BET 201
Experiment 8
Instrumentation Amplifiers
Objective:
To observe and understand the differential gain, common mode gain, and commonmode rejection ratio (CMRR) of an instrumentation amplifier.
Reference:
Carr/Brown, Chapter 7.7, 8.7
Equipment: Dual D.C. Power Supply 0-15 V; Function Generator; DMM; Analog trainer;
Oscilloscope; 3-LM741 Op-amps; resistors and capacitors as indicated.
Preparation:
1.
For the circuit of Figure 1, calculate the voltages at the outputs of each amplifier for
R2 = 10kΩ, V1 = – 0.1V, and V 2 = + 0.1V. Record the calculated values in Table 2A.
2.
For the circuit of Figure 1, calculate the voltages at the outputs of each amplifier for
R2 = 10kΩ, V1 = + 0.1V, and V 2 = + 0.1V. Record the calculated values in Table 2B.
3.
Calculate the peak-to-peak voltage at the output of each amplifier when the input signal at V1 is
100 mVp-p @ 1kHz, V2 = 0, and R 2 = ∞ (open). Record the calculated values in Table 3.
Procedure:
RESISTOR VALUES
1. Construct the circuit of Figure 1 with R2 = 10kΩ. Carefully remove each resistor (one at a
time) and measure its exact value. Return each resistor to the circuit and record the values in
Table 1.
DC INPUT
2. For R2 = 10kΩ, V1 = – 0.1V, and V 2 = + 0.1V, measure the voltages at the output of each
amplifier with respect to ground. Record the data in Table 2A.
3.
For R2 = 10kΩ, V1 = + 0.1V, and V 2 = + 0.1V, measure the voltages at the output of each
amplifier with respect to ground. Record the data in Table 2B.
AC INPUT
4.
Remove resistor R2 and put it in a safe place (you'll need this same resistor again later).
Apply input signals of V1 = 100 mVp-p @ 1kHz and V2 = 0 (connected to ground). With
R2 = ∞ (open), use an oscilloscope to measure the peak-to-peak voltage at the output of each
amplifier. Record the data in Table 3. Sketch the voltage waveforms at the circuit inputs (V1
& V2 ) and the output of each amplifier VA, VB, & V3 ).
5.
For resistor R2 , place a 10kΩ potentiometer in the circuit. Adjust the potentiometer to vary the
gain of the amplifier, and notice how the waveform at V3 changes as the potentiometer is
adjusted. Vary the potentiometer to obtain the largest symmetrical (undistorted) waveform.
Remove the potentiometer from the circuit, and measure this value for R2 .
COMMON MODE REJECTION RATIO
6. Replace the potentiometer with the 10kΩ resistor used in Step 1. Measure the peak-to-peak
voltage at the output with input signals V1 = 100 mVp-p @ 1kHz and V2 = 0. With an
oscilloscope, measure the voltages at the circuit inputs and output. Calculate the measured
differential gain (V3 /V1 ). Record the data in Table 4.
7.
Remove V2 from ground and connect it to the same 100 mVp-p @ 1kHz sine wave already
connected to V1 . With an oscilloscope, measure the peak-to-peak voltage at the circuit output.
Calculate the measured common-mode gain (V3 /V1 ). Record the data in Table 4.
Additional Calculations:
1.
Show the calculations to verify step 5 of the procedure. Assume that the maximum output
voltage will be 15V (rail voltage).
2.
Use the exact values of the resistors measured in Step 1, and the nominal input voltages used
in Steps 6 & 7 of the procedure. Calculate the common-mode gain (Acm), the differential gain
(Adiff), and the common-mode rejection ratio (CMRR) for the circuit.
Report Requirements:
1.
This experiment and results should be presented in accordance with the report guidelines.
2.
In the summary and conclusion section, briefly discuss how the instrumentation amplifier
operates.
3.
In the summary and conclusion section, explain why the output of the amplifier shows a nonsymmetrical AC waveform when the gain is increased past a certain point.
Experiment 8
Instrumentation Amplifiers
Data Tables
Table 1. Resistor Values
R1A
R1B
(Ω)
(Ω)
Nominal
47 kΩ
47 kΩ
R2
(Ω)
R3
(Ω)
R4
(Ω)
R5
(Ω)
R6
(Ω)
10 kΩ
10 kΩ
10 kΩ
47 kΩ
47 kΩ
Measured
Table 2A. DC Input
V1
(V)
VA
(V)
VB
(V)
V3
(V)
VA
(V)
VB
(V)
V3
(V)
VA
(Vp-p)
VB
(Vp-p)
V3
(Vp-p)
Table 4. Common Mode Rejection Ratio (CMRR)
V1
V2
V3(meas)
A(meas)
(Vp-p)
(Vp-p)
(Vp-p)
V3(calc)
(Vp-p)
Calculated
– 0.1
V2
(V)
+ 0.1
Measured
Table 2B. DC Input
V1
(V)
Calculated
+ 0.1
V2
(V)
+ 0.1
Measured
Table 3. AC Input
V1
(Vp-p)
Calculated
100 mVp-p
1 kHz
V2
(Vp-p)
0
Measured
Differential
100 mVp-p
1 kHz
0
Common
100 mVp-p
1 kHz
100 mVp-p
1 kHz
A(calc)
C1
560 pF
+15V
V1
3
7
2 LM741
4
R3
10 kΩ
6 VA
R5
47 kΩ
R1A
47 kΩ
+15V
-15V
2
R2
+15V
2
V2
3 LM741
4
3 LM741
4
-15V
R1B
47 kΩ
7
7
6
VB
R4
10 kΩ
-15V
R6
47 kΩ
Figure 1.
Offset Null
Inverting
Input
Non-Inverting
Input
–V
1
8
2
7
NC
+V
LM741
3
6
4
5
Output
Offset Null
6
V3