Download ECE 322L Lab 7: Differential Amplifiers

ECE 322L
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Lab 7: Differential Amplifiers
Lab 7
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 differences 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 noninverting (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 noninverting 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.
ECE 322L
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Lab 7: Differential Amplifiers
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
supplied by a BJT current mirror. Indicate the value of V out when both inputs are grounded.
Verify your design PSPICE®.
ECE 322L
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Lab 7: 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
supplied by an FET current mirror. Indicate the value of V out 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.
ECE 322L
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Lab 7: 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.