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Differential Amplifier with RE and Current Mirror Biasing Lab

Objectives:

1. Complete the design of two differential amplifiers, one of which uses emitter resistor (RE) biasing, and one of which uses current mirror biasing.

2. Predict, measure and record DC voltages and currents in differential amplifiers which employ two basic types of constant current biasing.

3. Predict, measure and record AC voltage gain, common mode rejection ratio, and input and output impedance characteristics of a differential amplifier.

4. Observe the impact on common mode voltage gain, Acm, and common mode rejection ratio, CMRR, as a current mirror is substituted for the emitter resistor (RE) of a simple differential amplifier.

Equipment:
1. Power Supply
2. Function Generator
3. Oscilloscope
4. Computer and PSpice Software
5. CA 3046 or LM 3046 Transistor Array
6. Matched 2.2 kΩ Resistor Pair
7. Assorted Resistors

Procedure:

1. From the specification sheet for the LM3046, determine typical values for the following terms at a quiescent collector current of 1mA and Vceq = 3V.

rπ = hie

β = hfe

ro = 1 / hoe

VBE

2. From the above typical values, determine values for PSpice parameters BF, VAF, NF, and IS. (Use default values for remaining parameters.)

3. Complete the design of the amplifier in Figure 1 by calculating the value of RE required to set the collector currents of Q1 and Q2 to 1mA. Assume matched transistors and neglect the DC voltage drop across the 10Ω resistor at the base of Q1.

4. Predict the DC node voltages, collector (given) and emitter currents, and IQ.

5. Calculate the single and double ended differential voltage gains (As and Ad), common mode gain (ACM), common mode rejection ratio (CMRR), single ended input resistance (Ri1), common mode input resistance (Ricm) and differential (Rod) and single ended (Ro2) output resistances for the amplifier.

6. Use PSpice to verify circuit parameter calculations.

7. Construct the circuit of Figure 1, leaving Vg (function generator) initially disconnected from the amplifier.

8. Measure and record the actual DC voltages and currents that were predicted for Step 4. Compare measurements to predictions and PSpice results.

9. Connect Vg to the circuit. Adjust the amplitude of Vg to some convenient value, noting that the signal at pin 2 will be 1/1000 of the function generator amplitude. Set the frequency of the function generator to 1kHz.




10. Measure and record the actual AC circuit parameters that were predicted for Step 5. Compare measurements to predictions and Pspice results.

a. Measure single ended output between the collector of Q2 and ground.

b. Use ADD, INVERT mode on the oscilloscope to measure the differential double ended (balanced) output between the collectors of Q1 and Q2

c. Use a potentiometer to determine input and output resistances. Be careful to not disturb the DC quiescent point. You may need to use the DC offset function of your function generator when determining input resistances, and you may want to use a coupling capacitor when measuring output resistances.


Figure 1: Differential Amplifier Employing Emitter Resistor Biasing

11. Complete the design of the amplifier in Figure 2 by calculating the value of R3 required to set the collector currents of Q1 and Q2 1mA. Consider the early factors in determining R3 for the required IQ.

Question: Are the values of R3 and RE (Figure 1) the same? Why / why not?

12. Repeat Steps 4 through 10 for the circuit of Figure 2, with the following exception. Predict and measure the current through R3 rather than RE.

13. Compare the AC circuit parameters (Acm , CMMR , Ricm , etc.) of the two amplifiers. Discuss differences in the conclusions section of your laboratory report.


Figure 2: Differential Amplifier Employing Simple Current Mirror Biasing


Lab Notes

Lab Data


Electrical Engineering lab key words: differential amplifier, current mirror, DC biasing, DC circuit analysis, AC circuits, collector, base, emitter, differential stage, function generator, computer simulation, Pspice, predictions, frequency, balance resistor, quiescent current, voltage gain.

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