Lab 7
ECE 312
Spring 2025
Differential Amplifier
This experiment uses five sub-circuits to demonstrate the capability of differential amplifiers to reject
common-mode noise while amplifying differential-mode signals. You will use five 741 op amp chips. The
op amp’s schematic and pin-outs are diagrammed below. (Be sure to test each sub-circuit independently
to verify its proper operation.)
Top View
1
4 (15V)
8
2
3
4

6

5
7 (+15V)
Prelab
Inverting Amplifier
1. An inverting op amp amplifier with a gain of 1 is used to generate fully-differential 1 kHz 20 mVpeak square wave signals, Vp and Vm. Build the circuit in Multisim and run a Transient analysis.
Note that the op amp output, Vm, exhibits an offset error due to the op amp’s VOS and IB. Use the
cursors to measure the offset error for Vm.
0-20 mV
R4
1kΩ
VEE
-15V
Vp
4
U4
R5
2
Should be 20 to 0 mV
Vm
1kΩ
6
V1
1kHz
20mV
3
7
1
5
741
VCC
15V
Offset error
Oscillator
2. An op amp relaxation oscillator is used to generate a common-mode “noise” signal, Vnoise. Build
the circuit in Multisim. Set the capacitor’s initial condition (IC) to 0 V, and run a Transient analysis
with “Initial conditions” set to “User-defined”. Use the cursors to measure the “noise” signal’s
frequency and peak-to-peak amplitude.
R16
100Ω
VCC
15V
7
R17
1
5
U2
3
100Ω
6
C5
0.01µF
IC=0V
4
741
VEE
-15V
R18
100Ω
1
R19 Vnoise
1kΩ
2
Lab 7
ECE 312
Spring 2025
Weighted Summer
3. Two op amp weighted summer circuits are used to add the “noise” signal Vnoise to each of the
fully-differential input signals, Vp and Vm, and generate the two differential amplifier inputs, Vi1
and Vi2. Thus Vi1 will have a common-mode component, Vnoise and a differential-mode
component, Vm. And Vi2 will have a common-mode component, Vnoise and a differential-mode
component, Vp. On-page connectors
,
,
,
and
allow for connections to
be made on the Multisim schematic without having a “rat’s nest” of wires. Any wires in SPICE that
are given the same name will be connected together by on-page connectors, so you can
automatically obtain on-page connectors just by clicking on a wire and giving it a name, then
clicking on another wire and giving it the same name. Build the Weighted Summer circuits in
Multisim, connecting them to the Inverting Amplifier and Oscillator and run a Transient analysis
to confirm that the noise is added to the signals as shown below.
R8
R11
1kΩ
1kΩ
VEE
VEE
-15V
Vp
Vi2
R9
4
741
1kΩ
R12
4
U3
2
R10
6
3
1kΩ
U1
1kΩ
2
Vnoise
Vi1
-15V
Vm
Vnoise
R13
1kΩ
7
1
5
6
3
7
VCC
1
5
741
VCC
15V
15V
Differential Amplifier
4. The differential amplifier attenuates common-mode signals while amplifying differential-mode
signals, thereby extracting the differential-mode signal from the common-mode “noise”.
R2
10kΩ
VEE
4
-15V
560Ω
560Ω
Vout
6
3
R6
10kΩ
15V
VCC
2
1
R3
2
5
Vi1
U5
R1
7
Vi2
741
Lab 7
ECE 312
Spring 2025
Note that this is the same circuit as in Lab 1, but with a nominal differential-mode gain of 𝐴𝑑 =
10 𝑘Ω⁄560 Ω = 17.86. For Lab 7 the differential-mode input signal should be Vp Vm = 40 mV
peak-to-peak. Thus the expected peak-to-peak output voltage is 17.86 × 40 𝑚𝑉 = 714.4 𝑚𝑉.
Build the Differential Amplifier in Multisim, connecting it to the Weighted Summer circuits and
run a Transient analysis. Use the cursors to measure the peak-to-peak output voltage, Vout.
Lab
5. Measure and record the actual values for all resistors prior to building the circuits on the
breadboard.
6. Build and test each sub-circuit independently to verify its proper operation. Connect the subcircuits together to form the system circuit.
Common-Mode Gain
7. Disconnect signals Vp and Vm from the weighted summer circuits. The summer circuits now pass
just the common-mode “noise” Vnoise to the differential amplifier’s inputs. Measure the peakto-peak voltage Vout, and calculate the common-mode gain 𝐴𝑐𝑚 = Vout/ Vnoise.
Differential-Mode Gain
8. Disconnect signal Vnoise from the weighted summer circuits, and reconnect signals Vp and Vm.
The summer circuits now pass just the fully differential-mode signals to the differential amplifier’s
inputs. Measure the peak-to-peak voltage Vout, and calculate the differential-mode gain: 𝐴𝑑 =
Vout/40 mV.
9. Calculate the Common-Mode Rejection Ratio, CMRR = 20 log |Ad/Acm| (dB).
Noise Immunity
Note: do not use the oscilloscope’s averaging function here; you want to see the differential amplifier’s
noise suppression capability apart from the scope’s noise cancellation by averaging.
10. Reconnect signal Vnoise to the weighted summer circuits. The summer circuits now add commonmode “noise” to the fully-differential input signals. Use the oscilloscope to observe the signals
plus noise at nodes Vi1 and Vi2; capture these wave forms for your report.
11. Observe the output signal Vout; capture this wave form for your report. Comment on the results.
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