This document summarizes a lecture on common source, common gate, and common drain amplifiers. It discusses: - The common source amplifier circuit and its small signal model parameters such as transconductance and output resistance. Maximizing gain by increasing the load resistance or transconductance is also covered. - The common gate amplifier, which acts as a current buffer with low input and high output impedance. Its voltage gain and output resistance are derived. - The common drain amplifier, which has high input and low output impedance, acting as a voltage buffer. Its voltage gain and output resistance are also derived.

1. Department of EECS University of California, Berkeley
EECS 105 Fall 2003, Lecture 17
Lecture 17:
Common Source/Gate/Drain
Amplifiers
Prof. Niknejad

2. EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad
Department of EECS University of California, Berkeley
Lecture Outline
 MOS Common Source Amp
 Current Source Active Load
 Common Gate Amp
 Common Drain Amp

3. EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad
Department of EECS University of California, Berkeley
Common-Source Amplifier
Isolate DC level

4. EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad
Department of EECS University of California, Berkeley
Load-Line Analysis to find Q
Q
D
DD out
R
D
V V
I
R


1
10k
slope 
0V
10k
D
I 
5V
10k
D
I 

5. EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad
Department of EECS University of California, Berkeley
Small-Signal Analysis
in
R  

6. EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad
Department of EECS University of California, Berkeley
s
v
s
R
in
R
out
R L
R
m in
G v
in
v


Two-Port Parameters:
Find Rin, Rout, Gm
in
R  
m m
G g
 ||
out o D
R r R

Generic Transconductance Amp

7. EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad
Department of EECS University of California, Berkeley
Two-Port CS Model
Reattach source and load one-ports:

8. EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad
Department of EECS University of California, Berkeley
Maximize Gain of CS Amp
 Increase the gm (more current)
 Increase RD (free? Don’t need to dissipate extra
power)
 Limit: Must keep the device in saturation
 For a fixed current, the load resistor can only be
chosen so large
 To have good swing we’d also like to avoid getting
to close to saturation
||
v m D o
A g R r
 
,
DS DD D D DS sat
V V I R V
  

9. EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad
Department of EECS University of California, Berkeley
Current Source Supply
 Solution: Use a
current source!
 Current independent
of voltage for ideal
source

10. EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad
Department of EECS University of California, Berkeley
CS Amp with Current Source Supply

11. EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad
Department of EECS University of California, Berkeley
Load Line for DC Biasing
Both the I-source and the transistor are idealized for DC bias
analysis

12. EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad
Department of EECS University of California, Berkeley
Two-Port Parameters
From current
source supply
in
R  
||
out o oc
R r r

m m
G g


13. EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad
Department of EECS University of California, Berkeley
P-Channel CS Amplifier
DC bias: VSG = VDD – VBIAS sets drain current –IDp = ISUP

14. EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad
Department of EECS University of California, Berkeley
Two-Port Model Parameters
Small-signal model for PMOS and for rest of circuit

15. EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad
Department of EECS University of California, Berkeley
Common Gate Amplifier
DC bias:
SUP BIAS DS
I I I
 

16. EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad
Department of EECS University of California, Berkeley
CG as a Current Amplifier: Find Ai
out d t
i i i
  
1
i
A  

17. EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad
Department of EECS University of California, Berkeley
CG Input Resistance
At input:
Output voltage:
t out
t m gs mb t
o
v v
i g v g v
r
 

    
 
( || ) ( || )
out d oc L t oc L
v i r R i r R
  
gs t
v v
 
 
||
t oc L t
t m t mb t
o
v r R i
i g v g v
r
 

   
 

18. EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad
Department of EECS University of California, Berkeley
Approximations…
 We have this messy result
 But we don’t need that much precision. Let’s start
approximating:
1
1
||
1
m mb
t o
oc L
in t
o
g g
i r
r R
R v
r
 
 

1
m mb
o
g g
r
  ||
oc L L
r R R
 0
L
o
R
r

1
in
m mb
R
g g



19. EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad
Department of EECS University of California, Berkeley
CG Output Resistance
( ) 0
s s t
m gs mb s
S o
v v v
g v g v
R r

    
1 1 t
s m mb
S o o
v
v g g
R r r
 
   
 
 

20. EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad
Department of EECS University of California, Berkeley
CG Output Resistance
Substituting vs = itRS
1 1 t
t S m mb
S o o
v
i R g g
R r r
 
   
 
 
The output resistance is (vt / it)|| roc
|| 1
o
out oc S m o mb o
S
r
R r R g r g r
R
 
 
   
 
 
 
 
 

21. EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad
Department of EECS University of California, Berkeley
Approximating the CG Rout
The exact result is complicated, so let’s try to
make it simpler:
S
gm 
500
 S
gmb 
50
 
 k
ro 200
]
[
|| S
S
o
mb
S
o
m
o
oc
out R
R
r
g
R
r
g
r
r
R 



]
[
|| S
S
o
m
o
oc
out R
R
r
g
r
r
R 


Assuming the source resistance is less than ro,
)]
1
(
[
||
]
[
|| S
m
o
oc
S
o
m
o
oc
out R
g
r
r
R
r
g
r
r
R 




22. EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad
Department of EECS University of California, Berkeley
CG Two-Port Model
Function: a current buffer
• Low Input Impedance
• High Output Impedance

23. EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad
Department of EECS University of California, Berkeley
Common-Drain Amplifier
2
1
( )
2
DS ox GS T
W
I C V V
L

 
2 DS
GS T
ox
I
V V
W
C
L

 
Weak IDS dependence

24. EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad
Department of EECS University of California, Berkeley
CD Voltage Gain
Note vgs = vt – vout
||
out
m gs mb out
oc o
v
g v g v
r r
 
 
||
out
m t out mb out
oc o
v
g v v g v
r r
  

25. EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad
Department of EECS University of California, Berkeley
CD Voltage Gain (Cont.)
KCL at source node:
Voltage gain (for vSB not zero):
 
||
out
m t out mb out
oc o
v
g v v g v
r r
  
1
||
mb m out m t
oc o
g g v g v
r r
 
  
 
 
1
||
out m
in
mb m
oc o
v g
v g g
r r

 
1
out m
in mb m
v g
v g g
 


26. EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad
Department of EECS University of California, Berkeley
CD Output Resistance
Sum currents at output (source) node:
|| || t
out o oc
t
v
R r r
i
 t m t mb t
i g v g v
 
1
out
m mb
R
g g



27. EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad
Department of EECS University of California, Berkeley
CD Output Resistance (Cont.)
ro || roc is much larger than the inverses of the
transconductances  ignore
1
out
m mb
R
g g


Function: a voltage buffer
• High Input Impedance
• Low Output Impedance

28. EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad
Department of EECS University of California, Berkeley