Common Collector Small-Signal Equivalent

1. AC Analysis of
BJT and MOSFET
Voltage Followers
Assoc Prof Chang Chip Hong email: echchang@ntu.edu.sg
EE2002 Analog Electronics

2. EE2002 Analog Electronics AC Analysis of BJT and MOSFET Voltage Followers 2
Lesson Objectives
At the end of this lesson, you should be able to:
• Identify BJT and MOSFET voltage follower circuits
• Draw small-signal AC equivalent circuits of C-C and C-D amplifiers
• Calculate the following performance characteristics of C-C and C-D amplifiers
- Voltage gain
- Input resistance
- Output resistance

3. EE2002 Analog Electronics AC Analysis of BJT and MOSFET Voltage Followers 3
RB2
RB1
RI
vi
+
-
RL
C2  
C1  
vo
Q
vi
+
-
RB
RL
+
vo
-
RE
RI
L o E LR r R R 
e
vt
b
v
A
v

1 2B B BR R R
ro
B E
C
rp
gmvbe
=  ib
+ vbe -ib (+1)ib
RE
VCC
KVL
C-C Amplifier (Voltage Follower):
Terminal Voltage Gain
 
 
1
1
L
b L
b
bi r i
R
R
i
p

  


 ( 1)
( 1) 1
L m L
L m L
R g R
r R g Rp


 
 
   
If

4. EE2002 Analog Electronics AC Analysis of BJT and MOSFET Voltage Followers 4
RB
RL
+
vo
-
RE
L o E LR r R R 1 2B B BR R R
ro
B E
C
rp
 ib
+ vbe -ib
Rin
 
 
1
1
b b b L
b
in L
b
v i r i R
v
R r R
i
p
p


  
    
in in BR R R
vb
+
-
(+1)ib
inR
KVL
C-C Amplifier (Voltage Follower):
Input Resistance
vi
+
-
RI
1 2B B BR R R
RB
RL
+
vo
-
REro
B E
C
rp
gmvbe
=  ib
+ vbe -
Rin

5. EE2002 Analog Electronics
VCC
AC Analysis of BJT and MOSFET Voltage Followers 5
RB2
RB1
RI
vi
+
- RL
C2  
C1  
vo
RE
Q
RB
RL
+
vo
-
RE
1 2B B BR R R
ro
B E
C
rp
gmvbe
+ vbe -
Rin
o o b
v
i b i
v v v
A
v v v
   vi
+
-
RI
vi
+
-
RI
vtA
C-C Amplifier (Voltage Follower):
Overall Voltage Gain
in
I in
R
R R


+
vb
-
Rin

6. EE2002 Analog Electronics
x x x
x m be
E o th
v v v
i g v
R r r Rp
  - 

1 2th I B BR R R R
be x
th
r
v v
r R
p
p
 
 - 
 
REro
B E
C
rp
gmvbe
+ vbe - ix
vx
+
-
Rout
1 1 1
x x m x x
x
E o th th
x
E o th th
v v g r v v
i
R r r R r R
v
R r r R r R
p
p p
p p

   
 
 
    
  
1
1 1 1
1
x
out
x E o th
th
E o
v
R
i R r r R
r R
R r
p
p


-
 
     
 
   
AC Analysis of BJT and MOSFET Voltage Followers 6
KCL at node E
C-C Amplifier (Voltage Follower):
Output Resistance
RL
+
vo
-
RI
1 2B B BR R R
RB
REro
B E
C
rp
gmvbe
=  ib
+ vbe -
Rout
vi
+
-

7. EE2002 Analog Electronics
VDD
AC Analysis of BJT and MOSFET Voltage Followers 7
RG2
RG1
RI
vi
+
-
RL
C2  
C1  
vo
RS
vi
+
- RG
RL
+
vo
-
RS
RI
L o S LR r R R 
gs
m
L
gs
gs
L
m
g v R
v g v R



1 2G G GR R R
ro
G S
D
gmvgs
+ vgs -
M
KVL
C-D Amplifier (Voltage Follower):
Terminal Voltage Gain
s
vt
g
v
A
v

1
m L
m L
g R
g R



If

8. EE2002 Analog Electronics AC Analysis of BJT and MOSFET Voltage Followers 8
RG
RL
+
vo
-
RS
1 2G G GR R R
ro
G S
D
gmvgs
+ vgs -
Rin
in GR R
C-D Amplifier (Voltage Follower):
Input Resistance
vi
+
-
RI

9. EE2002 Analog Electronics
VDD
AC Analysis of BJT and MOSFET Voltage Followers 9
RG2
RG1
RI
vi
+
-
RL
C2  
C1  
vo
RS
RG
RL
+
vo
-
RS
1 2G G GR R R
ro
G S
D
gmvgs
+ vgs -
Rin
vi
+
-
go o G
v vt
i g i I G
vv v R
A A
v v v R R
    

RI
M
C-D Amplifier (Voltage Follower):
Overall Voltage Gain

10. EE2002 Analog Electronics
C-D Amplifier (Voltage Follower):
Output Resistance
x x
x m gs
S o
v v
i g v
R r
  -
1 2th I G GR R R R
gs xv v -
RSro
G S
D
gmvgs
+ vgs -
ix
vx
+
-
Rout
1 1x x
x m x m x
S o S o
v v
i g v g v
R r R r
 
      
 
1
1 1
1
x
out m
x S o
S o
m
v
R g
i R r
R r
g
-
 
    
 

AC Analysis of BJT and MOSFET Voltage Followers 10
KCL at node S
RL
+
vo
-
RG
RS
RI
1 2G G GR R R
ro
G S
D
gmvgs
+ vgs -
Rout
vi
+
-