Chapter 6 BJT Amplifiers - University of Misan · Chapter 6 Basic Electronic Devices and Circuits EE 111 ... BJT Amplifiers. The CommonEmitter Amplifier In the commonemitter (CE)

Chapter 6

Basic Electronic Devices and Circuits

EE 111

Electrical Engineering

Majmaah University

2nd Semester 1432/1433 H

© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.

Electronic Devices, 9th edition

Thomas L. Floyd1

BJT Amplifiers

The Common-Emitter Amplifier

In the common-emitter (CE) amplifier, the input signal is applied to the base and the inverted output is taken from the collector.

The emitter is common to ac input & output signals.



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R2

RE

R1

Vin

RC

VCC

Vout

RL

C1

C2

C3

ac short; ZC = 1/( j ω C )[bypass capacitor]

2

=-5

-3

-1

1

3

5

7

9

11

13

AC + DC (in amplifier)



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+-5

-3

-1

1

3

5

7

9

11

13

Vout AC (Vp=5V)

-5

-3

-1

1

3

5

7

9

11

13

DC = 8 V (Q-point)

3


8.42



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2

4

DC Analysis



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8.42



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2

6

AC Analysis



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Signal (AC) Voltage at the Base



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Input Resistance at the Base



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Output ResistanceThe output resistance is the resistance seen looking back into the output terminal with Vin=0.



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(re' is much smaller than rc' )

(rc' is much larger than RC )

10

AmplifierSupply

AmplifierLoad

Why find Output Resistance?Why find Input Resistance?



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Voltage division.We prefer high Rin(tot)

Voltage division.We prefer low Rout

11


8.42This figure is mentioned in the next slide.



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2

12

(actually 3.58 mA on slide 18, but use 3.8 mA)



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< 10 mV !

There is significant attenuation (reduction) of the source (supply) voltage due to the voltage division between the source resistance (Rs) and the amplifier’s input resistance (Rin(tot)).

13


8.42



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2

14

Voltage Gain



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This is the voltage gain from base to collector.To get the overall gain of the amplifier from the supply voltage to collector, the attenuation of the input circuit must be included.

15

Attenuation is the reduction in signal voltage as it passes through a circuit and corresponds to a gain of less than 1.

gain = 1 / attenuationgain = output / input

attenuation = 1 / gain = input / output

Example: If the signal amplitude is reduced by half,

Attenuation



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Example: If the signal amplitude is reduced by half,gain = 0.5or attenuation = 1 / 0.5 = 2

Example:

A source (supply) produces a 10 mV input signal, and the source resistance combined with the load resistance results in a 2 mV output signal.

gain = output / input = 2 / 10 = 0.2attenuation = input / output = 10 / 2 = 5

16

Voltage gain from base to collector = Av = Vc / Vb

Attenuation from source (supply) to base = Vs / Vb

Overall Voltage Gain

(reciprocal of voltage division)

The overall voltage gain of the amplifier, is the



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of the amplifier, is the voltage gain from base to collector, Vc / Vb, times the reciprocal of the attenuation, Vb / Vs.

≥ 1 < 1 ≥ 1

17

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Chapter 6 BJT Amplifiers - University of Misan · Chapter 6 Basic Electronic Devices and Circuits EE 111 ... BJT Amplifiers. The CommonEmitter Amplifier In the commonemitter (CE)