and its output to a
Logic Indicator LED.

Verify that a HIGH on the input
will cause the corresponding LED
to be dark (LOW).

Verify that a LOW on the input
will cause the corresponding LED
to be bright (HIGH).

Write the truth table (i.e., function table) for the inverter.

Try using an LED in series with a 220 Ω resistor (instead of the LED built into
the prototyping board) to display the state of the gate’s output.

2. Quad 2-input NAND 74HC00 [CMOS] and 74LS00 [TTL]




110
This chip has four NAND gates, each with two inputs (hence the name
Quad 2-input NAND). Look up the pin configuration in a data sheet.
§§
The
output pins are 3, 6, 8, and 11. Connect V
cc
to + 5 V. If you are using a
prototyping board with built-in input-switches use these for the inputs, and
use the built-in LEDs to display the outputs. At first use the CMOS chip.

(a) NAND (Figure 8-1a)
(b) Inverter with NAND (Figure 8-1b)
(c) AND (Figure 8-1c)
(d) OR (Figure 8-1d)
(e) NOR (Figure 8-1e)
(f) Mystery NAND circuit (Figure 8-1f)


See the pin diagram, above.

Write down a Truth Table for each configuration listed above, and
check off every state as you test it. Draw the gates.

Finally, replace the CMOS chip with the TTL chip, and verify that the
chip works the same, i.e., that the truth table for (a) is the same as for the
CMOS chip.



For the remaining exercises, you may use either TTL or CMOS.

3. Quad 2-Input NOR (74LS02 or 74HC02)

(a) NOR (Figure 8-2a)

§§
It is a useful exercise to look up the pin diagram in the data sheet which can be found
either in a Data Book, or on a manufacturer’s website. This skill is necessary when you
design a circuit yourself.

111
(b) Inverter with NOR (Figure 8-2b)
(c) OR (Figure 8-2c)
(d) AND (Figure 8-2d)

See the pin diagram, above.

Write down a Truth Table and check off every state as you test it. Draw the
gates.


4. XOR (74LS86 or 74HC86)

(a) XOR from 7486 (Figure 8-3a)
(b) XOR using NAND only (Figure 8-3b)

See the pin diagram, above.

Write down a Truth Table and check off every state as you test it. Draw the
gates.

6. Half Adder

See Figure 8-4. Connect this circuit.

Write down a Truth Table and check off every state as you test it.
Draw the gates.


1. `151 Multiplexer


112
A multiplexer is a switch. It has multiple inputs and a single output. The
purpose of the multiplexer is to allow the user to determine which of the
inputs should be connected to the output.



Connect the 74LS151 as shown below.


DATA SELECTS OUTPUT

binary 2
2
2
1
2
0

state C B A Y


IC pin 9 10 11 5

board SW1 SW2 SW3 LED1

Set the following pins on the 74LS151 to LO:

Strobe (pin 7)
All inputs except D1

(a) The intended use of the chip: multiplexing

Predict, by examining the truth table, which combination of input states
A B and C will result in an output Y that is HI or LO if D1 is HI. Write your
predictions in the form of a table with four columns: C B A and Y. Repeat
for D1 LO.


113
Operating the circuit, with D1 high and then D1 low, verify the two
truth tables you predicted above (you can indicate a successful test with a
checkmark to the right of a row).


(b) Another use for the chip: XOR

Predict, by examining the truth table, which combination of data input
states D0-D7 will result in the output Y that is a two-input XOR, using A and
B as the inputs to the XOR. Write your prediction as a two-column list, with
D0 – D7 listed in the left column, and in the right column write either “A
XOR B” or “SOMETHING ELSE”.

Testing the circuit, apply the states you predicted to the data inputs D0
– D7, and try all four combinations of inputs A and B. Write your results in
the form aof a truth table, with three columns: A B and Y. State whether this
truth table is the same as a two-input XOR.

Repeat the previous two steps of predicting and testing, this time for a
3-input XOR using A B and C as the inputs of the XOR.




114
A
B
A A
_
a.
b.
A
B
A B
A
B
A + B
c.
d.
A + B
OR
See d
A
B
____
A
B
C
?
e.
f.
Figure 8-1 Nand gate circuits


115
A
B
A + B
____
A A
_
a. b.
A
B
A + B
A
B
A B
c.
d.
Figure 8-2 Circuits using NOR gates


116

A
B
+
A B
a.
B
A
+
A B
b.
Figure 8-3 Different realizations of XOR



A
B
Figure 8-4 Half adder
C
out

Copyright © 2011, John A. Goree Edited by John Goree 6 Jan 2011

REFERENCE: Horowitz and Hill Sections 8.16 -8.26
Section 9.10 [LED display]
Section 9.04 [De-bouncer]

INTRODUCTION

In this lab we study two versions of the Flip-Flop (bistable multivibrator). Then we
test some counters and learn about BCD (binary-coded decimal). Next, we take the
BCD output produced by the decade counter and convert it with a decoder-driver to
power a 7-segment LED display. After building a counter, we will use it to
demonstrate how a switch de-bouncer works. Finally, we try out a BCD-to-decimal
converter.
You will look up a few items in data sheets, to learn how to find information for
chips.

EQUIPMENT

Prototyping board
Digital Oscilloscope
Pulse Generator
Digital Multimeter
Digital ICs: [TTL (LS) or CMOS (HC)]
7400 Quad NAND
7442 BCD to Decimal Decoder
7447 BCD to 7-segment decoder driver (common anode)
7490 decade counter
74112 JK flip-flop
74175 D flip-flop
74161 4-Bit Synchronous Binary Counter
74195 4-Bit Shift Register

Capacitors 0.1 µF
Resistors 150 Ω, 10 k (2)
Seven-Segment LED (common anode) LSD322X-XX or LN513RA
Wires for Prototyping board (35) [for 7-segment]
Switch SPDT (momentary contact) with wires attached [for de-bounce]

Lab 9

Flip-Flops, Shift Registers, Counters,
Decoders, 7-Segment Display

118
PRELAB


Explain the difference between parallel and serial:
• what is meant by “parallel input”?
• what is meant by “serial shift”?

119
PROCEDURE

1. D Flip-Flop (74HC175 or 74LS175)

The D Flip-Flop is the simplest but most important Flip-Flop. It simply saves at
its output, Q, what it saw at its input, D, just before the most recent clocking edge.

The chip used below, the `175, has four flip flops, and they respond to a rising edge
on the clock input.



D Q
Q
+ 5V
CLOCK
GND
CLEAR
9
4
8 16
2
3
‘151 Flip-Flop

120
(a) Basic operation

Use Vcc = +5 V and GND to power this chip.

Connect CLR (pin 1) to a breadboard switch, set to HI.


Provide data to the D1 input (pin 4) from a prototyping board switch, by setting
the D1 input to HIGH .

Clock the flop (pin 9) with a momentary-contact switch (for example, “logic
switch A on the prototyping board).

Set up LED indicators of: the input D1 output Q1 (pin 2) and its complement Q
¯
1

(pin 3).


(i) Verify that the D flop ignores information presented to its input before a rising
edge on the clock, and that it then shifts that value to the output. Draw a timing
diagram, showing CLK, D1, and Q1.

(ii) Verify the Truth Table in the data sheet by copying the truth table (the figure
labeled “Function Table” above) by placing a check-mark by each entry that
you test and verify. Label the state on the timing diagram that corresponds to
each entry in the truth table.


(b) Shift Register

Leaving the first flip-flop set up as it is, apply its output Q1 to the D2 input of
another flip flop, and so on, for four flip-flops total, as shown below.

Connect the four outputs Q1, through Q4 to LED indicators.

Set CLR to HI using a switch on the prototyping board.

Use a clock frequency of about 1 to 10 Hz, or use a momentary switch (for
example, “Logic Switch A” on the prototyping board) as a manually-operated clock.



121

V = pin 16

GND = pin 8
cc
D Q
Q
D Q
Q
D Q
Q
D Q
Q
4 2 5 7 12 13 15 10
3 6 11 14
9
1
IN
clock
+ 5V
`175
shift register


Watch all the states. Observe that the input is shifted to the right once
each clock cycle. Draw a timing diagram showing the four D inputs, the final
output, and the clock.



2. The `195 4-Bit Shift Register



Shift Registers use a series of D flip-flops, with
the output of one flip-flop connected to the input of
the next. It accepts both series and parallel inputs and
produces both series and parallel outputs. Thus, the
`195 can be used to convert series input to parallel
output and vice versa.

When LOAD is LO, the outputs are set to the
inputs, on a rising edge of CLK.
When LOAD is HI, the register shifts.

‘195 Truth table

INPUTS at t
N
OUTPUTS at t
N+1


J K’ Q
A
Q
B
Q
C
Q
D


L L L Q
AN
Q
BN
Q
CN

H H H Q
AN
Q
BN
Q
CN

L H Q
AN
Q
AN
Q
BN
Q
CN

H L Q
AN
’ Q
AN
Q
BN
Q
CN




122
Connect the 74LS195 to switches (for input) and LEDs (to indicate output):

INPUTS OUTPUTS
(see note)

binary 2
3
2
2
2
1
2
0
2
3
2
2
2
1
2
0

state IN
D
IN
C
IN
B
IN
A
CLK Q
D
Q
C
Q
B
Q
A

IC pin 7 6 5 4 10 12 13 14 15

board SW1 SW2 SW3 SW4 SWA LED1 LED2 LED3 LED4


Set CLR (pin 1) to HI.

(a) Parallel data loading and serial shift

Set LOAD to LO to allow parallel loading. Input a number like “0010”, where
the first digit is for input A, the second digit for input B, etc. To load this input
number, clock the chip by pressing SWA to cause a rising clock edge. Confirm the
number shows up on the LED indicators.

(i) serial shifting
Now set LOAD to HI to allow serial shifting. Clock the register by pressing
SWA to advance the data (you will need to do this to move the data forward).

Connect J and K’ to one of the four possible input combinations shown in the
truth table.

Examine Q
B
, Q
C
, and Q
D
on the LED indicators to verify that the register
shifts data serially in the direction A to D, as indicated in the truth table.

Fill in the chart on the next page. Repeat for the four combinations of J and K’.
(The first combination J = LO and K’=LO is pre-completed to show you what is
expected; for this combination just verify that what is pre-completed is correct.)

(ii) truth table
Examining your chart, verify (by ticking a box in a skinny column) that the
part of the truth table that indicates that, upon a shift, state A is filled with either a
LO, a HI, the previous value, or the complement of the previous value, depending on
the values of the J and K’ input

123
Chart for ‘195 shift register

The lines of this table highlighted in gray have already been completed, as an
example of the desired style.

Input After 1
st
clk After 2nd clk After 3rd clk
J=LO K’=LO
QA 0 0 0 0
QB 1 0 0 0
QC 0 1 0 0
QD 0 0 1 0

J=HI K’=HI
QA
QB
QC
QD

J=LO K’=HI
QA
QB
QC
QD

J=HI K’=LO
QA
QB
QC
QD

124
Repeat all the steps of (i) for the ‘195 shift register, this time beginning by
inputting the number “1111”. Fill out the chart below.



Input After 1
st
clk After 2nd clk After 3rd clk
J=LO K’=LO
QA 1
QB 1
QC 1
QD 1

J=HI K’=HI
QA
QB
QC
QD

J=LO K’=HI
QA
QB
QC
QD

J=HI K’=LO
QA
QB
QC
QD

125
INSTRUCTOR NOTE: usually steps (b) and (c) below should be omitted, because
the corresponding topics are not taught in lecture.

(b) Serial to Parallel conversion S → P

Observe Q
A
to verify that when you present a datum to both the J and K’
inputs (first two lines of truth table) that the number is sent to the output Q
A
on the
clock.

Note that this allows 4 serial bits (entered on four clocks) to be readied for
parallel output.

(c) Parallel to Serial conversion P → S

Examining the data in your diagram, verify that the initial input data is
converted into serial data where the output QD shows the input data as the clock is
advanced.


3. The `161 Synchronous Binary Counter


This is a 4-bit binary counter with data inputs that can be preset, i.e.,
programmed, so that it begins counting from a desired number. Binary counters
contain JK flip-flops.

Connect V
cc
to pin 16 and GND to pin 8. Connect the inputs of the 74LS161 to
switches on the prototyping board, and the outputs to LED’s, as follows:


INPUTS OUTPUTS

binary 2
3
2
2
2
1
2
0
2
3
2
2
2
1
2
0

state IN
D
IN
C
IN
B
IN
A
LOAD Q
D
Q
C
Q
B
Q
A

IC pin 6 5 4 3 9 11 12 13 14

board SW1 SW2 SW3 SW4 SWA LED1 LED2 LED3 LED4



Connect pin 2 (CLK) to a 1 Hz clock on your prototyping board.

Set the following pins on the 74LS161 to HI (you may use data input switch on
your prototyping board to accomplish this).

Enable P (pin 7)
Enable T (pin 10)

126
Clear (pin 1)


When LOAD’ (pin 9) is LO, the counter can be loaded with a binary number on
data inputs IN
D
, IN
C
,

IN
B
,

IN
A
. When it is HI, clock signals are counted.

Before using the circuit, convert the decimal number “12” to binary.

Program the counter so that it will count from 12. Do this as follows: With
LOAD’ LO, input the decimal number “12” on the input switches. Then set LOAD’
to HI.

Now clock the CLK input and watch the chip count in binary from 0 to 15. Note
that it will start at the value 12 that you programmed, then count up to 15, then
start over at 0.

Draw a timing diagram for the counting, showing CLK and outputs Q
A
, Q
B
,

Q
C
,

Q
D
.




4. Decade Counter (7490)

Referring to the diagrams below from the data sheet, we see that the `90 has four
internal Flip-Flops. The first one is not internally connected to the others while the
other three are cascaded to form a divide-by-5 synchronous counter.


127
Connect the `90 as follows (note the unusual power supply pin assignments):

1. ÷5, ÷ 10 input
2. GND
3. GND
4. NC
5. +5 V
6. GND
7. GND
8. Q
C

9. Q
B

10. GND
11. Q
D
, ÷ 5 output
12. Q
A
, ÷ 2 output
13. NC
14. ÷ 2 input

For an input in parts (a) to (c), use the pulse generator to produce square pulses,
between 0 and 5 V, at 5 kHz. (If you have difficulties below in getting a desirable
signal from the outputs of the chip, check the input waveform: it sometimes helps to
set the pulse generator’s INT LOAD switch to the “in” position.)

Set up a scope for dual-trace operation to show the pulse input from the pulse
generator and an output (either pin 11 or 12 depending on what you are measuring)
of the `90. Trigger the scope internally, using as the trigger source the scope channel
CH1 or CH2 that you have connected to the counter’s output.

Use the `90 to perform the following operations:

(a) Divide by 2

Connect the pulse input to the ÷2 input.

Print the waveforms. Discuss briefly the meaning of the phrase “divided by
two” as it is shown by your waveforms, identifying which parameter is divided.

(b) Divide by 5

Connect the pulse input to the ÷5 input.

Print the waveforms. Explain how the waveform is “divided by five.” Note the
asymmetry of the output waveform.

(c) Divide by 10


128
Connect the ÷5 output to the ÷2 input. Use the scope to observe the ÷2 output.

Print the waveform. . Discuss briefly the meaning of the phrase “divided by
ten” as it is shown by your waveforms, identifying which parameter is divided.


(d) BCD Counting

Binary coded decimal (BCD) is the same as binary for numbers 0 to 9. For
larger numbers it is different: BCD is composed of groups of four binary digits,
where each group represents one digit in base 10.

Example
314 = decimal 3 1 4
BCD 0011 0001 0100
binary 100111010

For this chip, outputs Q
A
,

Q
B
,

Q
C
, and Q
D
are the four-bit BCD outputs.

Disconnect the pulse generator.

Use a switch on your prototyping board (SWA) to apply a pulse input on pin 14.

Connect Q
A
(pin 12) to the ÷ 5 input (pin 1) for BCD counting. (It may already
be connected this way from part c above.)

By means of LED indicator lights (you may use those built into the prototyping
board), test the levels of the D, C, B, and A outputs (pins 11, 8, 9, 12 respectively)
after each pulse.

(i) counting

Write a copy of Table 9-1, and check off each state 0 through 9 to confirm that
the outputs D C B and A of the counter accumulate in a BCD counting cycle. (Note:
the output may at first indicate an invalid number, such as 1011,when you first turn
on the power. If this happens, just apply several pulse inputs until a new cycle
begins.)

(ii) resetting

Verify (by copying the first two rows from the “Mode selection” table above
and placing check marks by the entries as you test and verify them) that setting both
the reset inputs, MR1 and MR2, to HI will zero the outputs at D, C, B, and A of the
decade counter.


129
To avoid wasting time, do not take this circuit apart yet. Leave this circuit wired
up for use with the 7-segment display in the next part.


Table 9-1

decimal BCD

D C B A

0 0 0 0 0
1 0 0 0 1
2 0 0 1 0
3 0 0 1 1
4 0 1 0 0
5 0 1 0 1
6 0 1 1 0
7 0 1 1 1
8 1 0 0 0
9 1 0 0 1
0 0 0 0 0
etc.


6. Decoder-Driver and 7-Segment LED Display (7447)

[Forewarning: this circuit is not hard, but it requires hooking up more wires than
most; thus it might take more time.]

Displays are popular output devices. LED’s are brighter, while LCD’s consume
far less power. Both types of displays require a decoder-driver to convert (decode)
from BCD input and to drive the display.

LED displays have 8 leads for the 7 LED’s. The displays and their
corresponding driver IC’s come in two varieties, common anode and common
cathode.

Ordinarily, when using LED’s, one uses a series resistor of about 150 Ω to limit
the current.

Leave the BCD counter hooked up as it was at the end of part 5, above, with a
switch for the input and the 4 BCD outputs indicated by 4 LED’s.

Check the part number on the 7-segment display. If it differs from the LSD322X-
XX, specified here, you will need to check the pin diagram for your display; this can
be done by searching the internet for the part number, and looking for a data sheet.


130
Connect the BCD outputs of the BCD counter to the BCD inputs of the 7447.
Connect the 7447 to the 7-segment LED display as shown. [Note: pin
configurations for LED displays is not standardized. Pin configurations are shown
here for the SENIOR SEA3210 display. (Your display might have a different model
number but the same pins.) This display has an additional LED for a decimal, which
we will not use.]

(a) Counting

Write a copy of Table 9-1 and check off each state 0 through 9 to confirm that
the LED outputs D C B & A of the counter are properly indicated by the display.

(b) Current consumption

Use your multimeter in the dc-current-mode to measure the current from the
common anode to the + 5 V V
cc
. How much current is consumed by the LED
display when the display reads “1” and when it reads“8”? [Note that LED displays
are power-hungry devices.]








a
b
c
d
e
f
g
7
1
2
6
+5 V V
cc
+5 V V
cc
A
B
C
D
e
f
g
a
b
c
d
13
12
11
10
9
15
14
16
8 GND
74LS47
(anode)
150
BCD inputs
.
a
b
c
d
e
f
g
8 d
9 right decimal
10 c
11 g
12 NC
13 b
14 NC
a 1
f 2
anode 3
NC 4
NC 5
left decimal 6
e 7
LSD3222-XX
7-segment common-anode LED
(top-view)
decimal
note: this LED display does not
actually have 14 leads,
but the package
is shaped like a 14-pin DIP,
with some leads missing.
Missing leads are indicated here as NC

131
7. Latch Used for Switch De-Bouncing

Note to instructor: this circuit must be used with one of the counter circuits
above. It is possible to use either the binary counter or the decade counter.

An ordinary mechanical switch “bounces” several times when it opens or closes.
This means that it opens and closes several times, on a time scale of about 100 µ s,
before reaching its final state. When using a switch to input data to a logic circuit,
bounce is unfortunate because it looks like real data that is changing several times.


(a) Bouncy switch

Use a momentary-contact switch (not the
switch built into your prototyping board; it
is already de-bounced). Use the multimeter
to check continuity, to see which two
terminals to use on the switch so that it is
normally open, and closed only when you
push.

Connect the simple input data switch shown below, so that the switch is normally
connected to LO.

Adjust a digital oscilloscope with DC coupling and approximately 100 usec / div.
Observe the waveform of the output of the switch, adjusting the trigger and timebase
so that you can see any bounce, as in the photo below.

what the switch does
what you really want
what it looks like to a digital input
output waveforms (voltage vs.
time) from switch
circuit intended to provide an
input to a digital circuit
out

132

Apply the output to the clock input of a counter circuit (you may use any of the
counter circuits you built above.

Press the switch, watch an LED indicator of the counter output. Does the
counter count more than one clock, due to bounce?

Observe and print the waveform produced by the switch. Use a digital storage
oscilloscope.



+5 V
10 k
counter
digital
scope
push-button switch is
normally closed, but open
when pushed

133

(b) De-bounced switch

The circuit shown below is called an “SR flip flop” or a “cross-coupled NAND
latch.” Wire it up using a 7400 quad NAND gate chip.





Now add a switch with pull-up resistors to make a data input, as shown below. Press
the switch, and observe that the counter counts only one clock, because the switch is
now de-bounced.




+ 5 V
10 k
10 k
+ 5 V
counter





134
8. The `042 BCD to Decimal Decoder Chip

Before wiring the circuit, convert the following decimal numbers to BCD:
0, 1, 8.

Wire the decoder chip with four inputs for BCD input. Use the switches on your
prototyping board to provide input, as shown below:


BCD INPUTS

binary 2
3
2
2
2
1
2
0

state D C B A


IC pin 12 13 14 15

board SW1 SW2 SW3 SW4

Apply the BCD input corresponding to “0” . Touch a wire connected to
an LED indicator to test the states of the 10 output pins. Verify that the output
pin corresponding to “0” is LO and the other outputs are HI. Repeat with data
inputs “1” and “8”.

Copyright © 2011, John A. Goree Edited by John Goree 6 Jan 2011

REFERENCE: Horowitz and Hill Sections 8.20 [one-shot]
Section 5.14 [555 timer]
Section 9.15-9.16, 9-20 [DAC& ADC]


INTRODUCTION

In this lab we will test IC’s used to interface digital and electronic
circuits. The first is a popular chip for making oscillators, the `555.
Next is the monostable multivibrator (One-Shot) which can produce a
jitter-free trigger signal from a rising or falling analog input. Finally we
will set up an digital-to-analog converter (DAC) and an analog-to-digital
converter (ADC).


EQUIPMENT

Digital Oscilloscope
Function generator
Pulse generator
DC power supply
Prototyping board
Capacitors 0.1 µF (3), 0.01 µF, 0.001 µF
Resistor 3k (2), 5.1k (2), 10k (2)

Integrated circuits:
74121 one shot
LM555 or 7555 timer
DAC0808 8-Bit Digital-to-Analog Converter
ADC0804 8-Bit Analog-to-Digital Converter

Temperature sensor
LM35DZ (not included in student kit, stored in cabinet)

Lab 10

Digital Meets Analog

136
The terms “pulse generator” and “square wave generator” refer to
different things.

A “square wave generator” is usually a bipolar oscillator. The
waveform swings from positive to negative voltages. Sometimes it
has a feature to add an arbitrary DC offset to the output.

A “pulse generator” is usually not bipolar, and there is usually no
DC offset adjustment. A pulse swings between zero and either a
positive voltage (as in the case of logic circuits) or a negative
voltage.

Pulse generators are used to clock digital circuits.



square wave
pulse train
(clock)
0 V
0 V
+5 V
-5 V
+5 V




137
Lab 10 circuit, as wired on the prototyping board at the end of the Lab.

138
PRELAB

1. Identify all formulas used in this lab.

2. While an analog voltage can have an unlimited number of values, a digital
signal has a limited number of discrete values. How many discrete values can be
represented by 8 bits of data?

3. What is the sensitivity (mV per degree C) of the temperature sensor you will
use?

139
PROCEDURE

1. 555 Timer, used as an oscillator

Timer circuits are common in many of the instruments that are
“home-built” in research laboratories. A favorite and easy-to-use chip is
the 555 timer, or a modernized version such as the 7555. With this
chip, you can easily build a simple oscillator or other waveform
generator.


Here you will build a square wave oscillator.

Using the prototyping board, build the circuit shown in the figure.
Use +5 V from your prototyping board for V
cc
. (V
cc
= +15 V should
work too.)

As with many circuits, this one can benefit from adding a filter
capacitor across the power supply leads to reduce problems arising
from voltage drops appearing on the power supply. It is recommended
to add a 10 uF capacitor across the power supply pins 8 and 1; position
this capacitor within a few millimeters of the 555 chip. Note the
polarity of the leads of the electrolytic capacitor.

(a) Use the oscilloscope to observe the waveforms at the output and at
pin TH.

Measure the duty cycle of the output waveform.

Duty cycle ≡ fraction of time the output voltage is “high”

Specify your answer in percent, e.g. 37.5 % ± 5%.

(b) Measure the frequency. Compare it to the predicted value:

f
osc
= 1
/
[ 0.7 ( R
A
+ 2 R
B
) C ] .

(c) Replace R
B
with a short circuit.

Describe the resulting waveform.
Explain your observation.

Leave this circuit hooked up. You will use it with the one-shot in the
next part.

140

V
CC
4 8
D
G
OUT
TR
TH
R
V
CC
OUT
R
A
10 k
R
B
C
0.1 µF
10 k
7
2
6
1
3



555 Timer square-wave oscillator




2. Monostable Multivibrator One Shot (74LS121)

The output of a one-shot is a digital pulse triggered by an analog
waveform at its input. Even if the input is noisy, the one-shot will
produce a clean pulse. The pulse has a width determined by an RC time,
and the pulse height is determined by the input voltage.


141
The One-Shot you will use has a Schmitt-trigger input (for the B input).
Schmitt trigger inputs fire when the input exceeds a certain threshold, and
reset when the input voltage drops below a lower threshold. Schmitt
triggers are widely used to produce a jitter-free pulse from a noisy input
signal.

Refering to the specification sheet, connect the '121 on the prototyping
board as follows. Connect an 0.1 µF capacitor between pins 10 and 11.
The one-shot you will use has an internal timing resistor with a nominal
value of R
int
= 2 kΩ (refer to the data sheet to confirm this value).

1. Q
¯

output 8. NC
2. NC 9. Timing pin: Short to pin 14
3. input A1 = GND 10. Timing pin: C+
4. input A2 = GND 11. Timing pin: C-
5. input B 12. NC
6. Q output 13. NC
7. GND 14. +5 V


(a) Set up the pulse generator to apply a ≈ 10 V peak square wave to the
input of the one-shot. Use a frequency f ≈ 1 kHz.

By varying the input amplitude, measure the trigger voltage.

Measure the pulse width. Compare to R
int
C ln 2.

(b) Reduce the pulse width and verify that the pulse width at the
output of the one-shot is unaffected by the pulsewidth at its input.


142
(c) Replace the shorting wire between pins 9 and 14 with a 10 k
resistor, and measure the pulse width.

(d) At the input, replace the signal from pulse generator with the
square wave signal from the 555 timer that you built in part 1. Verify
that this combination works as a pulse generator.


3. Digital to Analog Conversion (DAC0808LCN or equivalent)

The DAC0808 is an easy-to-use 8-bit D to A converter (DAC).

Wire up the DAC as shown in the figure. Use wires to connect the
inputs of the three most-significant bits (MSBs) on pins 5, 6, and 7 to
the input switches on your prototyping board. Connect the other four
inputs to LO by connecting pins 8 through 12 with wires directly to
ground. Connect a multimeter to the analog output, pin 4.

Make a table showing all eight possible combinations of the
digital input states on the three most-significant bits. Then measure the
analog output V
out
for all of these states.


143

5
6
7
8
9
10
11
12
3
16
2
13
14
15
V = - 12 V
EE
+5 V
DAC0808
0.1 µF
5 k
4
10 k
analog out
V = + 12 V
CC
5 k




4. Analog-to-Digital Converter

[Note: This circuit requires ≈ 23 wires to hook it up on your
prototyping board.]

An ADC is often used to interface digital circuits or computers to
the real world, especially to acquire data. Your digital multimeter, for
example, has an ADC. Among other things, an ADC contains
comparators and voltage reference levels. The incoming analog signal
is compared to the voltage reference levels to determine which is
closest. A corresponding digital output bit is then set to TRUE.

An ADC is rated by the number of bits and the maximum clock
speed.

Here we will use the ADC 0804, an inexpensive ADC that is
contained on one chip. It digitizes eight bits at a clock speed up to 1.4
MHz. It can be clocked by an external pulse source, or by connecting a

144
resistor and capacitor to make a simple relaxation oscillator (analogous
to the way the 555 works); we will do the latter here.

The ADC 0804 has two analog inputs for differential voltage
measurement. For convenience in testing this ADC, we will ground
one of the differential inputs (pin 7) and to the other one (pin 6) we
shall apply a simple dc voltage. One way of doing this is to use a
voltage divider as a voltage source, with an op-amp voltage follower to
assure a low output impedance. The op-amp follower is not critical here
if you use a voltage source with a low output impedance to drive it.


Connect the ADC0804 as shown. Use LED’s on your prototyping
board to indicate the three most significant bits (pins 11 to 13).

Pay careful attention to the grounds when using circuits with both
analog and digital parts. Although in this lab you may be asked to
connect them together, in general the analog ground and digital ground
are different, and some circuits might not work if you connect them
toghether

Use a bench power supply (see photo) as an adjustable voltage
source. Be sure its negative terminal is connected to its ground, and
connect that ground to your circuit’s ground



145
(a) Clock

Using an oscilloscope, look at the waveform on pin 19, and print
it. Confirm that there is a 5 V, 100 kHz clock signal present. [This
frequency is determined by R and C, as it was in the 555 timer.]


(b) A to D Conversion

Connect a DMM to measure dc voltage across the differential input
to the ADC (i.e., between pin 6 and ground).

If it looks like too many LEDs are lighting up, the input voltage may
be jittering near the borderline between digital levels, causing a fast
alternation between one digital level and another, and because of the
fast clock speed, it looks like all the LEDs are lit. Try adjusting the
input voltage slightly up or down.

Caution: in the next step, try not to exceed 5.0 V input voltage; the
chip will fail if you exceed its maximum input voltage rating, which is
about 15 V.

Vary the input voltage from 0 V to 5 V, and record V
in
(at the 8
levels) where the three most significant bits of digital output change
from LO to HI. Present your results in the form of a table.

Using the function generator, produce a triangle wave (or sine wave)
varying from 0 V to 5 V at approximately 0.1 Hz, and apply this signal
to the input. Watch the LEDs as they change, and be sure that you
understand why they change as they do.

146



Test circuit for the ADC 0804 analog-to-digital converter.

Use the GND on your prototyping board as both the analog and digital grounds.
(The manufacturer designed the chip so that it can be used with two separate
grounds, if desired, to reduce noise. Here we will not use this feature.)

The purpose of C2 is merely to reduce noise in the voltage divider’s output; it is
not a critical component.
analog gnd
digital gnd
note grounds:
1
2
3
4
5
6
7
8
17
18
MSB
LSB
digital
output
V +5 V
cc
ADC 0804LCN
C
0.001 µF
9
10
19
20
R 10 k
11
12
13
14
15
16
A GND
Vin
-
+
Vin
D GND
CLK IN
WR
INTR
CS
RD
V REF
DB7
DB0
V
cc
analog
input
CLKR
+5 V
3 k
3 k
+2.5 V
C2
0.01 µF

147

5. Using sensors


There are several ways to detect temperature. As is often the case, a
specialized integrated circuit is a good solution. Here you will test the
LM35DZ.

General Description (from the National Semiconductor datasheet):

The LM35 series are precision integrated-circuit temperature
sensors, whose output voltage is linearly proportional to the
Celsius (Centigrade) temperature. The LM35 thus has an
advantage over linear temperature sensors calibrated in
° Kelvin, as the user is not required to subtract a large
constant voltage from its output to obtain convenient Centigrade
scaling. The LM35 does not require any external
calibration or trimming to provide typical accuracies of ±1⁄4°C
at room temperature and ±3⁄4°C over a full −55 to +150°C
temperature range. Low cost is assured by trimming and
calibration at the wafer level. The LM35’s low output impedance,
linear output, and precise inherent calibration make
interfacing to readout or control circuitry especially easy. It
can be used with single power supplies, or with plus and
minus supplies. As it draws only 60 μA from its supply, it has
very low self-heating, less than 0.1°C in still air.

Features
• Calibrated directly in ° Celsius (Centigrade)
• Linear + 10.0 mV/°C scale factor
• 0.5°C accuracy guaranteeable (at +25° C)



Use the LM35 Precision Centigrade Temperature Sensor chip, which
has a sensitivity 10 mV per degree C. The diagrams below are from the
datasheet for the LM35DZ. Use a power supply voltage of Vs = +5V.
First connect the sensor output to a digital multimeter, and touch your
finger to the sensor and observe the change in the output voltage.
Describe your observations.
Connect the sensor output to the input of the ADC circuit. Can you
observe a change in the data output, in response to a change in
temperature?

Discuss how you could use the ADC circuit to make a digital
thermometer.

148



Copyright © 2011, John A. Goree Edited by John Goree 6 Jan 2011




Toward the end of the semester, you will design and build a circuit of your own, to meet
whatever purpose you like.

Project planning: This project is not like an ordinary lab assignment. It is more like a
part of a thesis project. To be successful, you must do project planning.

Project planning entails:
• making a schedule for yourself
• allowing time to receive purchased supplies
• allowing time to assemble, test, and debug your experiment, and finish it on time
• budgeting for costs.

An original idea: The circuit you design may be analog, digital, or both. The circuit
could arise from your thesis project, or it could complement an existing instrument, for
example your telephone or stereo. You could make a game, a circuit that demonstrates
some mathematical or scientific concept, or something for a hobby, such as a temperature
controller for a photographic darkroom. It is up to your imagination.

Warning: Your design must be your own. Be prepared to disclose which portion of
your circuit is your idea, and which portion is copied from another source. Students who
are unimaginative and copy entire designs (from books, the internet, or someone they
know) are usually unable to answer questions about their project at the time it is graded,
and they receive a poor grade.

Requirements: Your circuit must include a minimum number of electronic components
that will be agreed upon when you discuss your ideas with the instructor.

Money: Purchase your own components from vendors such as the following:
• UI Engineering Electronics Shop 2018 Seamans Center
http://www.engineering.uiowa.edu/~eshop/
• Radio Shack (2024 8th St. Coralville; 1654 Sycamore St. Iowa City)
• DigiKey (tel 1-800-DIGIKEY), http://www.digikey.com


SPECIAL PROJECT

150
Two ways to build: You must choose either:
o to build it on a prototype board (recommended)
o to build it hardwired in a box with a battery or power supply. If
your final project is built on a prototype board, it must be more
ambitious. If it is hardwired in a box, it must meet good standards
for safety and quality of construction. (This is not recommended
unless you already have the required skills and tools.)

Hardwired projects:

Safety: use a grounded box
use grommets & strain relief for power cord
cover all 110 VAC so that it is unreachable when the box is closed
Connectors: use a terminal strip with crimp-on lugs for 110 VAC

Holes: de-burr holes after drilling sheet metal

Wires to front panel:
bundle them with tie-wraps for neatness
Circuit boards:
line up ICs and other parts in neat rows
don’t cross wires over chips
use sockets for chips
Testing: test your ideas on prototype boards first
show the instructor that your prototype-board version works
(for partial credit, in case your final circuit doesn’t work)

Special Project Tips:

Planning

As with all research projects:

• Plan ahead.
• Procrastinators will learn a hard lesson.
• Expect your project to require 3× as long as you expect.
• Expect things to go wrong.

Designing

• A simple idea that works is better than a grand idea that fails.
• Avoid designing circuits with high speed, high frequency, high voltage, or
high current.
• Design a circuit mainly or entirely based on components that you already
understand, like op-amps and digital gates.

151
• Many successful projects begin with a fairly simple idea, then add “bells and
whistles.” After your basic circuit is tested successfully, extra features such
as displays or adjustable settings can be added. Do this if your basic circuit
idea does not have enough components.

Purchasing

• Buy from local sources wherever possible.
• Plan well ahead if you buy by mail.
• Buy several spares for every semiconductor, chip, or high-power item.
• Obtain data sheets for all semiconductors and chips.

Testing tips:

• Begin by testing your circuit, one section at a time, on a prototype board.
Don’t assemble a big circuit all at once and expect it to work. You will have
bugs to work out, and this is easiest to do if you assemble it in stages, one
section at a time.
• For analog circuits, use an oscilloscope to observe AC signals.
• Buy a small prototyping board of your own so that you can keep your
project assembled as you work on it. Otherwise you will have to
disassemble it so that other students can use your board. A single strip or
two may be enough.
• If your circuit’s inputs are to come from a transient source, such as
pushbuttons or random pulses, test your circuit with a repetitive waveform
from a function generator.
• A few students choose to test their design on a computer, using Spice-based
circuit simulation software (such as Multisym) before building it.


Power supply tips:

• Many students have problems because their circuit draws too much current
for their power supply. Give some thought to choosing which power supply
you use. If your circuit consumes more than 100 mA from a 5 VDC power
supply, use a bench power supply (HP or Lambda, for example), not the
prototyping board. Otherwise your circuit will stop working due to loading
the power supply.
• Some circuits that involve large currents that are not DC currents, for
example an LED display or a 555 timer with a large capacitor, will cause the
power supply voltage to droop momentarily, and this can cause errors in
logic chips elsewhere in the circuit. To avoid this, place an electrolytic
capacitor (a few microfarads) across the power supply leads on your
prototyping board, and locate this capacitor physically near the subcircuit
(e.g., the 555 timer or LED display) that demands a large current.

152
• If you use a 9-Volt battery, you can reduce the voltage to 5 VDC using the
3-pin voltage regulator from one of the labs in this manual.

Checklist for Special Project Grading:

At the end of your project, you will:

• Present a schematic diagram
• label every part (part number, and function if it is a chip)
• identify function of switches, LED indicators, etc.
o (e.g. reset, power)

• Present a list of specifications
• at least 3 numbers
• example:
o frequency bandwidth [Hz @ 3 dB flatness]
o max input voltage [V]
o input impedance [W]
• identify whether measured or computed
o measured is better
• include error estimate
***
and units

• Demonstrate how your circuit works
• plan how to show it in 5 minutes
• start by showing the schematic diagram, and disclosing
which features are not your own design
• in case it doesn’t entirely work:
o demonstrate that part of it works
o hope for partial credit for the part that does work
• be prepared to explain how you arrived at your error
estimates
Grading scheme:
grading factor prototype hardwired
design 80 % 60 %
cleverness of idea
how well it works
how ambitious it is
schematic diagram 10 10
specifications 10 10
quality of construction - 15
safety - 5

***
Error estimates require values from the manufacturer’s specifications. on the
instrument. For the multimeter, parameters are reproduced in the preface to this manual;
for other instruments (including digital oscilloscope) you should refer to the actual
manufacturer’s User Manual.