Download BJT Amplifier I

EKT104 ANALOG
ELECTRONIC CIRCUITS
[LITAR ELEKTRONIK ANALOG]
BASIC BJT AMPLIFIER
(PART I)
DR NIK ADILAH HANIN BINTI ZAHRI
[email protected]
1
Analog Signals & Linear Amplifiers
Analog signals
•
•
•
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
Analog circuits
•
•
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
Block diagram of a
compact disc
player system
DC power
(a)
Signal
source
a) Low signal power
b) High signal power
DC voltage
source
(b)
Amplifier
Load
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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 operating mode
Time-varying output voltage is directly proportional to & larger
3
than time-varying input voltage  linear amplifier
The Bipolar Linear Amplifier
Variable
iB, vBE
•
Meaning
Total instantaneous values
DC values
IB, VBE
Instantaneous ac values
ib, vbe
Summary of notation
Phasor values
Ib, Vbe
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Graphical Analysis & AC
Equivalent Circuit
VC
C
iC
vO
RB
vs
iB
RC
vCE
vBE
VBB
Figure (c)
(c) Common-emitter circuit with
time varying signal source in series
with base dc source
Figure (d)
(d) Common-emitter transistor characteristics, dc load line, and sinusoidal
variation in base current, collector current, and collector-emitter voltage
5
Graphical Analysis & AC
Equivalent Circuit
• Base on Figure (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)
6
Graphical Analysis & AC
Equivalent Circuit
• For B-E loop, considering time varying signals:
VBB  vs  iB RB  vBE
 ( I BQ  ib ) RB  (VBEQ  vbe )
• Rearrange:
VBB  I BQ RB  VBEQ  ib RB  vbe  vs
•
(7)
(8)
Base on (5), left side of (7) is 0. So:
vs  ib RB  vbe
(9)
7
Graphical Analysis & AC
Equivalent Circuit

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

(11)
Base on (6), left side of (11) is 0. So:
ic Rc  vce  0
(12)
8
Graphical Analysis & AC
Equivalent Circuit
• Definition of small signal
• Small signal : ac input signal voltages and currents which
are in the order of ±10 percent of Q-point voltages and
currents.
e.g. If dc current is 10 mA, the ac current (peak-to-peak) <
0.1 mA.
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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
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Graphical Analysis & AC
Equivalent Circuit
• Equations
RC
•
ic
vs  ib RB  vbe
vO
RB
vs
ib
+
vbe -
+
vce
-
 I BQ 
vbe
ib  
 VT 
•
AC equivalent circuit of C-E with npn
transistor
(ac equivalent circuit of Figure (c))
Base-emitter loop (Input
loop)
Thermal
voltage, 0.026
Collector emitter loop
(Output loop)
ic RC  vce  0
ic  ib
11
Small-signal Hybrid-
Equivalent Circuit
gm=ICQ/VT
vbe = ibrπ
rπ
= diffusion resistance /
base-emitter input
resistance
1/rπ
= slope of iB – VBE curve
r=VT/ICQ
vbe
V
 V
 r  T  F T ,
ib
I BQ
I CQ
gm 
Small signal hybrid-π equivalent circuit
for npn transistor using
transconductance (gm) parameter
I CQ
VT
ic  g m vbe
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Alternative Form of Small-signal
Hybrid- Equivalent Circuit
 ib
( I b )
Using common-emitter current gain (β) parameter
ic
 
ib
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Constructing 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
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Small-signal Voltage Gain
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
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Voltage Gain Measurement
VC
C
Example
RC
RB
Given :  = 100, VCC = 12V
VBE = 0.7V, RC = 6k,
RB = 50k, and VBB = 1.2V
vO
vs
VBB
Calculate the collector-emitter voltage
at q-point and small-signal voltage
gain.
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SOLUTIONS
1.
I BQ 
2.
3.
4.
VBB  VBE ( on)
RB
1.2  0.7

 10 A
50
I CQ  I BQ  100(10A)  1 mA
VCEQ  VCC  I CQ RC  12  (1)(6)  6V
r 
VT
I CQ
I CQ
(100)(0.026)

 2.6 k
1
5.
6.
 r 
Vo
  11.4
Av   g m RC 
Vs
 r  RB 
VT

1
 38.5 mA / V
0.026
gm 
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Hybrid- Model and Early Effect
Early Effect
• Collector voltage has some effect on collector current
• Collector current increases slightly with increases in
voltage  Early Effect
• Modeled as a linear increase in total current with
increases in VCE
18
Hybrid- Model and Early Effect
Early Voltage (pg 296)
Early Voltage
(VA)
19
Hybrid- Model and Early Effect
transconductance
parameter
ro=VA/ICQ
current gain
parameter
ro = small-signal transistor
output resistance
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VA = early voltage
Basic Common-Emitter
Amplifier Circuit
VCC
Example
Given :  = 100, VCC = 12V
R1
VBE(on) = 0.7V, RS = 0.5k,
RS
CC
RC
vO
RC = 6k, R1 = 93.7k, R2 = 6.3k
and VA = 100V.
vs
R2
Calculate the small-signal voltage gain.
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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

Ans: ICQ = 0.95mA, VCEQ =6.3V, Av =-163)
22
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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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.
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