Basic Electronic Devices and Circuits
EE 111
Electrical Engineering
Majmaah University
2nd Semester 1432/1433 H
Chapter 6
BJT Amplifiers
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
1
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.
VCC
RC
R1
C3
Vout
C1
Vin
RL
R2
RE
Electronic Devices, 9th edition
Thomas L. Floyd
C2
ac short; ZC = 1/( j ω C )
[bypass capacitor]
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
2
AC + DC (in amplifier)
13
11
9
7
5
3
1
-1
-3
-5
=
Vout AC (Vp=5V)
13
11
9
7
5
3
1
-1
-3
-5
Electronic Devices, 9th edition
Thomas L. Floyd
DC = 8 V (Q-point)
+
13
11
9
7
5
3
1
-1
-3
-5
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
3
The Common-Emitter Amplifier
8.42
2
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
4
DC Analysis
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
5
The Common-Emitter Amplifier
8.42
2
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
6
AC Analysis
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
7
Signal (AC) Voltage at the Base
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
8
Input Resistance at the Base
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
9
Output Resistance
The output resistance is the resistance seen
looking back into the output terminal with Vin=0.
(re' is much smaller than rc' )
(rc' is much larger than RC )
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
10
Why find Input Resistance?
Supply
Amplifier
Voltage division.
We prefer high Rin(tot)
Electronic Devices, 9th edition
Thomas L. Floyd
Why find Output Resistance?
Amplifier
Load
Voltage division.
We prefer low Rout
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
11
The Common-Emitter Amplifier
This figure is mentioned in the next slide.
8.42
2
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
12
(actually 3.58 mA on slide 18, but use 3.8 mA)
< 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)).
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
13
The Common-Emitter Amplifier
8.42
2
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
14
Voltage Gain
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.
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
15
Attenuation
Attenuation is the reduction in signal voltage as it passes through a circuit
and corresponds to a gain of less than 1.
gain = 1 / attenuation
gain = output / input
attenuation = 1 / gain = input / output
Example: If the signal amplitude is reduced by half,
gain = 0.5
or 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 = 0.2
attenuation = input / output = 10 / 2 = 5
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
16
Overall Voltage Gain
Voltage gain from base to collector = Av = Vc / Vb
Attenuation from source (supply) to base = Vs / Vb
(reciprocal of voltage division)
The overall voltage gain
of the amplifier, is the
voltage gain from base to
collector, Vc / Vb, times
the reciprocal of the
attenuation, Vb / Vs.
≥1
Electronic Devices, 9th edition
Thomas L. Floyd
<1
≥1
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
17