Download 10. Operational Amplifier Circuits

Villanova University
ECE 2053 Fundamentals of Electrical Engineering I Lab
Spring 2012
10 – Op Amp Circuits
Introduction
The electrical symbol for the operational amplifier, or op amp, is shown in Fig. 1. The
op amp is essentially a high-gain amplifier. It has two input terminals and one output
terminal. The – input terminal is the inverting input and the + input terminal is the
noninverting input.
Inverting input
OUT
+
Noninverting
input
OPAMP U1
Figure 1
The op amp is used in applications where amplification, integration, summation or wave
shaping are required.
The most important parameters of an op amp are listed below.
- very high voltage gain G, of the order, 105 to 107 volts/volt,
- very high input resistance Ri, e.g., 100 k,
- very low output resistance Ro, typically 10 .
When the op amp is used in a linear circuit, the output signal, or some portion of it, is
fed back to the inverting input. The circuit is said to have negative feedback.
Learning Objectives
At the end of this experiment, you should be able to:
1. Wire a positive and a negative dc supply to the protoboard and to the op amp.
2. Wire an inverting amplifier, a noninverting amplifier, and a summing amplifier.
3. And measure the basic characteristics of these amplifier types.
Exercise 1: Wiring the +/– DC Power Supplies to the Proto Board
The op amp we will use is the LM741. LM stands for Linear, Monolithic. The
LM741 is made on one integrated circuit chip, thus the term monolithic.
The manufacturer’s data sheet includes important parameters such as dc supply
voltage, power dissipation, input resistance and large signal voltage gain.
1. Go to the web site of the National Semiconductor Corporation at its URL
http://www.national.com/mpf/LM/LM741.html#Overview.
2. Look for the Adobe logo with the word Datasheet below it. Open the data sheet
and note the pin numbers for the chip, especially those for the dc power.
3. Figure A-1, below, shows the wiring for the +/– 25-V power supply.
4. Wire the power supply to your PB by following the check off steps below.
769859084
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Check Off List.
___ 1. Place your PB in landscape orientation as shown above.
___ 2. Connect a green wire from the COM terminal of your PS to terminal G on the PB.
___ 3. Connect another green wire from terminal G to a hole on the bottom row of the PB
(the row is indicated by a + sign) as shown above. This is your ground bus or reference
bus.
___ 4. Connect a red wire from the + terminal of your PS to terminal R on the PB.
___ 5. Connect another red wire from terminal R to a hole on the top row of the PB (the
row is indicated by a + sign) as shown above. This is your positive bus or positive rail.
___ 6. Connect a black wire from the  terminal of your PS to terminal B on the PB.
___ 7. Connect another black wire from terminal B to a hole on the row of the PB (the
row is indicated by a – sign) as shown above. This is your negative bus or negative rail.
___ 8. Do not connect the function generator.
___ 9. Set the + 25-volt sources to 12 volts each, and the current limits to 25 mA.
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Exercise 2: Inserting the Op amp
1. Obtain a 741 op amp. The top of the IC is shown below, with the pins pointing away
from you.
2. Note the location of pin 1 and the semicircle. The other pin numbers go, in sequence,
counter clockwise around the IC.
3. Place the IC over the gutter (slot) as shown in Figure A-1 and push in, gently.
4. The last bit of wiring is to connect the dc power that is on the PB to the op amp. As
shown in Fig. A-1, connect a short black wire from the negative rail to pin 4 of the
IC, and connect a short red wire from the positive rail to pin 7 of the IC.
5. Turn on the power supply. Connect the black clip lead of your DVM to the ground
bus and leave it there. Verify that the voltages at pins 4 and 7 are correct. Then turn
off the power.
Exercise 3: Inverting Amplifier
Theory
Figure 2 shows the inverting amplifier circuit. Note the following:
- pin 2 is the inverting input
- pin 3 is the noninverting input
- pin 6 is the output
- R1 is the input resistor
- Rf is the feedback resistor.
The FG supplies a signal, Vi, to the amplifier. The output voltage is
Vo = (− Rf / R1) Vi
As can be seen from the equation, the output signal, Vo, is an amplified and inverted
version of the input signal. In addition, the voltage gain, Gv, is
Gv = Vo / Vi = − Rf / R1 volts/volt
(1)
3
Note: pin 3 is grounded.
Rf
R=?
Vfg
U3
2
-
R=?
OS1
OUT
3
+
V+
500mVac-max
0Vdc
12Vdc
4
V1
V-
Ii
------>
R1
Vi
OS2
1
Vo
6
5
7
uA741
V2
12Vdc
Figure 2 Inverting amplifier
The input resistance, Rin, of the amplifier is the equivalent resistance as seen by the
source Vi and is given by
Rin = Vi / Ii = R1
(2)
Exercise
1. Design an inverting amplifier that meets all of the following specifications:
- a voltage gain of Gv = –10 V/V
- an input resistance of at least 5 kΩ
- resistors in the range of 4.7 kΩ to 82 kΩ.
2. Wire the resistors as shown in Figure A-2, and connect the FG.
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3. Turn on the power.
4. Set the FG for a 200 Hz, zero offset, sine wave with a 500 mV peak value.
Note: Set the FG display for the correct p-p voltage readout – see Appendix.
5. Connect the CH 1 scope probe to display the input signal, and connect the CH 2
probe to display the output signal. The displayed waveforms should be sinusoidal
and 180 degrees out of phase, indicating inversion. If everything is ok, sketch the
waveforms in your notebook and record the peak values of the signals. Determine the
voltage gain Gv from the peak values. Compare this gain with the value predicted by
Equation (1).
6. Slowly increase the FG amplitude to 3 volts peak (press Auto-scale often). Observe
how the output waveform starts to flatten out on the top and the bottom. This
flattening is called output saturation. Record the input and output waveforms.
Record the + saturation voltage, and the – saturation voltage. Theoretically, what
should the saturation voltages be?
7. Change R1 to 680 Ω and Rf to 10 kΩ. Adjust the amplitude of the input signal so the
output signal is not saturated. Record the peak values of the signals and determine
the voltage gain. Compare this gain with the value predicted by Equation (1).
8. Observe the waveforms if the input signal is a triangular wave.
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9. Turn off the power.
Exercise 4: Noninverting Amplifier
Theory
Figure 3 shows the noninverting amplifier circuit. The output voltage is an
amplified version of the input voltage but with no inversion. The output voltage is
Vo = 1+ (Rf / R1) Vi
The voltage gain is
Gv = Vo / Vi = 1+ (Rf / R1) V/V
(4)
The input resistance, Rin, of this amplifier is the equivalent resistance as seen by the
source Vi and is given by
Rin = Vi / Ii
But since the current into pin 3 is essentially zero,
Rin = Vi / Ii =  
(5)
Exercise
1. To wire this circuit, you will only have to make minor changes to the wiring of your
existing inverting amplifier. Follow the instructions below. (The resistors are the
same, i.e., R1 = 680 Ω, and Rf = 10 k Ω.)
Ii
------>
7
U2
3
Vfg
+
V+
Vi
OS2
OUT
2
-
OS1
6
Vo
1
4
0Vdc
V-
500 mVpeak
uA741
5
Rf
10k
R1
680
Figure 3 Noninverting amplifier
2. Remove the wire, which comes out of the red terminal of the FG, from R1. Remove
the ground wire at pin 3. Connect the left side of R1 to ground. In addition, connect
the wire of the FG to pin 3. This completes the wiring.
3. Set the FG for a 500 mV peak sine wave signal at 200 Hz with zero offset. Turn on
the dc power. Connect the scope probes to display the input and output signals. The
input and output waveforms should be sinusoidal and in phase. If everything is ok,
sketch the waveforms in your notebook and record the peak values of the signals.
Determine the voltage gain Gv from the peak values. Compare this gain with the
value predicted by Equation (4).
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4. Slowly increase the FG amplitude until the output waveform starts to flatten out on
the top and the bottom (saturation). Record the peak amplitude of the input
waveform. Are the +/– saturation voltages the same as before?
Exercise 5: Summing Amplifier
Theory
Figure 4 shows a summing amplifier circuit. If Vfg = Vi1 and Vsync = Vi2 the output
voltage is given by
Vo = Gv1 Vi1 + Gv2 Vi2
(5)
where
Gv1 = − Rf / R1
(5a)
and
Gv2 = − Rf / R2
(5b)
Exercise
1. Use these resistor values for the summing circuit:
R1 = 680 Ω, R2 = 12 kΩ and Rf = 10 k Ω.
Rf
10k
R2
V1
0 Vbaseline
Vfg
500 mVpeak
680
OS1
OUT
3
+
OS2
1
Vo
6
5
uA741
7
0Vdc
-
V+
4 V peak
U3
2
V-
R1
Vsync
12Vdc
4
12k
V2
12Vdc
Figure 4 Summing amplifier
2. Set the FG for a frequency of 200 Hz. Input 1 is Vfg and comes from the FG Output
terminals; set it to a 500 mV peak sine wave with zero offset. Input 2 is Vsync and
comes from the FG Sync Output terminals. It is a nonadjustable square wave, at 200
Hz, with a minimum value of 0 V and a maximum value of approximately 4 V.
3. Connect CH 1 probe to input 1 and connect CH 2 probe to input 2. Record the two
input waveforms, noting their baselines, peak values, and phase relations.
4. Move CH 2 probe to the output and record the output waveform noting the phase
relation with the input waveforms.
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Exercise 6: Summing Amplifier Using Student Designed Power Supply
1. Remove the three Agilent PS wires that attach at the Red, Black, and Green terminals
on the PB.
2. Retrieve the + 15 V power supply that you designed in the previous lab, and connect
the + 15 V wire to the Red terminal, the  15 V wire to the black terminal, and the
ground wire to the Green terminal.
3. Connect your power supply to the 30-volt Agilent supply in the same manner as last
week.
4. Check to see if the summing amplifier works as in Exercise 5.
Parts List
LM 741 op amp
Resistors: 680 Ω, 10 kΩ, 12 kΩ and others (5 %, ¼ W)
Binding post-to-BNC connector: 2
Appendix
Enter the following commands on the front panel of the FG so that the readout
gives the actual peak-to-peak value.
Keys to Press
Shift Menu
On/Off
→ → →
↓
↓
→
Enter
What is in the Display
A: MOD MENU
D: SYS MENU
1: OUT TERM
50 OHM
HIGH Z
ENTERED
Now the default FG display is the same as what is read on the scope. Note: If you
turn off the FG, you lose the above settings.
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