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
Introduction
• The things you learned about biasing a transistor in Chapter 5 are now
applied in this chapter where BJT circuits are used as small-signal
amplifiers.
• The term small-signal refers to the use of signals that take up a
relatively small percentage of an amplifier’s operational range.
• Additionally, you will learn how to reduce an amplifier to an equivalent
dc and ac circuit for easier analysis.
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
2
rms: 1/√2 ≈ 0.707
avg: 2/π ≈ 0.6366
AC Quantities
AC quantities are indicated
with a lowercase italic
subscript;
rms values are assumed
unless otherwise stated.
The figure shows an example of
a specific waveform for the
collector-emitter voltage. Notice
the DC component is VCE and the
ac component is Vce.
V
rms
avg
Vce
Vce
VCE
Vce
Vce
vce
0
t
0
Resistance is also identified with a lowercase subscript when analyzed
from an ac standpoint.
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
3
Linear Amplifier
A linear amplifier produces a replica of the input signal at
the output.
+V
CC
Ic
Vb
VBQ
ICQ
RC
R1
Vce
Rs
Vs
Ib
C1
I BQ
R2
C2
VCEQ
RE
RL
For the amplifier shown, notice that the voltage waveform is
inverted between the input and output but has the same shape.
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
4
IB
Q
VCC/(RC||RL +RE)
Ib
AC Load Line
IC
Ic
ICQ
Operation of the linear
amplifier can be illustrated
using an ac load line.
The ac load line is different
than the dc load line because
a capacitor looks open to dc
but effectively acts as a
short to ac.
Q
0
V CE
Vce
VCEQ
ac ground
ac short; ZC = 1/( j ω C )
Thus the collector resistor
RC appears to be in parallel
with the load resistor RL.
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
5
Solution:
Projections on the graph of
Figure 6–4 show the collector
current Ic varying from 6 mA
to 4 mA for a peak-to-peak
value of 6 – 4 = 2 mA
and the collector-to-emitter
voltage Vce varying from 1 V
to 2 V for a peak-to-peak
value of 2 – 1 = 1 V.
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
6
Transistor AC Model
very large
very small
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
7
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
8
Transistor AC Model
The five resistance parameters (r-parameters) can be used for
detailed analysis of a BJT circuit.
For most analysis work, the simplified r-parameters give good
results.
The simplified r-parameters are
shown in relation to the transistor
model.
An important r-parameter is re'. It
appears as a small ac resistance
between the base and emitter.
25 mV
r =
IE
'
e
assuming an abrupt junction between the n
and p regions, and an ambient temperature of
20°C
Electronic Devices, 9th edition
Thomas L. Floyd
C
C
βac Ib
B
βac Ib
B
re′
E
re′
Ib
E
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
9
Quiz
Q. The equation for finding the ac emitter resistance of a
BJT is
Electronic Devices, 9th edition
Thomas L. Floyd
a.
re' =
25 mV
IB
b.
re' =
25 mV
IE
c.
0.7 V
r =
IB
d.
re' =
'
e
0.7 V
IE
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
10
IE = 25 mV / 8 Ω = 3.125 mA
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
11
Comparison of βac to βDC
Curve of IC versus IB is
nonlinear!
slope
Electronic Devices, 9th edition
Thomas L. Floyd
slope
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
12
h (hybrid) parameters
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
13
Relationships of h Parameters and r Parameters
forward current gain, common-base
forward current gain, common-emitter
voltage feedback ratio, common-emitter
output admittance (conductance), common-emitter
input impedance (resistance), common-emitter
Electronic Devices, 9th edition
Thomas L. Floyd
© 2012 Pearson Education. Upper Saddle River, NJ, 07458.
All rights reserved.
14