Download EECE 322 Lab 8: Differential Amplifiers

EECE 322
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Lab 8: Differential Amplifiers
Laboratory Goals
This project will focus on single stage differential amplifiers. Both BJT and FET
amplifiers will be examined as will the use of resistor and current source biasing.
Reading
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Student Reference Manual for Electronic Instrumentation Laboratories by Stanley Wolf
and Richard Smith, Copyright 1990.
Oscilloscope User’s Guide (Copies of this reference book are available in the lab, or at
the website)
Tektronics 571 Curve Tracer Manual
BS170 Transistor Data Sheet
Read the pre-lab introduction below
Equipment needed
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Lab notebook, pencil
Oscilloscope (Agilent or Tektronics)
2 oscilloscope probes (already attached to the oscilloscope)
BNC/EZ Hook test leads
Tektronics 571 Curve Tracer
PB-503 Proto-Board
Workstation PC, with PSICE application
Parts needed
2N2222 BJT, 2N7000 FET
Lab safety concerns
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Make sure before you apply an input signal to a circuit, all connections are
correct, and no shorted wires exist.
Do not short the function generator signal and ground connections together
Do not touch the circuit wiring while power is applied to it
Ensure you connect the correct terminal of the transistor to prevent blowing the
transistor
1. Pre-Lab Introduction
The operational amplifier has had a dramatic impact on electronic circuit design, both
analog and digital, over the last 25 years. While the complexity, speed and capability of
EECE 322
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Lab 8: Differential Amplifiers
the Op-Amp have changed dramatically over this time, the basic operation still depends
heavily on the input differential amplifier stage. It is this differential amplifier stage that
will be examined in this project. The differential amplifier is designed to effectively shift
a constant current between two branches as a function of the difference between the two
input signals. Ideally, as a result of the changing current, the amplifier output reflects
only the difference between the inputs. The quality for the amplifier design is determined,
in part, by examining the output of the differential amplifier under two specific input
conditions. The ratio of the differential mode voltage gain [ADM] (inputs are equal in
magnitude and opposite in sign) to the common mode voltage gain [ACM] (both inputs
are equal) is used to determine the Common Mode Rejection Ratio (CMRR). The higher
the ratio, the better the differential amplifier stage is able to discriminate between the
actual difference in the signals present at the two input terminals. The input impedance is
another important measure of the quality of the differential amplifier stage. These two
items, CMRR and Zin, will be the primary focus of this project.
Figure 8-1 illustrates a BJT based differential amplifier and Figure 8-2 shows an FET
based stage. The differential output versions (Figures 8-1 (A) and 8-2 (B)) have a resistor
in each branch and the output is measured between the two collectors (drains). Many
differential amplifiers are designed as single ended outputs since the information
contained in either of the collector (drain) terminals is sufficient to determine the
differential input. Figures 8-1 (B) and 8-2 (B) illustrate the single ended designs. In the
single ended case one of the branch resistors (R1 for example) is removed and replaced
by a short circuit. The determination of the non-inverting (VP) and inverting (VN) input
terminal is made by looking at the relationship between a change on the input terminal
and the corresponding change in the output voltage. The non-inverting terminal causes an
increase in the output voltage for an increase in the input signal. There is a 180° phase
shift between the inverting input signal change and the output signal change. The current
source illustrated between each diagram is generally implemented using an appropriate
BJT or FET current mirror. A discussion of current mirrors can be found in Projects 8
and 13. The use of a resistor in place of the current mirror is also used to provide an
approximately constant current source. Both types of current supplies will be investigated
in this project.
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Lab 8: Differential Amplifiers
Figure 8 - 1: BJT Differential Amplifier
Figure 8 - 2: FET Differential Amplifier
2. Design
1. Design a single ended BJT differential amplifier capable of providing a ± 10 V output
swing across a 1 k
should be supplied by a BJT current mirror. Indicate the value of Vout when both inputs
are grounded. Verify your design PSPICE®.
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Lab 8: Differential Amplifiers
2. Repeat step 1 using a single resistor to provide the switching current.
3. Design a single ended FET differential amplifier capable of providing a ± 5 V output
swing across a 1 k-Ohm resistor using ± 15 V DC power supplies. The switching current
should be supplied by an FET current mirror. Indicate the value of Vout when both inputs
are grounded. Verify your design PSPICE®.
4. Repeat step 3 using a single resistor to provide the switching current .
3. Lab Procedure
1. Construct the differential amplifier designed in step 1 of the design procedures. Verify
the circuit operation with both inputs grounded. Be careful in making voltage
measurements so as not to effectively by-pass your current source.
2. Apply a differential voltage signal (VX) to each input. The individual input voltages
should be equal in magnitude but opposite in polarity. Measure the output voltage and
determine the differential mode voltage gain (ADM). Measure the input current for
terminal and determine the effective input impedance as seen by the total differential
input voltage (2VX). Be careful not to over drive the amplifier.
3. Apply a common mode signal (equal magnitude and same polarity) to the two inputs.
Adjust your common mode voltage to the total differential voltage used in step 2 (2 VX).
Measure the output voltage and determine the common mode voltage gain (ACM).
Again, be careful not to overdrive the amplifier. Determine the input impedance for this
input condition. Compare this input impedance with the impedance determined in step 2.
Discuss possible causes for any differences between the two values.
4. Determine the CMRR (ADM/ACM) for the amplifier.
5. Reverse the polarities on the inputs for steps 2 and 3 and determine the differential
mode gain, common mode gain, and CMRR for the revised inputs. Comment on any
similarities and/or differences.
6. Repeat steps 2 - 5 for each of the other three differential amplifier designs.
7. Prepare a summary of the various measurements and results for all the tests. Analyze
the summary data and provide a brief discussion of the differences/similarities between
the various designs.
4. Analysis
1. Could any, or all, of these circuits be designed using a single DC power supply?
Explain your answer.
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Lab 8: Differential Amplifiers
2. What, if any, limitations are there on the value of the common mode signal? Are they
different for the amplifiers designed using a current source when compared to those using
a single resistor for the current supply?
3. Are the two voltage gains, ADM and ACM affected by the value of the input voltages
used? Explain your answer.
4. Is the input impedance affected by the decision to use a current mirror versus the single
resistor? If so, how can the difference be explained?
5. Comment on the benefit/disadvantage of using an FET current mirror for the BJT
based differential amplifier. Repeat for the reversed situation.
6. Discuss the benefits/disadvantages of the single ended output versus the differential
output designs.
2. Why should you include a resistor in parallel with the capacitor in the integrator?
3. What is the purpose of the resistor in series with the input capacitor in the
differentiator?
4. Is it possible to design a circuit to perform the differentiation and integration functions
using the non-inverting input? Explain your answer.