American University of Sharjah
College of Engineering
Department of Electrical Engineering
Analog Electronics Lab (ELE342L)
Lab 11
Differential Amplifier
Outcomes: After completing this lab the student will be able to build and test a
Differential amplifier.
Background:
A differential amplifier is a high-gain electronic voltage amplifier with differential inputs
and usually, a single output. The differential amplifier is one of the most widely used
circuit building block in analog integrated circuits. The differential amplifier is an
important stage of a very large area of applications, including high-performances analog
/mixed ICs, such as operational amplifiers, voltage comparators, voltage regulators, video
amplifiers, modulators and demodulators or A/D and D/A converters. The differential
amplifiers can be implemented with BJTs, MOSFETs and MESFETs. We focus on the
differential amplifiers implemented with MOSFETs.
The circuit makes use of two matched transistors with sources shorted together and
connected to a current source. Devices must always be in saturation mode/ON state. The
circuit amplifies the difference between the two input voltages. But there is also common
mode amplification in the no ideal case. The discrimination against common mode signals
is greater if the common emitter node is driven by a constant current source. The
differential amplifier provides low noise amplification with high input impedance. The
differential inputs of the amplifier consist of an inverting input and a non-inverting input.
The amplifier amplifies only the difference in voltage between the two inputs. The gain
offered by the amplifier to the differential signal is called the "differential voltage gain”.
The circuit diagram of a discrete operation amplifier is given below.
The common-mode gain of the differential amplifier will be small (desirable) if the
resistance of the biasing current source is large. The biasing current source is synthesized
from other transistors. In most situations, the user will choose some form of current mirror
to produce the equivalent current source. In the current mirror circuit the reference current
is set by the resistor. Because the two MOSFETs are matched and have precisely the same
gate-source and threshold voltages, their drain currents will be equal.
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V1
9V
RD1
RD2
Vo
Q2
Q1
V1
V2
V2
9V
Rref
Q3
Q4
V3
-9V
Figure 1 MOS Differential Amplifier
The Q1 and Q2 together constitute the differential amplifier. The gain of this stage will
depend on the transconductance of the MOSFETs used and the RD resistance values. The
output is taken from the drain terminal of one of the transistors.
The differential amplifier driven by a current sources enhances the CMRR of the
amplifier. A simple current source can be implemented with the circuit given in Figure.2.
V2
9V
Rref
Io
Q3
Q4
V3
-9V
Figure 2. Constant current source
Current Io can be estimated with the following equation.
π2 − 1.0 − π3
πΌπππ =
π
πππ
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The circuit is implemented with matched pair of transistors. ALD1106 is general purpose
n-channel MOS transistor arrays. Each array consists of four independent transistors (Nchannel Enhancement type) on a common substrate, which has a separate connection. The
substrate terminal should be connected to the lowest value of the supply voltage used in
the circuit (-9,0 V in our case). Independent connections for each transistor permit
maximum flexibility in circuit design.
Figure 3 Pin configuration of ALD1106,
The ALD1106 is a monolithic quad/dual N-channel enhancement mode matched
MOSFET transistor arrays intended for a broad range of precision analog applications.
The ALD1106 offers high input impedance and negative current temperature coefficient.
The transistor pairs are matched for minimum offset voltage and differential thermal
response, and they are designed for precision analog switching and amplifying
applications in +2V to +10V systems where low input bias current, low input capacitance
and fast switching speed are desired. The ALD1106 commonly used as building blocks
for differential amplifier input stages, transmission gates, multiplexer applications, current
sources and many precision analog circuits. The threshold voltage of the MOSFET is
between 0.7V(min) and 1.0V(max).
Equipment and Materials:
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0-9 V dual DC Power Supply
Digital Multimeter
Breadboarding Socket
IC ALD1106
Resistors ( 14 W) 10k, 91k, etc.
Function Generator
Oscilloscope
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Lab Procedure:
1. Build and test the current mirror circuit. Ensure that the current mirror circuit gives
the expected bias current.
2. Construct the differential amplifier circuit as given in Figure.1. Verify the circuit
operation by measuring the currents through Q1, Q2 and also the dc voltages at
the drain terminals with both inputs V1 and V2 grounded.
3. Note down the measured currents through Q1, Q2, Q3 and Q4.
4. Apply a differential voltage signal to the input. Measure the output voltage and
determine the differential mode voltage gain (AVD). Be careful not to over drive
the amplifier.
5. Apply a common mode signal (equal magnitude and same polarity) to the two
inputs. Measure the output voltage and determine the common mode voltage gain
(AVCM). Again, be careful not to overdrive the amplifier.
6. Determine the CMRR for the amplifier.
Discussion and conclusion:
1. Is there any advantage in setting low current values for current mirror circuit used in
the differential amplifier? Explain your results.
2. How do you test the working of the current mirror circuit in the differential amplifier
implemented? Explain.
3. What is the preferred value for CMRR in a differential amplifier? Explain the
significance of CMRR of a differential amplifier.
References:
1
2
Neamen, D., Microlectronics 3rd edition, 2006, McGraw-Hill
Robert Boylestad Louis & Nashelsky., “Electronic Devices and Circuit Theory”
6th edition, printice-Hall international, INC c
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Rd1
Rd2
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Rref
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Rout
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Rd2 voltage
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Rref
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Rd1 voltage and Rd2 second part
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Rref voltage second part
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Q1 grounded and Q2 Vin
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Q2 grounded and Q1 Vin
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Both function generator
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