狠狠撸

Chapter 1:
Bipolar Junction Transistor (BJT)
Assoc. Prof. Pham Nguyen Thanh Loan
September, 2024

Contents
? Structure and operation of BJT
? Different configurations of BJT
? Characteristic curves
? DC biasing method and analysis
? Base bias
? Collector-feedback bias
? Voltage divider bias
? AC signal analysis
? Impact of other parameters (temperature, leakage
currents)
2
The content of these slides are based on the book titled “Electronics Devices and Circuit theory of
Robert Boylestad”

Structure and operation of BJT
?BJT structure
? 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
3

? So: IE = IC + IB
? 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
4

? 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
?BJT symbol
IB
IC
IE
5
?Explain the symbol of BJT?

6
Technical parameters
? IC = αIE + ICBO
? IC ≈ αIE (neglect leakage ICBO)
? α = 0.9 ÷0.998.
? α is DC current transfer coefficient
? IE = IC + IB
? IC = βIB
? β = 100 ÷ 200 (may be higher)
? β is DC current gain
α =
β
β + 1

7
BJT as an amplifier
Different amplifier configurations
? 3 configurations
? Common emitter (CE)
? Common base (CB)
? Common collector (CC)
? Look at the input and output to distinguish these
configurations
Configuration Input Output
BC E C
EC B C
CC B E

BJT as an amplifier
? CE with voltage
divider biasing
8
? CC with voltage
divider biasing
? CB with fixed
biasing
B
E
C
E
B
C

9
CE configuration
? E is used in common for
in and out
? Input: re is considered as
AC resistor of diode BE
re=26mV/IE
? Output: Ic= βIb
? Definition of internal emitter re:

10
CE configuration – small signal
? Zi = Ube/Ib ≈ βIbre/Ib ≈ βre
(~ n100Ω – nKΩ)
? Zo = ro → ∞
(ignore in re model)
? Av = - RL/re (ro→ ∞)
? Ai = Ic/Ib = β
? Characteristics
+ Zi, Zo average
+ Av, Ai high

11
Characteristic curves: CE
? Input and output characteristic curves of CE configuration
11
?Explain why VCE increases, IB decreases for a constant VBE?

12
Characteristic curves: CE
? 0<VCE<0.7V: Junction BE
starts moving to forward
bias→ IC increases gradually
? VCE >0.7V: Junction BE is in
FB and Junction BC in reverse
→ IC = β*IB

13
CB configuration
? B is used in common for
in and out
? Input: re is considered as
AC resistor of diode BE
re=26mV/IE
? Isolation between in and
out
? Output: Ic=αIe

14
1) Zi = re (nΩ-50 Ω)
2) Zo = ro ≈ ∞ (nMΩ)
3) Av = αRL/re ≈ RL/re quite big, Uo & Ui in phase
4) Ai = -α ≈ 1
CB configuration

15
Characteristic curves: CB
? Input and output characteristic curves of CB configuration

16
CC configuration
? Similar to CE configuration
? Refer to Electronic Devices – Thomas Floyd

Limits of operation
? Two limits:
? cut-off region
? Saturation region
17

Cutoff and saturation
? Cutoff state ? Saturation state
18

19
? Q_point deplacement when Rc, Vcc, IB vary respectively
DC loadline & Q point
Variation of RC Variation of VCC Variation of IB

DC power vs. AC signal
20
DCpower AC signal

Example of CE configuration
21
? Output and input signal is out of phase
? Output signal is amplified

DC bias:
DC operating point & DC load line
22

DC bias
? A transistor must be properly biased in order to operate as
an amplifier
? 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
23

DC bias
? NOTES: REMEMBER some equations:
VBE ≈ 0,6 ÷ 0,7V (Si) ; 0,2 ÷ 0,3(Ge)
IC ≈ αIE
IE = IC + IBIC = βIB
? There 3 types of bias circuits
? Base bias
? Collector-feedback bias
? Voltage divider bias
? Question: How many amplifier circuits can be
designed?
24

3 types of baising
Base bias
25
Collector feedback bias
Voltage divider bias

Example of DC bias
26
? Q1. What are the amplifier configuration of these circuits?
? Q2. What kind of DC bias? And then draw DC equivalent circuit.
(a) (b) (c)
? Question 3: How many amplifier circuits can be designed?

? Consider the analysis for only EC configuration (similar
analysis can be obtained for BC and CC)
27
Base bias

Base bias
BE loop:
Vcc – IBRB – UBE = 0
? IB= (Vcc - UBE)/RB
IC=β*IB
CE loop:
? UCE = Vcc - ICRC
28

Voltage divider bias
Current and voltage do not
depend on β
29
Method 1: Thevenin equivalent circuit:
* Group R1, R2 and Vcc can be considered as
follows:
RBB=R1//R2
VBB= Vcc * R2/(R1+R2)
? Now it is similar to base-bias
analysis
Method 2: Approximative analysis
If β*R ≥ 10R -> I ≈ I
2 2 2 1
? VB=Vcc*R2/(R1+R2)
? VE=VB-VBE ? Ic ≈ IE=VE/RE
? VCE=Vcc- IC(RC+RE)

Collector-feedback bias
BE loop:
‘
(1) Vcc- Ic RC – IBRB – UBE – IERE =0
(2) IC= β *IB ; IE ? IC
c c
(3) KCL at C: IC= IB + I ‘ → I ‘ = IC - IB
= (β-1)IB
(1)+(2)+(3)
→ IB= (Vcc - UBE)/[RB+ β(Rc+Re)]
CE loop:
UCE = Vcc – IC (RC+RE)
Quite stable
30

Example 1
31
?Analyze the following circuit and then determine its Q-
point and DC loadline?

Example 2
33
?Analyze the following circuit and then determine its Q-point
and DC loadline?

34
Example 2 (cont’d)
Analysis by method 1

Homework
35
and DC loadline?

Homework
36
and DC loadline?

Homework
37
and DC loadline?

Working on your
1st in-class assignment
38

AC analysis
(Small signal analysis)
39

Small signal analysis
? Small signal analysis:
? Small signal refers to AC signal with small amplitude that take up
a relatively small percentage of an amplifier’s operation range
(compared to DC power supply)
? The operation region on amplifier should be in linear
? BJT model for small signal analysis
? Represent the BJT by an equivalent circuit that allows to visualize
and analyze the operation of BJT as an amplifier
40

Example of CE configuration
41
? Output and input signal is out of phase
? Output signal is amplified

AC equivalent circuit
? 1. Setting all DC sources to zero
? 2. Replacing all capacitors by a
short-circuit equivalent (wire)
? 3. Regrouping all elements
(resistors) in parallel (introduced
by step 1 and 2)
? 4. Redrawing the network in a
more convenient and logical form
43

AC analysis
? BJT amplifier is considered linear → be able to be
analyzed DC and AC separately (using
superposition theorem)
? Different approaches
? Using graphical determination method
? Using equivalent circuits
? T model
? rE model
? Hybrid equivalent model (quite popular in the past)
44

? Q-point and DC load-line
? Quiescent point (Q-point) is fixed on the output characteristic curve
and corresponding to a fixed collector-to-emitter voltage (VCE)
? DC load-line is used to describe the DC operation of BJT, a straight
line from saturation point (IC=ICmax, y-axe) to cutoff point
(VCE=VCEmax, x-axe)
? Q-point : intersect between DC load line and
characteristic curve
? DC load line vs. AC load line
? DC load line: VCE = VCC – ICRC
? AC load line: VCE = VCC - Ic(RC//RL)
45
AC analysis methods
Graphical Analysis

46
AC analysis methods
Graphical determination
? Input and output characteristic curves of EC config.

AC load line determination
47
AC load line
(Slope_AC: 1/(Rc //Rtai)
DC load line
(slope= 1/Rc)
Q N
? AC loadline is steeper than DC loadline
?Graphically: ON = OQ + QN where QN = IC-Q/Slope_AC =
IQ*(Rc//Rtai)
? A straight line through Q_point and N : AC load line

48
? Q_point deplacement when Rc, Vcc, IB vary respectively
AC analysis methods
Variation of RC Variation of VCC Variation of IB

49
? Basing on input and output characteristic curves→ determine
small signal input and output waveform
AC analysis methods

50
? Δvbe → Δib
? Δvce → Δic
? Ai = io/ii = Δic/Δib
? AV= vo/vi = Δvce/Δvbe
? Zin = vi/ii = Δvbe/Δib
? Zout = vo/io = Δvce/Δic
AC analysis methods

? Impact of Q point on AC output
signal
? Q closed to cutoff → BJT is closed to
OFF operation, with a very small AC
input amplitude → output voltage is
distorsed (is cut) at upper-part
? Q closed to saturation → BJT is
closed to saturation operation, with a
very small AC input amplitude →
output volage is distorsed (is cut) at
lower-part
? Large-signal may be cut at upper and
lower part
51
AC analysis methods

AC analysis
? T model
? rE model
52

Two-port model
53
? Most used for small signal analysis
? Characterized by 2 input terminals and 2 output terminals (4
-terminals model)
? The common terminal is used for input and output

Remind: AC equivalent circuit
? 1. Setting all DC sources to zero
? 2. Replacing all capacitors by a
short-circuit equivalent (wire)
? 3. Regrouping all elements
(resistors) in parallel (introduced
by step 1 and 2)
? 4. Redrawing the network in a
more convenient and logical
form
54
`
`
`

55
? Equivalent circuit
after step 1 and 2
Remind: AC equivalent circuit
?????????
?????????
? Equivalent circuit
after step 3 and 4

AC analysis
? rE model
56

? BJT is modeled by a diode and current source
? Input : BE junction is characterized by a diode in Forward bias
? Output: dependent current source where controlled current is input current
that is expressed by Ic = βIb or Ic=αIe.
? 3 configurations: EC; BC và CC
57
AC analysis methods
rE model
Common Base
(CE)

58
AC analysis methods
rE model
Common Emitter
(CE)

59
AC analysis methods
rE model
Common Collector
(CC)

60
? EC ? BC ? CC
c
e
b
e c
e
c
60
AC analysis methods
rE model

61
? EC ? BC ? CC
c
e
b
e c
c
e
Input: ib, vb
Output: ic, vc
Rin = vb/ib = βre
iout = ic = βiin= βib
?Refer to T model as learnt in Electronics Devices Course?
Determine Rin & Iout =f(Iin) to obtain re model
Input: ie, ve
Output: ic, vc
Rin = ve/ie = re
iout = ic = αiin = αie
Input: ib, vb
Output: ie, ve
Rin = vb/ib = βre
AC analysis methods
rE model

? U & I relation:
Ui=h11Ii+h12Uo
Io=h21Ii+h22Uo
? hij is determined at a given operating point (can be
different from Q_point)
? Index e (or b, c) illustrated for CE topology (or CB, CC)
? Hybrid parameters for 2N4400
62
AC analysis methods
Hybrid equivalent model
2 ports
Iv Ir
Uv Ur

63
AC analysis methods
Parameters EC BC CC
h11 (hi) 1kΩ 20Ω 1kΩ
h12 (hr) 2,5x10-4 3x10-4 ≈1
h21 (hf) 50 -0,98 -50
h22 (ho) 25μA/V 0,5μA/V 25μA/V
1/h22 40kΩ 2MΩ 40kΩ

? Other names of hij
? Read part 7.6, chapter 7 for further understanding
64
AC analysis methods
hi
h V
r i
hfIin h0

66
Analyze EC
AC analysis methods
rE model

67
? EC
EC configuration with fixed biasing

68
68

69
69

1) Zi = Rb||βre
2) Zo = Rc||ro
if Rb ≥ 10βre,
if ro ≥ 10Rc,
Zi ≈ βre
Zo ≈ Rc
3) Av = - (Rc||ro)/re ≈ - Rc/re
(β appered in re)
Ui & Uo out of phase180o
4) Ai = βRbro / [(ro+Rc)(Rb+βre)] ≈ β
(Ii current source. Io collector current)
70

EC configuration with different biasing
71

EC configuration with voltage divider
72

73

1) Zi = R1||R2||βre = R’|| βre
2) Zo = Rc||ro (If ro ≥ 10Rc, Zo ≈ Rc)
3) Av = - (Rc||ro)/re ≈ - Rc/re
Similar to EC with fixed biasing
≈ βR’/(R’+ βre)
≈ β
if ro ≥ 10Rc
if R’ ≥ 10 βre
74

75

EC configuration with feedback biasing
76
1) Zi = re/(1/β+Rc/Rf)
2) Zo = Rc//Rf
3) Av = -Rc/re
4) Ai = βRf/(Rf+ βRc)
≈ Rf/Rc
if βRc >> Rf
When ro≠∞ → ro in equation

77

78

79

80
Analyze BC
AC analysis methods
rE model

81
BC configuration
T model (learnt in Electronics Devices Courses)

85
Analyze BC configuration
85
1) Zi = Re||re Tr? kháng vào t??ng ??i nh?
2) Zo = Rc Tr? kháng ra l?n
3) Av = αRc/re ≈ Rc/re T??ng ??i l?n
Ui & Uo cùng pha
4) Ai = - α ≈ -1 Kh?ng khu?ch ??i dòng

87
87
CC configuration with fixed biasing

88
88

Analyze output impedance
89

Vo
90

1) Zi = Rb || [βre+(β+1)Re] ≈ Rb || β(re+Re)
High input impedance
2) Zo = Re||re ≈ re where Re >> re
Low output impedance
3) Av = Re/(Re+re) ≈ 1
Inphase with input and smaller amplitude
=> “emitter connection”
4) Ai = - βRb/[Rb+ β(re+Re)]
Application: Buffer
91

Quizz
? 1. eq/. Circuit
? 2. Av; Ai
92

Homework 1: Determine Ai, Av, Zi, Zo?
93

97
Homework 2: : Determine Ai, Av, Zi, Zo?

99
Homework 3: : Determine Ai, Av, Zi, Zo?

101
Homework 4: Determine Ai, Av, Zi, Zo?

102
Example: Determine Ai, Av, Zi, Zo?

106
Ki?m tra nhanh (l?n 1)
? ?? 1
? V? m?ch BC s? dung
b? chia áp
? V? d?ng song c?a tín
hi?u t?i B, C khi bi?t VB
= 3.7V, vB có biên ??
10mV, VC =9.3V v?i
biên ?? vC = 65mV.
? Xác ??nh Av c?a m?ch
? Xác ??nh ?i?m ho?t
??ng t?nh Q bi?t IB =
40uA, beta =95
? ?? 2
? V? m?ch CC s? dung
ph?n c?c bang h?i ti?p
? V? d?ng song c?a tín
hi?u t?i B, C khi bi?t VB
= 4.5 V, vB có biên ??
5mV, VC =8.7V v?i
biên ?? vC = 45mV.
? Xác ??nh Av c?a m?ch
? Xác ??nh ?i?m ho?t
??ng t?nh Q bi?t IB =
20uA, beta =90

狠狠撸

20241_Chapter 1eg regdfgdfhdfh dfhdf hd_BJT.pdf

More Related Content

Similar to 20241_Chapter 1eg regdfgdfhdfh dfhdf hd_BJT.pdf (20)

Recently uploaded (20)

20241_Chapter 1eg regdfgdfhdfh dfhdf hd_BJT.pdf