Download Basic BJT Amplifier(I)

ANALOG ELECTRONIC
CIRCUITS 1
EKT 204
Basic BJT Amplifiers (Part 1)
1
Analog Signals & Linear
Amplifiers

Analog signals




Analog circuits



Natural analog signals: physical sense (hearing, touch, vision)
Electrical analog signals: e.g. output from microphone, output signal from
compact disc – form of time-varying currents & voltages
Magnitude: any value which vary continuously with time
Electronic circuits which produce analog signals
E.g. linear amplifier
Linear amplifier

Magnifies input signal & produce output signal that is larger & directly
proportional to input signal
DC power
Block diagram of a
compact disc
player system
(a)
Signal
source
a) Low signal power
b) High signal power
DC voltage
source
(b)
Amplifier
Load
2
The Bipolar Linear Amplifier
(a) Bipolar transistor inverter circuit; (b) inverter transfer characteristics

To use circuit as an amplifier, transistor needs to be biased
with DC voltage at quiescent point (Q-point)  transistor
is biased in forward active region

Time-varying output voltage is directly proportional to &
larger than time-varying input voltage  linear amplifier 3
The Bipolar Linear Amplifier

Summary of notation
Variable
iB, vBE
Meaning
Total instantaneous values
IB, VBE
ib, vbe
DC values
Instantaneous ac values
Ib, Vbe
Phasor values
4
Graphical Analysis & AC
Equivalent Circuit
Fig. D
VC
C
Fig. C
iC
vO
RB
vs
iB
RC
vCE
vBE
VBB
(C) Common-emitter circuit
with time varying signal
source in series with base dc
source
(D) Common-emitter transistor characteristics, dc load line, and sinusoidal
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variation in base current, collector current, and collector-emitter voltage
Graphical Analysis & AC
Equivalent Circuit

Base on Fig. C & D
(time-varying signals linearly related & superimposed on
dc values)

iB  I BQ  ib
(1)
iC  I CQ  ic
(2)
vCE  VCEQ  vce
(3)
vBE  VBEQ  vbe
(4)
If signal source, vs = 0:
VBB  I BQ RB  VBEQ (B - E loop)
(5)
VCC  I CQ RC  VCEQ (C - E loop)
(6)
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Graphical Analysis & AC
Equivalent Circuit


For B-E loop, considering time varying signals:
VBB  vs  iB RB  vBE  ( I BQ  ib ) RB  (VBEQ  vbe ) (7)
Rearrange:
VBB  I BQ RB  VBEQ  ib RB  vbe  vs
(8)
Base on (5), left side of (7) is 0. So:
vs  ib RB  vbe
(9)
For C-E loop, considering time varying signals:
VCC  iC RC  vCE  ( I CQ  ic ) RC  (VCEQ  vce )
(10)
VCC  I CQ RC  VCEQ  ic Rc  vce

Base on (6), left side of (11) is 0. So:
ic Rc  vce  0
(11)
(12)
7
Graphical Analysis & AC
Equivalent Circuit

Definition of small signal

Small signal : ac input signal voltages and
currents are in the order of ±10 percent of Q-point
voltages and currents.
e.g. If dc current is 10 mA, the ac current (peakto-peak) < 0.1 mA.
8
Graphical Analysis & AC
Equivalent Circuit

Rules for ac analysis




Replacing all capacitors by short circuits
Replacing all inductors by open circuits
Replacing dc voltage sources by ground
connections
Replacing dc current sources by open circuits
9
Graphical Analysis & AC
Equivalent Circuit

RC
Equations

ic
vs  ib RB  vbe
vO
RB
vs
ib
+
vbe -
Input loop
 I BQ 
vbe
ib  
 VT 
+
vce

0.026 V
Output loop
ic RC  vce  0
AC equivalent circuit of C-E with npn
transistor
ic  ib
10
Small-signal hybrid-
equivalent circuit
gm=ICQ/VT
vbe = ibrπ
rπ
= diffusion resistance /
base-emitter input
resistance
1/rπ
r=VT/ICQ
= slope of iB – VBE
curve
vbe
VT
 FVT
 r 

,
ib
I BQ
I CQ
Using transconductance (gm) parameter
I CQ   F I BQ
11
Small-signal hybrid-
equivalent circuit
 ib
( I b )
Using common-emitter current gain (β) parameter
12
How to construct Small-signal hybrid-
VCC
RC
vO
RB
We know that
 i across B  ib
vs
 i across C βib
VBB
 i across E  (β+1)ib
 rπ between B -E
Place a terminal for the transistor
 Common Terminal as ground
B
C
B
C
βib
rπ
E
E
13
Small-signal hybrid-
equivalent circuit
Small-signal equivalent circuit
RB
Ic
B
C
Vo
+
Vs
Ib
Vbe
-
 r 
Vs
Vbe  
 r  RB 
+
r
gmVbe
E
RC
Vce
-
Vo  Vce  g mVbe RC
Output signal voltage
 r 
Vo

Small signal voltage gain, Av   g m RC 
Vs
 r  RB 
Input signal voltage
14
Small-signal hybrid-
equivalent circuit
Example
VC
C
RC
Given :  = 100, VCC = 12V
RB
VBE = 0.7V, RC = 6k,
RB = 50k, and VBB = 1.2V
Calculate the small-signal voltage
gain.
vO
vs
VBB
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Solutions
1.
I BQ 
VBB  VBE ( on)
RB

1.2  0.7
 10 A
50
2.
I CQ  I BQ  100(10A)  1 mA
3.
VCEQ  VCC  I CQ RC  12  (1)(6)  6V
4.
r 
5.
6.
VT
I CQ

(100)(0.026)
 2.6 k
1
I CQ
1
gm 

 38.5 mA / V
VT
0.026
 r 
Vo
  11.4
Av   g m RC 
Vs
 r  RB 
16
Hybrid- Model and Early
Effect
transconductance
parameter
ro=VA/ICQ
current gain
parameter
ro = small-signal transistor
output resistance
17
VA = early voltage
Hybrid- Model and Early
Effect
Early Voltage (pg 299)
Early Voltage
(VA)
18
Basic Common-Emitter
Amplifier Circuit
Example
VCC
Given :  = 100, VCC = 12V
R1
VBE(on) = 0.7V, RS = 0.5k,
RS
RC = 6k, R1 = 93.7k, R2 = 6.3k
and VA = 100V.
Calculate the small-signal voltage
gain.
vs
CC
RC
vO
R2
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Solution
Small-signal equivalent circuit
Ri
RS
Vs
R1 \\
R2
Ro
B
C
r
gmV
rO
Vo
RC
E
Ri  R1 R2 r
 R1 R2 r


V
V  
 R1 R2 r  RS  s


Ro  ro RC
Vo  g mV ro RC 
 R1 R2 r
Vo

Av    g m 
 R1 R2 r  RS
Vs


r R 
 o C

20
Self-Reading
Textbook: Donald A. Neamen,
‘MICROELECTRONICS Circuit Analysis &
Design’,3rd Edition’, McGraw Hill International
Edition, 2007
Chapter 5:The Bipolar Junction Transistor
Page: 334-339
Chapter 6: Basic BJT Amplifiers
Page: 370-388.
21
Exercise
The circuit parameters in Figure are changed to VCC = 5V,
R1=35.2kΩ, R2=5.83kΩ, RC=10kΩ and RS =0, β =100,
VBE(on) =0.7V and VA =100V. Determine the quiescent
collector current and collector-emitter voltage and find
the small-signal voltage gain.
Ans: ICQ = 0.21mA, VCEQ =2.9V, Av =-79.1)
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