EE315_Exp4_BJTssAmp

8/19/2019 EE315_Exp4_BJTssAmp

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İ YTE - Department of Electrical and Electronics Engineering

EE315 - Electronics Laboratory - Experiment 4 1

EE315 - Electronics Laboratory

Experiment 4BJT Small Signal Analysis

Preliminary Work

1.a) Calculate Av, Ai, Rin, and Rout on the circuit given in Figure 1. Assume that CCi

and CCo behave as open circuit for DC signals and as short circuit for AC signals.

1.b) Determine limits of the signal source, vS, for the transistor to remain in forward-

active region.

VCC=15V

R2

27kΩ

vo

R1

110kΩRC

3.6kΩ

Q1: BC237

or BC238

β=200VBE(on)=0.7V

RE

1.2kΩ

Figure 1. Common-emitter amplifier.

vS RL 12kΩ

CCo

4.7μF

CCi

4.7μF

Rin Rout

Q1

RS

100Ω

2.a) Calculate Av, Ai, Rin, and Rout on the circuit given in Figure 2. Assume that the

coupling capacitors, CCi and CCo, behave as open circuit for DC signals and as short

circuit for AC signals.

2.b) Determine limits of the signal source, vS, for the transistor to remain in forward-

active region. (Hint: AC current on RL will be added onto the quiescent emitter

current.)

VCC=15V

vo

RB

220kΩ

Q1: BC237

or BC238

β=200

VBE(on)=0.7V

RE1.2kΩ

Figure 2. Common-collector amplifier

(emitter follower).

vS RL

100Ω

CCo

4.7μF

CCi

4.7μF

Rin Rout

Q1

RS 100Ω


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Procedure

1. Build the circuit given in Figure 1. Use the function generator to obtain 1 Vp-p,

1 KHz as vS. Note the polarity of the capacitors on the circuit diagram.

1.a) Measure the voltage gain, Av. Use the oscilloscope in X-Y mode, probing vS

on Ch-1 (X) and vO on Ch-2 (Y).

VSp-p = VOp-p = Av =

1.b) Determine the current gain, Ai, by measuring the peak-to-peak AC voltages

across RS and RL.

VRSp-p = VRLp-p =

ISp-p = ILp-p = Ai =

1.c) Disconnect RL and measure the input resistance, Rin. Use the oscilloscope in

X-Y mode, probing iS (voltage across RS) on Ch-1 (X) and vS on Ch-2 (Y). Note

that, the two oscilloscope channels must share a single, common node for proper

ground connection.

VSp-p = VRSp-p = ISp-p = Rin =

1.d) Measure peak-to-peak output voltage while RL is disconnected. Connect a

4.7 kΩ variable resistor in place of RL, and adjust the variable resistor to obtain halfthe output voltage measured without any load. Measure the value of variable resistor

that will give the Rout of the amplifier.

VOp-p(NoLoad) = Rout =

1.e) Find the maximum peak vS amplitudes before the transistor goes into cut-off or

saturation. Compare the results with your calculations in the preliminary work.

VSpeak(cut-off) = VSpeak(sat) =

1.f) Connect a 4.7 μF capacitor in parallel to RE. Make sure that the polarity of thecapacitor matches the DC voltage polarity on RE. Measure the voltage gain, Av,

again as described in step 1.a.


1.g) Draw the small signal model of the common-emitter amplifier with the capacitor

in parallel to RE, and calculate the voltage gain, Av, based on your model.


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2. Build the emitter-follower circuit given in Figure 2. Verify that the function

generator output is still set to 1 Vp-p, 1 KHz, and repeat the steps 1.a, 1.b, and 1.c.

2.a) Measure the voltage gain, Av.


2.b) Determine the current gain, Ai.

VRSp-p = VRLp-p =

ISp-p = ILp-p = Ai =

2.c) Disconnect RL and measure the input resistance, Rin.

VSp-p = VRSp-p = ISp-p = Rin =

2.d) Increase vS amplitude to 2.0 Vp-p and observe vO on the oscilloscope. How do

you explain the distortion in the vO waveform?

2.e) Find a way to restore the vO waveform while keeping vS amplitude at 2.0 Vp-p,

and RL = 100 Ω. Explain how you can restore the vO waveform.

2.f) Set the function generator output back to 1 Vp-p, and measure the output

resistance, Rout. You should use a smaller variable resistor such as 100Ω

to obtaina better precision with emitter-follower. Make sure that the output waveform is not

distorted while you measure Rout.

VOp-p(NoLoad) = Rout =

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