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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

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.

• 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.

CE AMP WITH A UNBYPASSED RE

• This causes two AC voltage drops: one

is between the base and emitter, and

the other is across RE.

• As the base voltage swings positive,

the base current increases.

• This causes the collector-to-emitter

current to increase.

• The emitter current increases as the

base swings positive, and the voltage

across RE 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.

REQUIRED PARTS

2 1200W, ½ W resistors (brown-red-red)

1 6800W, ½ W resistor (blue-grey-red)

2 10kW, ½ W resistors (brown-black-orange)

3 10mF electrolytic capacitors

1 MPSA-20 (NPN) silicon transistor

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.

COMMON EMITTER AMP FOR EXP. 9

PICTORIAL OF THE CE AMP FOR EXP. 9

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 R1 until the

output voltage is 1.5 V rms. (AC)

5. Measure the input voltage Vi going into

the amplifier.

a) Record your voltage measurement.

6. Use the formula AV = VO/Vi, and

calculate the voltage gain.

a) Record your gain calculation.

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 R7 = 0 W.)

EXP. 9 MODIFICATION CKT. FOR STEP 7

EXP. 9 MODIFICATION CKT. FOR STEP 7

b) Adjust the 1 kW potentiometer R1 so that

the output is 1.5 V rms.

c) Adjust the 100 kW potentiometer R7 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 R7.

1. Record this input impedance.

8. Use the following schematic diagram /

Exp. 9, 2nd Modification Circuit for Step

8.

a) Disconnect (lift) the ground wire on the

100 kW potentiometer R7 at pin 2.

b) Adjust the 1 kW potentiometer R1 so

that the output voltage is 1.5 V.

EXP. 9 MODIFICATION CKT. FOR STEP 8

PICTORIAL FOR EXP. 9 FOR STEP 8

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 R7.

1. Record this output impedance

measurement.

9. Use the formula IO = VO/RL, and the

values of VO (1.5 V) and RL (10 kW) to

calculate the output current.

a) Record your current calculation

b) Use the formula Ii = Vi /Zi, 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

c) Use the formula AI = IO/Ii, 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

CIE RESULTS

5. 0.50 V

6. 3.0…AV = 1.5 V/0.50V = 3.0

7. 1300W

8. 5000W

9. 0.15 mA; 0.35 mA and 0.39

IO = 1.5 V/104 W = 1.5 x 10-4 A

Ii = 0.50 V / 1300W = 0.38 mA

AI = IO/Ii = 0.15 x 10-3 A/0.38 x 10-3 A

AI = 0.39

10. 1.17… AP = AVAI = 3.0 x 0.39 = 1.17

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).

•(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)

•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.

• 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.

•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, RE1 and RE2, as shown in the

following schematic diagram.

EMITTER RESISTANCE AS RE1 AND RE2

•The DC bias current sees RE1 and RE2 in

series which results in good temperature

stability, since RE is seen by DC bias as a

large value.

•The AC signal current only sees RE1. (RE2 is

bypassed by a CE) Thus the AC current

doesn’t see R2.

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.

QUESTIONS?

RESOURCES

Casebeer, J.L., Cunningham, J.E. (2001).

Lesson 1430: Transistors, Part 1.

Cleveland: Cleveland Institute of

Electronics.

THE END

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Instructors in the CIE Instruction

Department.

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All Rights Reserved / Nov. 2012