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
