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2 OBJECTIVES 1. To measure the voltage, current, and power gains of the CE amplifier 2. To measure the input and output impedance of the CE amplifier CE = Common Emitter

3 INTRODUCTION In the previous experiment, we bypassed the emitter resistor with a capacitor to avoid degenerative feedback. The resulting voltage, current and power gains were high.

4 In this experiment, we will be leaving the emitter resistor unbypassed. We want to observe how that affects the performance of the amplifier The following slide shows a basic CE amplifier with an unbiased emitter resistor. The input signal is applied between the base and ground.

5 CE AMP WITH A UNBYPASSED RE

6 This causes two AC voltage drops: one is between the base and emitter, and the other is across R E. As the base voltage swings positive, the base current increases. This causes the collector-to-emitter current to increase.

7 The emitter current increases as the base swings positive, and the voltage across R E rises. Thus the AC signal on the base is in phase with the AC signal on the emitter. This reduces the AC signal between the base and the emitter, thus reducing the gain of the amplifier.

8 REQUIRED PARTS W, ½ W resistors (brown-red-red) W, ½ W resistor (blue-grey-red) 2 10kW, ½ W resistors (brown-black-orange) 3 10mF electrolytic capacitors 1 MPSA-20 (NPN) silicon transistor

9 PROCEDURE 1. Construct the circuit on the following slide. 2. Turn the trainer on and adjust the positive power supply to 15V. a) Adjust the FREQUENCY control so the output is 1 khz.

10 COMMON EMITTER AMP FOR EXP. 9

11 PICTORIAL OF THE CE AMP FOR EXP. 9

12 3. Measure the DC voltage on the emitter, base and collector, and check them against the values shown Schematic Diagram for Experiment 9 on the previous slide. 4. Adjust potentiometer R 1 until the output voltage is 1.5 V rms. (AC)

13 5. Measure the input voltage V i going into the amplifier. a) Record your voltage measurement. 6. Use the formula A V = V O /V i, and calculate the voltage gain. a) Record your gain calculation.

14 7. Use the input circuit/exp. 9, modification circuit for Step 7, and complete this step. The modification circuit is on the following slide. a) Turn the knob on the 100 KW pot fully counterclockwise (thus making R 7 = 0 W.)

15 EXP. 9 MODIFICATION CKT. FOR STEP 7

16 EXP. 9 MODIFICATION CKT. FOR STEP 7

17 b) Adjust the 1 kw potentiometer R 1 so that the output is 1.5 V rms. c) Adjust the 100 kw potentiometer R 7 so that the output is 0.75 V rms. d) Turn the trainer off and measure the resistance between terminals 1 and 2 of the 100 kw pot R Record this input impedance.

18 8. Use the following schematic diagram / Exp. 9, 2 nd Modification Circuit for Step 8. a) Disconnect (lift) the ground wire on the 100 kw potentiometer R 7 at pin 2. b) Adjust the 1 kw potentiometer R1 so that the output voltage is 1.5 V.

19 EXP. 9 MODIFICATION CKT. FOR STEP 8

20 PICTORIAL FOR EXP. 9 FOR STEP 8

21 c) Reconnect the ground wire on the 100 kw potentiometer, and adjust it so the output is 0.75 V rms. d) Turn the trainer off and measure the resistance between terminals 1 and 2 of the 100 kw pot R Record this output impedance measurement.

22 9. Use the formula I O = V O /R L, and the values of V O (1.5 V) and R L (10 kw) to calculate the output current. a) Record your current calculation b) Use the formula I i = V i /Z i, and the values of Vi (step 5) and Zi (step 7) that you recorded earlier, calculate the input current of the amplifier. 1. Record your current calculation

23 c) Use the formula A I = I O /I i, to calculate the current gain of the amplifier. 1. Record your current gain calculation 10. Use the formula AP = AIAV, using AV (step 6) and AI (step 9) and calculate the power gain of the amplifier. a) Record your power gain calculation

24 CIE RESULTS V A V = 1.5 V/0.50V = W W ma; 0.35 ma and 0.39 I O = 1.5 V/10 4 W = 1.5 x 10-4 A

25 I i = 0.50 V / 1300W = 0.38 ma A I = I O /I i = 0.15 x 10-3 A/0.38 x 10-3 A A I = A P = A V A I = 3.0 x 0.39 = 1.17

26 FINAL DISCUSSION It was not necessary to use a voltage divider at the input, since the gain of the amplifier was so low. We measured the input directly (step 5). We measured the input and output impedances (steps 7 and 8).

27 (Note that these were fairly close to the input and output impedances of the common emitter amplifier using the bypass capacitor in the last experiment) We calculated the current gain in step 9 and the power gain in step 10. (Note that the voltage, current ad power gains of the amplifier were much less than the amplifier with the bypass capacitor)

28 The difference between the two amplifiers is that the one with emitter feedback has much less distortion. We cannot measure distortion without special test equipment, but the improvement in performance is often worth adding a second stage of amplification to get the required gain.

29 The value of emitter resistance chosen for a particular amplifier depends on a compromise between opposite requirements. Good temperature stability requires a high value of emitter resistance, while good AC gain requires a low value of emitter resistance.

30 However, to reduce distortion, the value of the emitter resistance must be increased. In practice, a compromise is often made between these conflicting requirements. The emitter resistance is often broken up into two parts, R E1 and R E2, as shown in the following schematic diagram.

31 EMITTER RESISTANCE AS RE1 AND RE2

32 The DC bias current sees R E1 and R E2 in series which results in good temperature stability, since R E is seen by DC bias as a large value. The AC signal current only sees R E1. (R E2 is bypassed by a C E ) Thus the AC current doesn t see R 2.

33 The value of RE1 is chosen t provide the required compromise between gain and distortion; while the value of CE is chosen to provide a low reactance at the lowest signal frequency to be encountered.

34 QUESTIONS?

35 RESOURCES Casebeer, J.L., Cunningham, J.E. (2001). Lesson 1430: Transistors, Part 1. Cleveland: Cleveland Institute of Electronics.

36 THE END Developed and Produced by the Instructors in the CIE Instruction Department. Copyright 11/2012 All Rights Reserved / Nov. 2012