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## Chapter 7: BJT Transistor Modeling

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### ... to the ac domain where the conversion will become as ?=Po(ac)/Pi(dc) ... base BJT transistor. re model. re equivalent cct. 23. 23. isolation part, Zi=re. Zo ... – PowerPoint PPT presentation

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Title: Chapter 7: BJT Transistor Modeling

1
Chapter 7 BJT Transistor Modeling
Faculty of Electrical and Electronic Eng.
2
Topic objectives
• At the end of the course you will be able to
• Understand about the small signal analysis of
circuit network using re model and hybrid
equivalent model
• Understand the relationship between those two
available model for small signal analysis

3
• INTRODUCTIONTRANSISTOR MODELING
• To begin analyze of small-signal AC response of
BJT amplifier the knowledge of modeling the
transistor is important.
• The input signal will determine whether its a
small signal (AC) or large signal (DC) analysis.
• The goal when modeling small-signal behavior is
to make of a transistor that work for
small-signal enough to keep things linear
(i.e. not distort too much) 3
• There are two models commonly used in the small
signal analysis
• a) re model
• b) hybrid equivalent model

4
How does the amplification be done?
• Conservation output power of a system cannot be
large than its input and the efficiency cannot be
greater than 1
• The input dc plays the important role for the
amplification to contribute its level to the ac
domain where the conversion will become as
?Po(ac)/Pi(dc)
• Simply speaking

5
• Re model
• Fails to account the output impedance level of
device and feedback effect from output to input
• Hybrid equivalent model
• Limited to specified operating condition in order
to obtain accurate result

6
DC supply ? 0 potential
• O/p coupling capacitor ? s/c
• Large values
• Block DC and pass AC signal
• I/p coupling capacitor ? s/c
• Large values
• Block DC and pass AC signal
• Bypass
• capacitor ? s/c
• Large values

Voltage-divider configuration under AC analysis
Redraw the voltage-divider configuration after
removing dc supply and insert s/c for the
capacitors
7
Modeling of BJT begin HERE!
8
AC bias analysis 1) Kill all DC sources 2)
Coupling and Bypass capacitors are short cct. The
effect of there capacitors is to set a lower
cut-off frequency for the cct. 3) Inspect the
cct (replace BJTs with its small signal modelre
or hybrid). 4) Solve for voltage and current
transfer function, i/o and o/p impedances.
9
• IMPORTANT PARAMETERS
• Input impedance, Zi
• Output impedance, Zo
• Voltage gain, Av
• Current gain, Ai
• Input Impedance, Zi(few ohms ? M?)
• The input impedance of an amplifier is the value
as a load when connecting a single source to the
I/p of terminal of the amplifier.

10
Two port system-determining input impedance Zi
• The input impedance of transistor can be
approximately determined using dc biasing because
it doesnt simply change when the magnitude of
applied ac signal is change.

11
Demonstrating the impact of Zi
12
Example 6.1 For the system of Fig. Below,
determine the level of input impedance
13
Output Impedance, Zo (few ohms ? 2M?) The output
impedance of an amplifier is determined at the
output terminals looking back into the system
with the applied signal set to zero.
14
Example 6.2 For the system of Fig. below,
determine the level of output impedance
15
Example 6.3 For the system of Fig. below,
determine Zo if V600mV, Rsense10k? and Io10?A
16
Example 6.4 Using the Zo obtained in example
6.3, determine IL for the configuration of Fig
below if RL2.2 k? and Iamplifier6 mA.
17
• Voltage Gain, AV
• DC biasing operate the transistor as an
amplifier. Amplifier is a system that having the
gain behavior.
• The amplifier can amplify current, voltage and
power.
• Its the ratio of circuits output to circuits
input.
• The small-signal AC voltage gain can be
determined by

18
By referring the network below the analysis are
19
Example 6.5 For the BJT amplifier of fig. below,
determine a)Vi b) Ii c) Zi d) Avs
20
• Current Gain, Ai
• This characteristic can be determined by

21
• re TRANSISTOR MODEL
• employs a diode and controlled current source to
duplicate the behavior of a transistor.
• BJT amplifiers are referred to as
current-controlled devices.
• Common-Base Configuration
• ?Common-base BJT transistor
• ?re model
• ?re equivalent cct.

22
Therefore, the input impedance, Zi re that less
than 50O. For the output impedance, it will be as
follows
isolation part, Zire
Zo ? ??
23
The common-base characteristics
24
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25
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26
• Example 6.6 For a common-base configuration in
figure
• below with IE4mA, ?0.98 and AC signal of 2mV is
• applied between the base and emitter terminal
• Determine the Zi b) Calculate Av if RL0.56k?
• c) Find Zo and Ai

27
Solution
28
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29
• Example 6.7 For a common-base configuration in
previous
• example with Ie0.5mA, ?0.98 and AC signal of
10mV is
• applied, determine
• Zi b) Vo if RL1.2k? c) Av d)Ai e) Ib

30
• Common-Emitter Configuration
• ?Common-emitter BJT transistor
• ?re model
• ?re equivalent cct.
• Still remain controlled-current source (conducted
between collector and base terminal)
• Diode conducted between base and emitter terminal

31
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32
The output graph
33
Output impedance Zo
34
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35
• Example 6.8 Given ?120 and IE(dc)3.2mA for a
common-
• emitter configuration with ro ? ?, determine
• Zi b)Av if a load of 2 k? is applied c) Ai with

36
• Example 6.9 Using the npn common-emitter
configuration,
• determine the following if ?80, IE(dc)2 mA and
ro40 k?
• Zi b) Ai if RL 1.2k ? c) Av if RL1.2k ?

37
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38
Hybrid Equivalent Model
• re model is sensitive to the dc level of
operation that result input resistance vary with
the dc operating point
• Hybrid model parameter are defined at an
operating point that may or may not reflect the
actual operating point of the amplifier

39
Hybrid Equivalent Model
The hybrid parameters hie, hre, hfe, hoe are
developed and used to model the transistor. These
parameters can be found in a specification sheet
for a transistor.
40
Determination of parameter
H22 is a conductance!
41
General h-Parameters for any Transistor
Configuration
hi input resistance hr reverse transfer
voltage ratio (Vi/Vo) hf forward transfer
current ratio (Io/Ii) ho output conductance
42
Common emitter hybrid equivalent circuit
43
Common base hybrid equivalent circuit
44
Simplified General h-Parameter Model
The model can be simplified based on these
approximations hr ? 0 therefore hrVo 0 and
ho ? ? (high resistance on the output)
Simplified
45
Common-Emitter re vs. h-Parameter Model
hie ?re hfe ? hoe 1/ro
46
Common-Emitter h-Parameters
Formula 7.28 Formula 7.29
47
Common-Base re vs. h-Parameter Model
hib re hfb -?
48
Common-Base h-Parameters
Formula 7.30 Formula 7.31