Chapter 5: BJT SmallSignal Analysis

1
Chapter 5BJT Small-Signal Analysis
2
Contents

• Common-Emitter fixed-bias configuration
• Voltage divider bias
• CE Emitter bias
• Emitter-follower configuration
• Common-base configuration
• Collector-feedback configuration
• Hybrid equivalent circuit and model

3

BJT Small Signal Analysis

• re transistor model employs a diode and
  controlled current source to duplicate the
  behavior of a transistor in the region of
  interest.
• The re and hybrid models will be used to analyze
  small-signal AC analysis of standard transistor
  network configurations.
• Ex Common-base, common-emitter and
  common-collector configurations.
• The network analyzed represent the majority of
  those appearing in practice today.

4
AC equivalent of a network is obtained by

• Setting all DC sources to zero
• Replacing all capacitors by s/c equiv.
• Redraw the network in more convenient and logical
  form

5
(No Transcript)
6
(No Transcript)
7
(No Transcript)
8
Common-Emitter (CE) Fixed-Bias Configuration
The input (Vi) is applied to the base and the
output (Vo) is from the collector. The
Common-Emitter is characterized as having high
input impedance and low output impedance with a
high voltage and current gain.
9

Common-Emitter (CE) Fixed-Bias Configuration
Removing DC effects of VCC and Capacitors
10
Common-Emitter (CE) Fixed-Bias Configuration
re Model
Determine ?, re, and ro ? and ro look in the
specification sheet for the transistor or test
the transistor using a curve tracer. re
calculate re using dc analysis
11
Common-Emitter (CE) Fixed-Bias Configuration
Impedance Calculations
Input Impedance Output Impedance

12

Common-Emitter (CE) Fixed-Bias Configuration
Gain Calculations
Voltage Gain (Av) Current
Gain (Ai) Current Gain from
Voltage Gain
13
Voltage Gain
Common-Emitter (CE) Fixed-Bias Configuration
14
Current gain
Common-Emitter (CE) Fixed-Bias Configuration
15
Common-Emitter (CE) Fixed-Bias Configuration
Phase Relationship
The phase relationship between input and output
is 180 degrees. The negative sign used in the
voltage gain formulas indicates the inversion.
16
CE Voltage-Divider Bias Configuration
17
CE Voltage-Divider Bias Configuration
re Model
You still need to determine ?, re, and ro.
18
CE Voltage-Divider Bias Configuration
Impedance Calculations
Input Impedance Output Impedance

19

CE Voltage-Divider Bias Configuration
Gain Calculations
Voltage Gain (Av) Current
Gain (Ai) Current Gain
from Voltage Gain
20
Voltage Gain
CE Voltage-Divider Bias Configuration
21
Current gain
CE Voltage-Divider Bias Configuration
22
CE Voltage-Divider Bias Configuration
23
CE Voltage-Divider Bias Configuration
Phase Relationship
A CE amplifier configuration will always have a
phase relationship between input and output is
180 degrees. This is independent of the DC bias.
24

CE Emitter-Bias Configuration
Unbypassed RE
25
CE Emitter-Bias Configuration
re Model
Again you need to determine ?, re.
26

CE Emitter-Bias Configuration
Impedance Calculations
Input Impedance Output Impedance
27
Defining the input impedance of a transistor with
an unbypassed emitter resistor
CE Emitter-Bias Configuration
28
CE Emitter-Bias Configuration
Gain Calculations
Voltage Gain (Av)
Current Gain (Ai) Current
Gain from Voltage Gain
or
29
Voltage Gain
CE Emitter-Bias Configuration
30
Current Gain
CE Emitter-Bias Configuration
31
CE Emitter-Bias Configuration
Phase Relationship
A CE amplifier configuration will always have a
phase relationship between input and output is
180 degrees. This is independent of the DC bias.
32
CE Emitter-Bias Configuration
Bypassed RE This is the same
circuit as the CE fixed-bias configuration and
therefore can be solved using the same re model.
33
Emitter-Follower Configuration
You may recognize this as the Common-Collector
configuration. Indeed they are the same circuit.
Note the input is on the base and the output is
from the emitter.
34
Emitter-Follower Configuration
re Model
You still need to determine ? and re.
35
Emitter-Follower Configuration
Impedance Calculations
Input Impedance
36
Calculation for the current Ie
Emitter-Follower Configuration
37
Emitter-Follower Configuration
Impedance Calculations (contd)
Output Impedance
38
Emitter-Follower Configuration
Gain Calculations
Voltage Gain (Av) Current
Gain (Ai) Current Gain from Voltage
Gain
39
Voltage gain
Emitter-Follower Configuration
40
Current Gain
Emitter-Follower Configuration
41
Emitter-Follower Configuration
Phase Relationship
A CC amplifier or Emitter Follower configuration
has no phase shift between input and output.
42
Common-Base (CB) Configuration
The input (Vi) is applied to the emitter and the
output (Vo) is from the collector. The
Common-Base is characterized as having low input
impedance and high output impedance with a
current gain less than 1 and a very high voltage
gain.
43
Common-Base (CB) Configuration
re Model
You will need to determine ? and re.
44
Common-Base (CB) Configuration
Impedance Calculations
Input Impedance Output Impedance
45
Common-Base (CB) Configuration
Gain Calculations
Voltage Gain (Av) Current Gain
(Ai)
46
Voltage Current gain
Common-Base (CB) Configuration
47
Common-Base (CB) Configuration
Phase Relationship
A CB amplifier configuration has no phase shift
between input and output.
48
Collector DC Feedback Configuration
The network has a dc feedback resistor for
increased stability, yet the capacitor C3
will shift portions of the feedback resistance to
the input and output sections of the network in
the ac domain. The portion of RF shifted to the
input or output side will be determined by the
desired ac input and output resistance levels.
49
Collector DC Feedback Configuration
Impedance Calculations
Substituting the re equivalent circuit into the
ac equivalent network
Input Impedance Output Impedance
50
Voltage Gain
Collector DC Feedback Configuration
51
Current Gain
Collector DC Feedback Configuration
52
Approximate Hybrid Equivalent Circuit
The h-parameters can be derived from the re
model hie ?re hib re hfe ? hfb
-? hoe 1/ro The h-parameters are also found in
the specification sheet for the transistor.
53
Approximate Common-Emitter Equivalent Circuit
Hybrid equivalent model
re equivalent model
54
Approximate Common-Base Equivalent Circuit
re equivalent model
Hybrid equivalent model
55
Troubleshooting
1. Check the DC bias voltages if not correct
check power supply, resistors, transistor. Also
check to ensure that the coupling capacitor
between amplifier stages is OK. 2. Check the AC
voltages if not correct check transistor,
capacitors and the loading effect of the next
stage.
56
Practical Applications
Audio Mixer  Preamplifier Random-Noise
Generator Sound Modulated Light Source