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Lab 5
Differential Amplifiers
Objective: 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.
Components: 2N2222 BJT, 2N7000 FET
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 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 17-1 illustrates a BJT based differential amplifier and Figure 17-2 shows an FET based stage. The
differential output versions (Figures 17-1 (A) and 17-2 (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 17-1 (B) and 17-2 (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.
Figure 17 - 1: BJT Differential Amplifier
Figure 17 - 2: FET Differential Amplifier
Design:
1. Design a single ended BJT differential amplifier capable of providing a ± 10 V output swing across a 1
k
mirror. Indicate the value of Vout when both inputs are grounded. Verify your design PSPICE®.
2. Repeat step 1 using a single resistor to provide the switching current.
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 .
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.
Questions:
1. Could any, or all, of these circuits be designed using a single DC power supply? Explain your answer.
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.