Lecture no 2 to 5
THE BASIC BJT AMPLIFIER
CONFIGURATIONS
Prepared by
Engr:Sarfaraz Khan Turk
Lecturer at IBT LUMHS Jamshoro
Basic BJT Amplifiers Circuits
Single-Stage BJT Amplifiers
Key Words:
Common-Emitter Amplifier
Graphical Analysis
Small-Signal Models Analysis
Common-Collector Amplifier
Common-Base Amplifier
Basic BJT Amplifiers Circuits
Single-Stage BJT Amplifiers
C-E Amplifiers
To operate as an amplifier, the BJT must be biased to operate in active
mode and then superimpose a small voltage signal vbe to the base.
DC + small signal
coupling capacitor
(only passes ac signals)
C2
iC iB
RC
C1
vi 
 vBE   iB  

 ic 
vCE 
vo
vi  iB
iB  iC
iC  vO
Basic BJT Amplifiers Circuits
Single-Stage BJT Amplifiers
C-E Amplifiers
+
Vi
Vi
Vi
Vi
Basic BJT Amplifiers Circuits
Single-Stage BJT Amplifiers
C-E Amplifiers
Apply a small signal
input voltage and see ib
iB  I B  ib
vBE=vi+VBE
Basic BJT Amplifiers Circuits
Single-Stage BJT Amplifiers
C-E Amplifiers
See how ib translates into vce.
• vi = 0  IB、IC、VCE
vi  0
iC=ic+IC
i B  I B  ib


iC  I C  iC 
vCE  VCE  vce 
• VoM  ViM
f ( o)  f (i )
• vo out of phase with vi
vCE=vce+VCE
BJT Amplifiers Circuits
Single-Stage BJT Amplifiers
C-E Amplifiers
Considering VC (all the capacitors are replaced
by open circuits)
Considering Vi (all the capacitors are replaced
by short circuits)
Basic BJT Amplifiers Circuits
Single-Stage BJT Amplifiers
C-E Amplifiers
Considering VC (all the capacitors are replaced
by open circuits)
Considering Vi (all the capacitors are replaced
by short circuits)
Basic BJT Amplifiers Circuits
Single-Stage BJT Amplifiers
Graphical Analysis
• Can be useful to understand the operation of BJT
circuits.
• First, establish DC conditions by finding IB (or VBE)
• Second, figure out the DC operating point for IC
VCC
Can get a feel for whether the BJT will stay in active region of operation
– What happens if RC is larger or smaller?
Basic BJT Amplifiers Circuits
Single-Stage BJT Amplifiers
Graphical Analysis
vce  ic ( RC // RL )  ic RL'
VCC
Basic BJT Amplifiers Circuits
Single-Stage BJT Amplifiers
Graphical Analysis
Q-point is centered on the ac load line:
VCC
Basic BJT Amplifiers Circuits
Single-Stage BJT Amplifiers
Graphical Analysis
Q-point closer to cutoff:
VCC
Clipped at cutoff
(cutoff distortion)
Basic BJT Amplifiers Circuits
Single-Stage BJT Amplifiers
Graphical Analysis
Q-point closer to saturation:
VCC
Clipped at cutoff
(saturation distortion)
Basic BJT Amplifiers Circuits
Single-Stage BJT Amplifiers
Graphical Analysis
Basic BJT Amplifiers Circuits
Single-Stage BJT Amplifiers
Small-Signal Models Analysis
Steps for using small-signal models
1. Determine the DC operating point of the BJT
－ in particular, the collector current
2. Calculate small-signal model parameters: rbe
3. Eliminate DC sources
– replace voltage sources with short circuits and
current sources with open circuits
4. Replace BJT with equivalent small-signal models
5. Analysis
Basic BJT Amplifiers Circuits
Single-Stage BJT Amplifiers
Small-Signal Models Analysis
Example 1
VC  (I B  I C )R  I B R b  VBE  I E R e
VC  VBE
 IB 
Rb  (1   )(R  Re )
IC ≈ βIB,
IE = IC + IB = (1+β)IB
VCE  VC  I C RC  I E ( R  Re )
Basic BJT Amplifiers Circuits
Single-Stage BJT Amplifiers
Small-Signal Models Analysis
Example 2
VB 
Rb 2
VCC
Rb1  Rb 2
IC  I E 
vs
IB 
VB  VBE
V B/ Re
Re
IC

VCE  VCC  I C ( R C  R e )
Basic BJT Amplifiers Circuits
Single-Stage BJT Amplifiers
Small-Signal Models Analysis
There are three basic configurations for single-stage
BJT amplifiers:
– Common-Emitter
– Common-Base
– Common-Collector
e
N P N
b
VBB
e
c
c
b
RC
VCC
(a)
VE  VB  VC
VBB
N
P
N
e
Rc
b
VCC
(b)
VE  VB  VC
VBB
N
P
N
Re
c
VCC
(c)
VE  VB  VC
Basic BJT Amplifiers Circuits
Single-Stage BJT Amplifiers
Common-Collector Amplifier
VCC  I B Rb  VBE  I E Re  I B Rb  VBE  (1   ) I B Re
IB 
VCC  VBE
VCC

Rb  (1   ) Re Rb  1   Re
I C  I B
VCC  VCE  I E Re  VCE  I C Re
VCE  VCC  I C Re
(a) 共集电极电路
Note : Vo is slightly less than Vi due to the voltage drop introduced by VBE
AV  1
Basic BJT Amplifiers Circuits
Single-Stage BJT Amplifiers
Common-Collector Amplifier
The last basic configuration is to tie the collector to a fixed voltage, drive
an input signal into the base and observe the output at the emitter.
(a) 共集电极电路
Basic BJT Amplifiers Circuits
Single-Stage BJT Amplifiers
Common-Collector Amplifier
Let’s find Av， Ai：


Vo  I e ( Re // RL )  I b (1   )( Re // RL )


Vi  I b [rbe  (1   )(Re // RL )]  I b rbe  I e ( Re // RL )

(1   )(Re // RL )
 ( Re // RL )
 AV   

1
Vi rbe  (1   )(Re // RL ) rbe  (1   )(Re // RL )
VO
Basic BJT Amplifiers Circuits
Single-Stage BJT Amplifiers
Common-Collector Amplifier
Let’s find Av，
AiI：( R // R )  (1   ) I ( R // R )
I o RL 
e
e
L
b
e
L
(1   )(Re // RL )
Io  Ib
RL
Ib (rbe  (1   )(Re // RL ))  ( I i  Ib ) Rb
rbe  (1   )(Re // RL )  Rb
(1   )(Re // RL )  Rb
Ii  Ib
 Ib
Rb
Rb
Ai 
(1   )(Re // RL )
Rb
(1   )(Re // RL )


RL
(1   )(Re // RL )  Rb
RL
Ai 
(1   )(Re // RL )
>>1
RL
Ii
Io
(1   )(Re // RL ) << Rb
Basic BJT Amplifiers Circuits
Single-Stage BJT Amplifiers
Common-Collector Amplifier
Let’s find Ri：
Ri
vi  ib rbe  ie ( Re // R L )  ib rbe  (1   )( Re // R L )
R i 
vi
 rbe  (1   )( Re // R L )
ib
Ri  Ri // Rb  [rbe  (1   )( Re // RL )] // Rb  Rb //  ( Re // RL )
Basic BJT Amplifiers Circuits
Single-Stage BJT Amplifiers
Common-Collector Amplifier
Let’s find Ro：
I Re  I  I e
I  I Re  I e
I  I Re  Ie  I Re  1   Ib
I
I
Re 

Ro
(b)
Ie
I Re
I
I  I Re  Ib  Ib
v
v

 (1   )
Re
rbe  Rs // Rb
v
1
Ro  
1
i 1 
Re (rbe  Rs // Rb ) (1   )
(rbe  Rs // Rb )
 Re //
1 
Basic BJT Amplifiers Circuits
Single-Stage BJT Amplifiers
Common-Collector Amplifier
I
I
Ri
(a)
Ri  [rbe  (1   )( Re // RL )] // Rb
Re 

Ro
(b)
(rbe  Rs // Rb )
Ro  Re //
1 
Basic BJT Amplifiers Circuits
Single-Stage BJT Amplifiers
Common-Collector Amplifier

AV 

VO

Vi
Ai 

 ( Re // RL )
1
rbe  (1   )(Re // RL )
(1   )(Re // RL )
>>1
RL
Ri  [rbe  (1   )( Re // RL )] // Rb
(a) 共集电极电路
(rbe  Rs // Rb )
Ro  Re //
1 
C-C amp characteristics:
• Voltage gain is less than unity, but close (to unity) since β is large and rbe is small.
• Also called an emitter follower since the emitter follows the input signal.
• Input resistance is higher, output resistance is lower.
- Used for connecting a source with a large Rs to a load with low
resistance.
Basic BJT Amplifiers Circuits
Single-Stage BJT Amplifiers
Common-Base Amplifier
Ground the base and drive the input signal into the emitter
Rc
(a) 共基极电路
VB  VBE  I E R e
VB 
VCC
R b2
R b1  R b 2
VCE  VCC  I C RC  I E Re  VCC  I C ( RC  Re )
IC  I E 
IB 
IC

VB  VBE VB

Re
Re
Basic BJT Amplifiers Circuits
Single-Stage BJT Amplifiers
Common-Base Amplifier
Ro
Ri
(a) 共基极电路
i ( R // RL )  ( Rc // RL )
 Av  c c

 ib rbe
rbe
R
R
 C
 C (R  R ) I

(
R

R
)
C
L
C
L
  Io 

 E 1
A
i
(1   )
IC
Ii  rbe r
be
rbe
// Re
R i=
(1   )
Ro≈RC
(1   )
For RL<<RC, Ai 
// Re

(1   )
 1sinceI E  I C 
Basic BJT Amplifiers Circuits
Single-Stage BJT Amplifiers
Common-Base Amplifier
 RC ( R  R )
I
RC
C
L
A i  o 

(1   )
RC  RL
I
i
For RL<<RC, Ai 
Av 
 ( Rc // RL )

(1   )
1
rbe
rbe
rbe
//
R

R i=
e
(1   )
(1   )
Ro≈RC
(a) 共基极电路
CB amp characteristics:
• current gain has little dependence on β
• is non-inverting
• most commonly used as a unity-gain current amplifier or current buffer and not
as a voltage amplifier: accepts an input signal current with low input resistance
and delivers a nearly equal current with high output impedance
• most significant advantage is its excellent frequency response
Basic BJT Amplifiers Circuits
Summary for three types of diodes:
Input
Output
Functions
C-C
C-E
C-B
IB
IB
IB
IE
IC
IC
Zout < Zin Zout > Zin Zout > Zin
Vout ≈ Vin Vout > Vin Vout > Vin
Basic BJT Amplifiers Circuits
Frequency Response
Key Words:
Basic Concepts
High-Frequency BJT Model
Frequency Response of the CE Amplifier
Basic BJT Amplifiers Circuits
Frequency Response
Basic Concepts
1.0V
0.5V
0V
-0.5V
-1.0V
0.5ms
V(1)
1.0ms
1.5ms
2.0ms
2.5ms
V(2)
Time
3.0ms
3.5ms
4.0ms
Basic BJT Amplifiers Circuits
Frequency Response
Basic Concepts
800mV
600mV
400mV
200mV
0V
0Hz
V(2)
2KHz
V(1)
4KHz
6KHz
8KHz
10KHz
Frequency
12KHz
14KHz
16KHz
18KHz
1.0V
0.5V
0V
10Hz
V(2)
100Hz
1.0KHz
10KHz
100KHz
1.0MHz
20KHz
Basic BJT Amplifiers Circuits
Frequency Response
Basic Concepts
Av  Av ( f ) ( f )
Lower cut off frequency
or
A  Av ( ) ( )
Upper cut off frequency
The drops of voltage gain (output/input) is mainly due to:
1、Increasing reactance of Cs , Cc , Ce (at low f)
2、Parasitic capacitive elements of the network (at high f)
3、Dissappearance of changing current (for transformer coupled amp.)
Basic BJT Amplifiers Circuits
Frequency Response
High-Frequency BJT Model
In BJTs, the PN junctions (EBJ and CBJ) also have capacitances associated
with them
C
rbe
C
C
C
rbe
C'
C'
Basic BJT Amplifiers Circuits
Frequency Response
Frequency Response of the CE Amplifier
rbe
C'
vs
There are three capacitors in the circuit.
At the mid frequency band, these are considered to be short circuits
and internal capacitors C',and C'are considered to be open circuits.
C'
Basic BJT Amplifiers Circuits
Frequency Response
Frequency Response of the CE Amplifier
At low frequencies, C1, C2 are an
open circuit and the gain is zero.
Thus C1 has a high pass effect on the
gain, i.e. it affects the lower cutoff
frequency of the amplifier.
vs
1  C1 ( Rs  Rb1 // Rb2 // rbe )
f L1 
2 is the time constant for C2.
 2   1
1
2 1
---is neglected
Basic BJT Amplifiers Circuits
Frequency Response
Frequency Response of the CE Amplifier
1  C1 ( Rs  Rb1 // Rb2 // rbe )
 2   1
---is neglected
Capacitor Ce is an open circuit. The
pole time constant is given by the
resistance multiplied by Ce.
vs
2
2
2
f L  1.1 f L1  f L 2   f Le
 ( Rb // Rs  rbe )

 e  
// Re Ce
1 


1
f Le 
2 e
Basic BJT Amplifiers Circuits
Frequency Response
Frequency Response of the CE Amplifier
At high frequencies, C1, C2 Ce are all
short circuit.
The frequency that dominates is the
lowest pole frequency.
vs
The time constant is neglected for C'
( RL  1 jC' )
rbe
 C  ( Rb // Rs // rbe )C
C'
C'
fH 
1
2 C
In summary:the lower cut off frequency is determined by network capacitence.
e.g. C1 C2 , Ce  The higher cut off frequency is determined by the parasitic
ferquency of the BJT. e.g.
C
Basic BJT Amplifiers Circuits
Frequency Response
Frequency Response of the CE Amplifier
rbe
C'
j

A v  Avm 
vs
C'
(1  j
f
fL
f
f
)(1  j
)
fL
fH

f
For f L  f  f H ,
 0  Av  Avm — mid - frequency
fH
f
j

f
fL
For f  f L ( f  f H ),
 0,  Av  Avm
— low - frequency
f
fH
1 j
fL

fL
1
For f  f H ( f  f L )  0,  Av  Avm
— High  frequency
f
f
1 j
fH
f
 ,
fL
Basic BJT Amplifiers Circuits
Frequency Response
Frequency Response of the CE Amplifier
rbe
vs
j

A v  Avm 
fL 
(1  j
L
1

2 2 L
f
fL
f
f
)(1  j
)
fL
fH
fH 
H
1

2 2 H
C'
C'
Basic BJT Amplifiers Circuits
Frequency Response
Frequency Response of the CE Amplifier
decade
decade
0
For student References.
 Chapter 9 amplifier fundamentals (9.1) (9.2) from the book Electronic
devices, circuit and systems (Micheal M cirovic)
 Chapter 8 introduction to amplifiers (8.1) (8.2) from the book introductory
electronic devices and circuits by author (Robert T .paynter).
 Wikipedia and world wide web