PHYS 343 Experimental Techniques
Laboratory #6
Operational Amplifiers I
Due November 24
1
Preliminary Reading
PARAMETER
Input bias current
Input offset voltage
Slew rate
Gain-bandwidth product fT
Open-loop voltage gain
CMRR
Maximum output current
Horowitz & Hill, chapter 4.
2
Equipment
±15 v power supply, variable DC power supply, signal generator, 1.5 v cell in holder, linear slide pot, oscilloscope, DMM, 741C &
LM301A op amps, and assorted capacitors
and resistors.
3
Preliminary
We will use two different op amps in these experiments: the industry-standard LM741C
and the LM301A. The former is internally
compensated, while the latter requires an external capacitor for frequency compensation
in order to be stable. Look up the spec sheets
for each of these op amps. A good source
is the National Semiconductor linear IC catalog. You can get some information from
web. For example, go to www.google.com
and type “LM741C data sheet”. Several
.PDF files will show up. Draw the pin-out diagram (mini-DIP package) for each op amp.
To compare a few parameters for these
devices, complete a table as follows, using
“typical” values. If you must take a parameter from a graph, assume a supply voltage
of ±15 v.
301A 741C
You might have to obtain the GainBandwidth Product from a plot of open-loop
gain as a function of frequency (Bode plot).
Otherwise, use the value of fT for the 741C
to make a plot of the open-loop frequency
response for that op amp. That is, make a
Bode plot: open-loop gain as a function of
frequency on a log-log scale. You will make
use of the fact that the roll-off is -6 dB per
octave. A sketch in your notebook is adequate.
4
4.1
The inverting amplifier
DC measurements
Assemble the circuit shown on a solderless
breadboard, using an LM741C op amp.
Rf
1.5 V
or
3.0 V
R1
IC1
+
Vout
R1 = 1.5 K
Rf = 150 K
Measure Vin
Pot: 1 to 3 K
IC1 = 741C or 301A
here.
Measure Vout for the following values of Vin
(adjusted using the potentiometer): 100 mV,
50 mV, 25 mV, 0 V, -25 mV, -50 mV, and
-100 mV. (How do you get the negative voltages? You just change the ground connection on the battery!) You may include some
other values if you wish. Make a graph of
Vout as a function of Vin and determine the
voltage gain by a linear regression best fit
to the line. Compare experimental and theoretical values for the gain and explain any
discrepancies. (Note: it would be a good
idea to measure the resistor values with an
ohmmeter, to get three significant figures.)
Disconnect the slide wire and ground the
input resistor R1 (so that Vin = 0). Measure
the output voltage and compare it to your
“0 v” reading above. How does this measurement relate to the input offset voltage,
VOS , of the op amp? To accurately zero the
output, we have to use a nulling circuit, covered in a later lab.
4.2
Frequency response
Disconnect the potentiometer and use the
signal generator to supply a 100 mV p-p sine
wave to the amplifier input. Measure the
voltage gain (magnitude) as a function of frequency, using frequencies of 100 Hz, 300 Hz,
1000 Hz, 3000 Hz, and so on, up to 2 MHz.
Record your data in a table.
Make a log-log plot of the magnitude of
gain as a function of frequency. The easiest
way is to use Excel and use the log-log graph
option. From your earlier plot of open-loop
frequency response (Bode plot) you should
have been able to predict when a roll-off
would begin. Compare the earlier plot with
the actual data you have obtained.
Now turn off the power supply and replace the 741C with a 301A op amp. Turn
on the power supply. You should observe
that the op amp is in oscillation; it is unstable without an external compensating capacitor. Hook a small capacitor between 5 and
50 pf (record the value) between pins 1 and
8. The oscillation should cease.
Again measure gain as a function of frequency, as you did for the 741C. On the same
graph you used before, plot the gain versus
frequency. How does the performance of the
two op amps compare?
Replace the small capacitor with a larger
value, 150 pf or some other available value.
Take a few readings of the gain to determine
the roll-off under these conditions and the
new unity-gain frequency. (Take points up
to at least 200 KHz.) Plot these points on
the graph. What is the effect of a larger compensating capacitor?
5
Summing amplifier
Assemble the circuit shown and measure the
output voltage on the scope. Make a sketch
of the waveform. Be sure the scope is DCcoupled. While observing the waveform, disconnect the signal generator. Hook it back
up and disconnect the battery. Do you understand what is happening? Calculate what
the output voltage should be and compare
this with your measured (time-dependent)
voltage. Don’t calculate some maximum and
minimum values for Vout . Rather, find the
average value (DC component) and the amplitude of the sine wave that is superimposed
on this DC level. After all, that is really what
is happening!
Rf
R1
1.0 V
p-p,
100 Hz
R2
+ 1.5 V
+
Vout
741C
R1 = R2 = 2.2 K
Rf = 12 K
To increase the precision of your calculations, you should (1) measure the battery
voltage directly with the voltmeter and (2)
measure the values of the individual resistors.
What would the output voltage waveform
look like if R1 were replaced with a value
twice as large? Illustrate with a sketch.
6
Question
The gain of an inverting amplifier is proportional to Rf /R1 (referring to the diagram of
section 3) In the first circuit, would it work
just as well to use values of R1 = 2.2 Ω and
Rf = 220 Ω ? Explain.
of time. For each circuit, plot Vout and V in
on the same graph.
Now, hook up the circuits and observe
the results. Were your predictions correct?
If not, what was the problem? (Note: don’t
“fudge” your predictions. You will not be
graded on whether you were right or wrong.
I will be more impressed if you can give me
a clear description of what your misconceptions (if any) were.)
Zener diodes (especially low-voltage Zeners) often have a “soft” knee around the
Zener voltage. Did the effects of this show
up in your observations? If so, exactly what
were the effects you observed?
Vin
7
A final activity (review)
Obtain two low-voltage Zener diodes; the
Zener voltage should be less than 8 volts or
so. Consider the circuits below.
For each circuit, predict the output voltage Vout . Draw graphs in your notebook of
V in (a triangle wave) and Vout as a function
Vin
1K
Vout
Vout
1K
Vin is a triangle wave
with 10-volt amplitude.