INDIANA UNIVERSITY, DEPT. OF PHYSICS, P400/540 LABORATORY
FALL 2008
Laboratory #6: Operational Amplifiers
Goal: Study the use of the operational amplifier in a number of different
configurations: inverting amplifier, non-inverting amplifier, summing amplifier,
follower, current source, and current to voltage converter (transconductance amplifier)
1. Introduction
This is the first integrated circuit ("IC") that we will be using. The particular
package style is called "DIP" for "dual in-line package." We will be using the LM741, the
"original" low-cost operational amplifier. There are now op amps with lower noise, faster
response ("slewing"), etc., but this one illustrates generic op amp behavior.
The chip should "straddle" the trench in the breadboard, making connections to
each pin very easy. The pin numbering scheme for all chips is as shown:
8 7 6 5
LM741
1 2 3 4
+15 V
2 !
3 +
7
6
out
4
!15 V
The "dot" always designates pin 1, and the pins are numbered counter-clockwise
looking down on the chip. Note that in the above, not all pins are used ("NC" or "not
connected").
Remember to use a DVM to set the adjustable +15 V supply to +15 V and ditto
for the –15 V supply. Connect these to "bus bars" on the breadboard, with the +15 V
across the top and the –15 V across the bottom. Immediately make the power
connections to the IC chip, and before any other connections, use a DVM right at the
pins to make sure that the +15 and –15 Volts are present. This is one of the dominant
reasons for IC circuits not working: lack of proper power!
2. Open-loop gain
Wire up the circuit below. Watch the output voltage with a DVM as you slowly
twiddle the potentiometer, trying to apply zero volts (when the "wiper" is exactly at the
center and you have exactly the midpoint between +/–15 V).
Describe the behavior. Is it consistent with gain claims of "100's of volts/mV'?
+15 V
+15 V
3 +
10k
7
LM741
2 !
!15 V
6
out
4
!15 V
3. Inverting Amplifier
10k
1k
in
2 !
LM741
6
out
3 +
Construct the inverting amplifier drawn above. Drive the amplifier with a 1 kHz
sine wave. Measure the gain? What is the maximum output swing? How about linearity
(try a triangle wave)? Try sine waves of different frequencies. At some high frequency,
the amplifier fails to work well. Figure out a way to estimate the slew rate, i.e., the fastest
"slope" in V/ms or V/ns to which the amplifier can respond (N.B. if your signal is
Asin(2πft), differentiating this once gives the slope of the signal (in volts per unit time) for
any time, and you should be able to find the maximum slope. Alternatively, try inputting a
square wave and see how the device tries to duplicate the "infinite" input slope.).
Drive the circuit with a sine wave at 1 kHz again. Measure the input impedance of
this amplifier circuit by adding a 1 kΩ resistor in series with the input.
4. Summing Amplifier
10k
10k
2
in
!
LM741
15k
3
6
out
+
10k
+15 V
!15 V
Adjustable
"DC Offset"
Let's get the "operational" of the operational amplifier: adding signals (currents
really): Build the circuit above that allows you to sum a DC level with an input signal. For
the input, choose a small amplitude sine wave. Describe its operation and the range of
DC offsets you were able to achieve.
5. Non-inverting Amplifier
Wire up the non-inverting amplifier below. Measure the voltage gain and compare
it to prediction.
Try to measure the circuit's input impedance, at 1 kHz, by putting a 1 Meg
resistor in series with the input. How is this different from the inverting amplifier?
3
in
+
LM741
2 !
1k
10k
6
out
6. Follower
in
3 +
LM741
6
out
2 !
Build the follower shown above. Input a sine wave of some frequency and
examine the output and measure the gain. The ultimate boring circuit! What is it good
for?
Different Op Amp Uses
Because they can accept either voltages or currents as inputs, and produce voltage
or current outputs, we can label amplifiers as one four types:
Input
Voltage
Voltage
Current
Current
Output
Voltage
Current
Voltage
Current
Amplifier Name
Voltage
Transconductance
Transresistance
Current
In the next section, you will construct a current source that is controlled by an input
voltage (a "transconductance" amplifier). It is similar to the non-inverting amplifier, with
the input voltage set with a voltage divider.
7. Current Source
+15 V
15k
3
+
6
LM741
1k
2
out
!
A
180
10k "load"
Try out the op-amp current source shown above. What should the current be,
and what do you measure it as? Vary the load potentiometer and watch the current using
the DVM. Determine the range of load resistances over which the current source varies
by less than 10%. This is called its compliance.
This current source, although far more precise and stable than our simple
transistor current source, has the disadvantage of requiring a "floating" load (neither side
connected to ground). It also has significant speed limitations when the output current or
load impedance varies at microsecond speeds.
8. Current to Voltage Converter
Phototransistor
(no collector
connection)
light
10 M!
2 "
LM741
3 +
6
out
(a) Photodiode: Use the supplied phototransistor (feel free to peel back some of
the rubber coating if not enough light is getting into the fiber optic lead) as a photodiode
in the circuit above. To determine the pins, try using the DVM to distinguish them as
learned previously. Examine the output signal (if the DC level is more than 10 V, reduce
the feedback resistor to 1 MΩ).
If you see fuzz on the output (oscillations), put a small capacitor (~100 pF) in
parallel with the feedback resistor.
What is the average DC output level, and what is the percentage "modulation"?
This can be quite large due to the fluorescent lights in the lab... What input photocurrent
does the output level correspond to? Try covering the phototransistor with your hand.
Look at the "summing junction" (the inverting input pin) with the scope, as the output
voltage varies. What should you see?
(b) Phototransistor: Now connect the device as a phototransistor, as shown
below (the base is to be left open, as shown). What is the average input photocurrent
now? What about the percentage modulation? Look again at the summing junction.
+15 V
100k (or more)
light
2 !
Phototransistor
LM741
6
out
3 +
If you are curious and/or interested, try out the "game" on pg. 182 of your
manual!