Lab 10
Operational Amplifier Applications II
Purpose
This lab studies some of the advanced uses of op amps. The circuits studied will
include the inverting integrator, and the differential amplifier.
Material and Equipment
741 Op Amp
Assorted Resistors (2k (2), 39k (2))
Capacitor (1 µF)
Theory
This lab investigates several amplifier circuits.
Integrator
The circuit in Figure 10-1 is an inverting or a Miller integrator. The device can be
analyzed using standard op-amp analysis techniques.
Figure 10-1: The inverting integrator
The result of the output is described as :
So one can see that the output is proportional to the integral of the input signal. A
real integrator circuit requires a large resistor in parallel with the capacitor. This
configuration is known as a “lossy” integrator. The shunt resistor prevents the capacitor
from storing charge due to small offset currents and voltages at the input. It also limits
the minimum input frequency allowed. The integrator can be used to “accumulate” an
input signal over time.
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Differential Amplifier
The differential amplifier is designed to amplify the difference between the two
input signals. A simple differential amplifier is shown in Figure 10-2.
Figure 10-2: The Differential Amplifier
If the four resistors satisfy the relationship, R2/R1 = R4/R3, then the gain of this
amplifier is given by:
Because the amplifier only amplifies the difference between the two input signals,
it rejects common mode signals(signals which are common to the two inputs). Therefore,
if common noise appears at both inputs, it will be rejected. For this reason, the
differential amplifier is used in very noisy environments to reject noise.
If the same input signal is applied to both inputs, the voltage gain of that signal
(which should be very small) is denoted as ACM. One can define what is known as
common mode rejection ratio(CMRR) as:
CMRR = 20log | AD / ACM |
For a good differential amplifier, this number will be very large (80-100 dB).
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Procedure
Inverting Integrator
1) Build the amplifier in Figure 10-1. Take R=39k, R1=2k and C2=1µF.
Bias the amplifier with +15V or -15V.
2) Apply a 200Hz, 1Vp-p sinusoidal signal to the input. Measure and capture the
input and output signals. Be sure to measure them simultaneously.
3) Perform the same for a 1 kHz square wave input.
Differential Amplifier
1) Build the amplifier in Figure 10-2. Take R2=R4=39k and R1=R3=2k. Then,
bias the amplifier with +15V or -15V. By applying a 1Vp-p, 200 Hz signal
between two inputs, measure the differential gain of this circuit
(See Figure 10-3 below).
Figure 10-3: The Differential Amplifier differential gain calculation
2) Apply a common mode signal to the amplifier (this is done by connecting the
function generator simultaneously to both non-inverting and inverting inputs
of the op-amp) as shown in Figure 10-4. Measure the common mode gain of
this amplifier.
Figure 10-4: The Differential Amplifier Common Mode gain calculation
3) From steps above calculate the Common Mode Rejection Ratio CMRR.
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Questions for the Lab Report
Define CMRR. Make a web search and from the data sheet, specify the value of
CMRR for 741 circuit.
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