Slide điện tử tương tự chapter 2 bjt

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CHAPTER 2:
BIPOLAR JUNCION TRANSISTOR
DR. PHAM NGUYEN THANH LOAN

Hanoi, 9/24/2012
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Contents

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Structure and operation of BJT
Different configurations of BJT
Characteristic curves
DC biasing method and analysis

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AC signal analysis
Impact of other parameters (temperature, leakage
currents)

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Base bias
Collector-feedback bias
Voltage divider bias

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The content of these slides are based on the book titled “Electronics Devices and Circuit theory of
Robert Boylestad”
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Structure and operation of BJT
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BJT structure

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BJT :Bipolar Junction Transistor
2 kinds of BJT: NPN & PNP
3 terminals: E, B và C
E: Emitter; B: Base, C: Collector
Base located in the middle:
thinner than E & C; and lower
dope

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Structure and operation of BJT
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 Bias condition for 2 junctions: JBE & JBC

Junction BE in forward bias:
electrons (e) move from E region
to B region to create the current IE
(diffusion current; flow of
majority carriers)
Junction BC in reverse bias: e
that moved from E to B then
move from B to C to create the
current IC (drift current, flow of
minority carriers)
The combination of some

electrons with holes in B region
creates the current IB
So: IE = IC + IB

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Structure and operation of BJT
BJT symbol

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IE
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IB

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IC

3 terminals: B, E và C
Arrow instructs the current
direction between B & E
Conventional current is the
flow of positive charges
(holes)
NPN: B  E
PNP: E  B

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Technical parameters
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IE = IC + IB

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IC = βIB

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β = 100 ÷ 200 (may be higher)

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β is DC current gain

IC = αIE + ICBO

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IC ≈ αIE (neglect leakage ICBO)

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α = 0.9 ÷0.998.

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α is DC current transfer coefficient

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Technical parameters
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IE = IC + IB

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IC = β*IB

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β = 100 ÷ 200 (may be higher)

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β is DC current gain

IC = αIE + ICBO

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IC ≈ αIE (neglect leakage ICBO)

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α = 0.9 ÷0.998.

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α is DC current transfer coefficient

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BJT as an amplifier
Different amplifier configurations
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Look at the input and output to distinguish these
configurations

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Configuration
BC
EC
CC

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Common emitter (CB)
Common base (CB)
Common collector (CC)

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3 configurations

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Input
E
B
B

Output
C
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CE configuration
E is used in common for
in and out

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re=26mV/IE
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Output: Ic= βIb

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Input: re is considered as
AC resistor of diode BE

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CE configuration – small signal
Zi = Ube/Ib ≈ βIbre/Ib ≈ βre
(~ n100Ω – nKΩ)
 Z o = ro  ∞
(ignore in re model)
 Av = - RL/re (ro ∞)
 Ai = Ic/Ib = β
 Characteristics
+ Zi, Zo average
+ Av, Ai high

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Characteristic curves: CE

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Input and output characteristic curves of CE configuration

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Characteristic curves: CE
0starts moving to forward
bias IC increases gradually

VCE >0.7V: Junction BE is in
FB and Junction BC in reverse
 IC = β*IB

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CB configuration
B is used in common for
in and out

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Input: re is considered as
AC resistor of diode BE
re=26mV/IE

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Isolation between in and
out

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Output: Ic=αIe

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CB configuration

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(nΩ-50 Ω)

Z i = re

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Zo = ro ≈ ∞ (nMΩ)

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Av = αRL/re ≈ RL/re quite big, Uo & Ui in phase

4)

Ai = -α ≈ 1

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Characteristic curves: CB

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Input and output characteristic curves of CB configuration

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CC configuration
Similar to CE configuration

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Refer to Electronic Devices – Thomas Floyd

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Limits of operation
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Two limits:
 cut-off

region
 Saturation region

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Cutoff and saturation
Cutoff state

Saturation state

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DC bias:
DC operating point & DC load line

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DC bias
A transistor must be properly biased in order to operate as

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DC bias can be considered as supply power to BJT so that
 NPN: VE < VB < VC (JE: in Forward; JC: in Reverse bias)
 PNP: VE > VB > VC
DC bias is characterized by Q-point (DC operating point)
and DC load line

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an amplifier

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DC bias
NOTES: REMEMBER some equations:
VBE ≈ 0,6 ÷ 0,7V (Si) ; 0,2 ÷ 0,3(Ge)
IE = IC + IB IC = βIB
IC ≈ αIE
There 3 types of bias circuits

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Base bias
 Collector-feedback bias
 Voltage divider bias

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Question: How many amplifier circuits can be
designed?

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3 types of baising
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Base bias

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Voltage divider bias

Collector feedback bias
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Example of DC bias

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Q1. What are the amplifier configuration of these circuits?

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Q2. What kind of DC bias? And then draw DC equivalent circuit.

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Question 3: How many amplifier circuits can be designed?
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Base bias

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Consider the analysis for only EC configuration (similar
analysis can be obtained for BC and CC)

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Base bias

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BE loop:
Vcc – IBRB – UBE = 0
 IB= (Vcc - UBE)/RB
IC=β*IB
CE loop:
 UCE = Vcc - ICRC

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