UNIVERSITY OF MASSACHUSETTS DARTMOUTH
DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING
ECE 201
CIRCUIT THEORY I
OPERATIONAL AMPLIFIER -- OP AMP
INTRODUCTION
An Operational Amplifier, or OP AMP, is an electronic circuit that can be configured into
circuits that perform mathematical operations such as addition, subtraction, integration, and
differentiation. This device is fabricated as an integrated circuit and is very easy to use. “Under
the hood”, the OP AMP consists of several transistors, resistors, and a capacitor or two. The
circuit diagram for a typical OP AMP is shown in Figure 1.
Figure 1. An OP AMP configured as a Comparator
The operation of the circuit can be studied by applying the following condition. Let’s
consider that the voltage Vplus is larger than voltage Vminus . The voltage on the non-inverting input
of the OP AMP is larger than the voltage on the inverting input. The OP AMP will try to amplify
the difference between the input voltages with a very large non-inverting voltage gain, causing the
OP AMP output voltage to saturate at the positive supply voltage VCC (+ 15 Volts).
When the situation is reversed and voltage Vminus is larger than voltage Vplus , the voltage
on the inverting input of the OP AMP is larger than the voltage on the non-inverting input. The
OP AMP will try to amplify the difference between the input voltages but now with a very large
inverting voltage gain, and the OP AMP output voltage saturates at the negative power supply
voltage VEE (-15 Volts).
Let’s add some more components to the circuit and see what happens. Consider the
circuit shown in Figure 2.
Figure 1. The circuit diagram of a 741 OP AMP.
The circuit symbol for the 741 OP AMP is shown here in Figure 2.
7
1
5
U1
3
6
2
4
741
Figure 2. The circuit symbol and pin connections for a 741 OP AMP.
Figure 2 is repeated here for easy reference.
7
1
5
U1
3
6
2
741
4
The 741 OP AMP terminals/pins are defined as follows:
Pin #2
Pin #3
Pin #6
Inverting input
Non-Inverting input
Output
Pin #7
Pin #4
Positive Supply Voltage, VCC
Negative Supply Voltage, VEE
Pins # 1,5
Offset Null Adjustment (Not needed for now)
Briefly, here’s how things go. When a sinusoidal voltage is applied to the inverting input
and the non-inverting input is grounded (zero volts), the output voltage will be an inverted, or, out
of phase by 180, sinusoid. When the reverse is true, that is, a sinusoidal voltage is applied to
the non-inverting input and the inverting input is grounded, the output voltage will be a noninverted, or not-out-of-phase sinusoid. The sections that follow show the practical configurations
that produce these results.
INVERTING AMPLIFIER
The circuit shown here in Figure 3 is a typical OP AMP inverting amplifier.
Vee
15 V
Rf
10kOhm
4
U1
R1
1kOhm
2
Vout
6
3
V1
1V
1kHz
0Deg
7
1
5
741
Vcc
15 V
Figure 3. A 741 OP AMP as an Inverting Amplifier
The relationship between the output voltage, Vout and the input voltage to the circuit, V1 is
Vout = -(Rf /R1)V1
In this case, Rf = 10 kΩ, R1 = 1 kΩ, and Vout = -10V1, or we have a voltage gain, G = 10, and then
Vout = -GV1 = -10sin(2000πt) volts.
NON-INVERTING AMPLIFIER
A non-inverting OP AMP amplifier configuration is shown below in Figure 4.
Vee
15 V
Rf
10kOhm
4
U1
R1
2
1kOhm
Vout
6
3
V1
1V
1kHz
0Deg
7
1
5
741
Vcc
15 V
Figure 4. A 741 OP AMP as a non-inverting amplifier
Note that the feedback connection remains between the output and the inverting input as
before, but now the input voltage to the amplifier is applied to the non-inverting input. The output
and input voltages are related by the expression
Vout = {1 + (Rf /R1)}V1
Here, Rf = 10 kΩ, R1 = 1 kΩ, and Vout = 11V1, or the voltage gain G = 11. The output voltage is
given by
Vout = 11sin(2000πt) volts.
PRELIMINARY WORK / DESIGN
1. Design an OP AMP inverting amplifier that has a voltage gain, G, numerically equal to
G = 0.002(ABCD) + 5
where ABCD = the last four digits of your Student ID Number. Try to make the value of Rf at least
2 kΩ and the values of the other resistor between 100 Ω and 1 MΩ.
2. Design an OP AMP non-inverting amplifier that has a voltage gain, G, numerically
equal to
G = 0.002(EFG) + 12
where EFG = the first three digits of your Student ID Number.
3. Simulate each of your designs using MultiSim. Use the standard 741 model for the OP
AMP and power it with +/- 15 volts. Use an input voltage of 1 kHz at an amplitude that does not
cause your amplifier to saturate. Check your input and output voltages (amplitudes and phases)
with the oscilloscope.
EACH GROUP MEMBER MUST HAVE HIS/HER OWN AMPLIFIERS DESIGNED AND
SIMULATED BEFORE COMING INTO THE LAB!
LABORATORY PROCEDURE / RESULTS
Each group member will construct the circuits that they have designed and simulated.
For each circuit, measure the input and output voltages using the oscilloscope. Calculate the
voltage gain of each configuration and compare with the theoretical values. Be sure to sketch
each of the input and output voltages in your laboratory notebook. Use the oscilloscope’s storage
option to save a copy of your results for publishing in your lab report.
Your lab report should include all design calculations as well as any modifications that
were necessary. Be sure to comment on how well your results agreed with the theoretical
calculations. If things didn’t go as they should have, explain what may have gone wrong.