ch16 (1) electronics
TRANSCRIPT
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Chapter 16 CMOS Amplifiers
16.1 General Considerations
16.2 Operating Point Analysis and Design 16.3 CMOS Amplifier Topologies
16.4 Common-Source Topology
16.5 Summary and Additional Examples
16.6 Chapter Summary
1
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Chapter Outline
2CH 16 CMOS Amplifiers
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Example: Desired I/O Impedances
3CH 16 CMOS Amplifiers
inR 0ampR
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Method to Measure the I/O Impedances
4CH 16 CMOS Amplifiers
To measure Rin(Rout), deactivate all the other independent
sources in the circuit and find the ratio of vX/iX.
X
Xini
vR
X
Xout
i
vR
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Example: Input Impedance of a Simple Amplifier
5CH 16 CMOS Amplifiers
inX Ri 0
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The Concept of Impedance at a Node
6CH 16 CMOS Amplifiers
When the other node of a port is grounded, it is more
convenient to use the concept of impedance at a node.
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Example: Impedance Seen at Drain
7CH 16 CMOS Amplifiers
Oout rR
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Example: Impedance Seen at Source
8CH 16 CMOS Amplifiers
m
outg
R 1
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Impedance Summary
9CH 16 CMOS Amplifiers
Looking into the gate, we see infinity. Looking into the drain, we see rOif the source is (ac) grounded.
Looking into the source, we see 1/gmif the gate is (ac)
grounded and rOis neglected.
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Bias and Signal Levels for a MOS Transistor
10CH 16 CMOS Amplifiers
Bias point analysis establishes the region of operation and the
small-signal parameters.
On top of the bias point, small signals are applied to the circuit.
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General Steps in Circuit Analysis
11CH 16 CMOS Amplifiers
First, the effects of constant voltage/current sources are
analyzed when signal sources are deactivated.
Second, small-signal analysis is done when constant sources
are set to zero.
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Simplification of Supply Voltage Notation
12CH 16 CMOS Amplifiers
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Example: Amplifier Driven by a Microphone
13CH 16 CMOS Amplifiers
20mV
0V
Microphone Output
Since the DC (average) value is at zero, and 20mV is not
sufficient to turn on M1, M1 is off and Voutis at VDD.
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Example: Amplifier with Gate Tied to VDD
14CH 16 CMOS Amplifiers
Since the gate voltage level is fixed at VDD, no signal current
will be produced my M1, leading to no amplification.
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Example: Amplifier with Gate Bias
15CH 16 CMOS Amplifiers
With proper value of VB, M1can operate in the desired
saturation region and amplify the incoming voice signal.
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Simple Biasing
16CH 16 CMOS Amplifiers
In (a), VGS=VDD, whereas in (b) VGSequals to a fraction of VDD.
DDGS V
RR
RV
21
2
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Example: Bias Current and Maximum RD
17CH 16 CMOS Amplifiers
KR
KR
LW
VAC
VV
oxn
TH
15
20
0
18.05
/100
5.0
2
1
2
KRVVVVV
AVVRR
RLWCI
DRDTHGSD
THDDoxnD
15529.1271.0
10221
maxmin
2
21
2
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Capacitive Coupling
18CH 16 CMOS Amplifiers
Capacitive coupling is used to block the zero DC output value
of the microphone and pass the voice signal to the amplifier.
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Biasing with Source Degeneration
19CH 16 CMOS Amplifiers
Soxn
THDD
THGS
RL
WC
V
VRR
VRVVVVV
1
2
1
21
21
2
11
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Example: ID and Maximum RDfor Source Degeneration Biasing
20CH 16 CMOS Amplifiers
0
18.0/5/
/1005.0
2
LW
VACVV
oxn
TH
KI
VVVR
VVRR
VRVVVVV
V
RL
WC
V
D
THXDDD
THDD
THGS
Soxn
25.3)(
974.02
36.01
21
21
2
11
1
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Self-Biased MOS Stage
22CH 16 CMOS Amplifiers
22
1THDDSDDoxnD VIRRV
L
WCI
The gate voltage is provided by the drain with no voltage drop
across RGand M1is always in saturation.
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Example: Self-Biased MOS Stage
23CH 16 CMOS Amplifiers
0
5.0
/100 2
VV
VAC
TH
oxn
KRAI
AI
DD
D
867.2278
556
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Example: PMOS Stage with Biasing
VVKR
KR
LW
VAC
TH
oxp
5.015
20
0
18.05
/50
2
1
2
KRSaturation
AVVL
WCI
VVRR
RV
D
THGSoxpD
DDGS
3.27
562
1
771.0
max
2
21
2
24CH 16 CMOS Amplifiers
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Example: PMOS Stage with Self-Biasing
VV
LW
VAC
TH
oxp
5.0
0
18.05
/50 2
AI
VRIVLWCI
D
THDDDDoxpD
418
21 2
25CH 16 CMOS Amplifiers
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Good Example of Current Source
As long as a MOS transistor is in saturation region and =0, the
current is independent of the drain voltage and it behaves as an
ideal current source seen from the drain terminal.
26CH 16 CMOS Amplifiers
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Bad Example of Current Source
Since the variation of the source voltage directly affects the
current of a MOS transistor, it does not operate as a good
current source if seen from the source terminal
27CH 16 CMOS Amplifiers
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Possible I/O Connections to a MOS Transistor
Of all the possible I/O connections to a MOS transistor, only
(a,d), (a,e) and (b,d) are functional.
28CH 16 CMOS Amplifiers
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Common Source (CS) Stage
If the input is applied to the gate and the output is sensed at the
drain, the circuit is called a common-source (CS) stage.
29CH 16 CMOS Amplifiers
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Small-Signal Model of CS Stage
30CH 16 CMOS Amplifiers
Dmv
m
D
out
RgA
vgR
v
1
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Example: CS Stage
31
33.3
300
1
2
Dmv
Doxnm
RgA
IL
W
Cg
CH 16 CMOS Amplifiers
0
5.0
/100
1
2
VV
VAC
mAI
TH
oxn
D
Saturation
VIRVVVV
V
L
WC
IVV
DDDDTHGS
oxn
DTHGS
6.08.0
8.0,6.0
1.12
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Example: Faulty CS Stage Design
32CH 16 CMOS Amplifiers
0
18.05
/100
5.0
5
8.1
1
2
LW
VAC
VV
A
VV
mWPower
oxn
TH
v
DD
28455
56915561
Dv
mD
RA
gAImWPower
However, no solution exists since M1is out of the saturation
region (VDD-IDRD
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CS Stage I/O Impedance Calculation
x
xin
i
vR
33CH 16 CMOS Amplifiers
D
x
xout R
i
vR
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CS Stage Including Channel-Length Modulation
34CH 16 CMOS Amplifiers
ODout
ODmv
rRR
rRgA
||
||
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xAv
Example: Gain
2xAv
35CH 16 CMOS Amplifiers
No Channel-Length Modu lat ion With Channel-Length Modu lat ion
DD
D
D
O
RI
R
I
r
1
1
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Example: RD
36CH 16 CMOS Amplifiers
D
oxn
D
oxn
v
Omv
I
WLC
I
LWC
A
rgA
2
2
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CS Stage with Current Source Load
37CH 16 CMOS Amplifiers
21
211
||
||
OOout
OOmv
rrR
rrgA
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Example: CS Stage with Current Source Load
38CH 16 CMOS Amplifiers
211
||OOmv
rrgA
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CS Stage with Diode-Connected Load
39CH 16 CMOS Amplifiers
12
2
12
2
1
||||1
||||1
OO
m
out
OO
m
mv
rrg
R
rrg
gA
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Example: CS Stage with Diode-Connected PMOS
40CH 16 CMOS Amplifiers
121
2 ||||
1OO
mmv rrggA
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CS Stage with Source Degeneration
41CH 16 CMOS Amplifiers
S
m
Dv
Rg
RA
1
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Example: CS Stage with Source Degeneration
42CH 16 CMOS Amplifiers
21
11
mm
D
v
gg
R
A
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Example: Degeneration Resistor
8
2001
v
m
A
g
4
2001
v
m
A
g
43CH 16 CMOS Amplifiers
200141
6.18
mS
Sm
Dm
DDm
gRRg
Rg
KRRg
Withou t Degenerat ion With Degenerat ion
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Effective Transconductance
44CH 16 CMOS Amplifiers
Sm
m
in
outm
Rg
g
v
iG
1
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Effect of Transistor Output Resistance
45CH 16 CMOS Amplifiers
SmOoutSOmOout
RgrR
RrgrR
1
1
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Stage with Explicit Depiction of rO
46CH 16 CMOS Amplifiers
Sometimes, the transistors output resistance is explicitly
drawn to emphasize its significance.
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Example: NMOS Current Source Design
VV
V
VAC
KR
mAI
DS
oxn
out
D
3.0
25.0
/100
20
1
min
1
2
47CH 16 CMOS Amplifiers
578
201
150
12
3.0min
S
SOSm
THGS
D
m
THGSDS
R
KRrRg
VV
I
g
VVV
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Example: Output Resistance of CS Stage with Degeneration I
48CH 16 CMOS Amplifiers
121
22
11
2
111
Ooutmmm
mm
mOout
rRggg
gggrR
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Example: Output Resistance of CS Stage with Degeneration II
49CH 16 CMOS Amplifiers
211
12111OOmout
OOOmout
rrgRrrrgR
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Example: Failing Microphone Amplifier
50CH 16 CMOS Amplifiers
No Ampli f icat ion!!
Because of the microphones small low-frequency output
resistance (100), the bias voltage at the gate is not sufficient
to turn on M1.
mVVKK
KVX 5.25.2
50||100100
50||100
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Capacitive Coupling
51CH 16 CMOS Amplifiers
To fix the problem in the previous example, a method known as
capacit ive cou pl ingis used to block the DC content of the
microphone and pass the AC signal to the amplifier.
C C
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Capacitive Coupling: Bias Analysis
52CH 16 CMOS Amplifiers
Since a capacitor is an open at DC, it can be replaced by an
open during bias point analysis.
2
21
2
2
1
THDDoxnD VV
RR
R
L
WCI
C iti C li AC A l i
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Capacitive Coupling: AC Analysis
53CH 16 CMOS Amplifiers
Since a capacitor is a short at AC, it can be replaced by a short
during AC analysis.
ODmin
out rRgv
v||
C iti C li I/O I d
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Capacitive Coupling: I/O Impedances
212
1
||RRR
R
in
in
54CH 16 CMOS Amplifiers
ODout rRR ||
E l A lifi ith Di ti C ti f S k
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Example: Amplifier with Direction Connection of Speaker
55CH 16 CMOS Amplifiers
This amplifier design still fails because the solenoid of the
speaker shorts the drain to ground.
E l A lifi ith C iti C li t I/O
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Example: Amplifier with Capacitive Coupling at I/O
56CH 16 CMOS Amplifiers
This amplifier design produces very little gain because its
equivalent output resistance is too small.
08.0||8||
spDmv
spDeq
RRgA
RRR
S D ti ith B C it
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Source Degeneration with Bypass Capacitor
57CH 16 CMOS Amplifiers
It is possible to utilize degeneration for biasing but eliminate its
effect on the small-signal by adding a bypass capacitor.
Dm
G
v RgRRR
RRA
21
21
||
||
E l S D ti ith B C it D i
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Example: Source Degeneration with Bypass Capacitor Design
mVV
VV
VV
VAC
mWPower
KR
A
SR
DD
TH
oxn
in
v
400
8.1
0
5.0
/100
5
50
5
2
58CH 16 CMOS Amplifiers
KRKR
R
LW
g
R
D
m
S
225,3.64
2463
864
3.461
148
21
Concept S mmar
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Concept Summary
59CH 16 CMOS Amplifiers
Common Gate Stage
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Common-Gate Stage
60CH 16 CMOS Amplifiers
In a common-gate stage, the input is applied at the source
while the output is taken at the drain.
Small Signal Analysis of Common Gate Stage
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Small Signal Analysis of Common-Gate Stage
61CH 16 CMOS Amplifiers
Dmv RgA
Example: Common Gate Stage Design
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Example: Common-Gate Stage Design
VV
VV
VAC
LW
mAI
DD
TH
oxn
D
8.1
5.0
/100
50
5.0
2
62CH 16 CMOS Amplifiers
06.6447171.2
vm
DTHbDDDD
AgkRVVRIV
Input Impedance of Common Gate Stage
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Input Impedance of Common-Gate Stage
63CH 16 CMOS Amplifiers
m
ing
R 1
The Use of Low Input Impedance
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The Use of Low Input Impedance
64CH 16 CMOS Amplifiers
The low input impedance of a common-gate stage can be used
to impedance match a 50-transmission line.
Output Impedance of Common Gate Stage
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Output Impedance of Common-Gate Stage
65CH 16 CMOS Amplifiers
DOout RrR ||
Example: Alternate A Expression of CG Stage
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Example: Alternate Av Expression of CG Stage
66CH 16 CMOS Amplifiers
in
outvR
RA
CG Stage in the Presence of Finite Source
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Resistance
67CH 16 CMOS Amplifiers
S
m
D
in
out
Rg
Rvv
1
Output Impedance of a General CG Stage
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Output Impedance of a General CG Stage
68CH 16 CMOS Amplifiers
SOSmDout RrRgRR 1||
CG and CS Stages Output Impedance Comparison
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CG and CS Stages Output Impedance Comparison
69CH 16 CMOS Amplifiers
SOSmDoutCSoutCG RrRgRRR 1||
Since when calculating the output impedance, the input voltage
source of the CG stage is grounded, the result will be identical
to that of a CS stage if the same assumptions are made for both
circuits.
Example: AV and R t
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Example: AVand Rout
70CH 16 CMOS Amplifiers
SmmDm
in
out
Rgg
Rg
v
v
21
1
1
= 0
DOSm
Omout
RrRg
rgR ||||1
12
11
> 0
Example: CG Stage Lacking Bias Current
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Example: CG Stage Lacking Bias Current
71CH 16 CMOS Amplifiers
Although the capacitor C1isolates the DC content of the signal
source, it also blocks the bias current of M1,hence turning it
OFF.
Example: CG Stage with Source Shorted to Ground
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Example: CG Stage with Source Shorted to Ground
72CH 16 CMOS Amplifiers
Although there is now a path for bias current to flow to ground,
the signal current also goes with it, hence producing no gain.
CG Stage with Proper Bias Circuitry
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CG Stage with Proper Bias Circuitry
73CH 16 CMOS Amplifiers
R1 is used to provide a path for bias current to flow without
directly shorting the source to ground.
However, it also lowers the input impedance of the circuit
1||1
Rg
Rm
in DmSmv Rg
RRgA
111
1
Input Current Flowing Paths
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Input Current Flowing Paths
74CH 16 CMOS Amplifiers
To maximize the useful current i2, R1needs to be much larger
than 1/gm.
mg
R 1
1
Example: CG with Complete Bias Network
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Example: CG with Complete Bias Network
75CH 16 CMOS Amplifiers
VVmWPower
g
RR
A
VV
VAC
DD
m
S
v
TH
oxn
8.1
2
50/1
500,0
5
0
5.0
/100
1
2
kRkR
R
VVIg
LW
VV
GG
D
THGSDm
GS
135,45
682
4.136/2
244
8.0
21
1
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Source Followers Response to an Input Change
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Sou ce o o e s espo se to a put C a ge
78CH 16 CMOS Amplifiers
As the input changes by a small amount, the output will follow
the input and changes by a smaller amount, hence the name
source follower.
Small-Signal Model and Voltage Gain for Source Follower
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g g
79CH 16 CMOS Amplifiers
m
S
S
in
out
gR
R
v
v
1
Example: Source Follower with Current Source
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p
80CH 16 CMOS Amplifiers
AV
1vA
Source Follower Acting as a Voltage Divider
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g g
81CH 16 CMOS Amplifiers
m
S
S
in
out
gR
R
v
v
1
Complete Small-Signal Model with rO
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p g O
82CH 16 CMOS Amplifiers
m
SO
SO
in
out
gRr
Rr
v
v
1||
||
Example: Source Follower with a Real Current Source
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83CH 16 CMOS Amplifiers
m
OO
OO
v
grr
rrA
1||
||
21
21
Example: Source Follower with a Real Current Source
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84CH 16 CMOS Amplifiers
VV
VV
VAC
mWPowerA
R
DD
TH
oxn
v
S
8.1
0
5.0
/100
105.0
50
2
360
50
1
5.01
LW
g
Rg
RA
m
S
m
Sv
Output Resistance of Source Follower
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85CH 16 CMOS Amplifiers
SOm
out RrgR ||||
1
Example: Source Follower with Biasing
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86CH 16 CMOS Amplifiers
kR
VV
VV
VACA
mAI
G
DD
TH
oxn
v
D
50
8.1
0
5.0
/100
8.0
1
2
107
933.0
867
2
LW
VRIVV
RR
I
VV
R
A
SDDDGS
S
S
D
THGS
S
v
Source Follower with Current Source Biasing
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87CH 16 CMOS Amplifiers
In IC technology, source follower is often biased by a current
source to avoid the bias currents dependence on the supply
voltage.
Summary of MOS Amplifier Topologies
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88CH 16 CMOS Amplifiers
Example: Common Source Stage I
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89CH 16 CMOS Amplifiers
321
3
321
3
1
||||||1
||||||1
OOO
m
out
OOO
m
mv
rrrg
R
rrrg
gA
Example: Common Source Stage II
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90CH 16 CMOS Amplifiers
3
31
2
||11 Omm
Ov
rgg
rA
Example: CS and CG Stages
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91CH 16 CMOS Amplifiers
S
m
OvCG
OOSOmmvCS
Rg
rA
rrRrggA
1
2
11112
1
||1
Example: Composite Stage I
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92CH 16 CMOS Amplifiers
21
11
mm
D
v
gg
RA
Example: Composite Stage II
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93CH 16 CMOS Amplifiers
221
43
32
1
2
2
2
21
||11
||||1
1||
1
||1
Omm
OO
m
in
out
m
O
m
O
m
in
out
rgg
rrg
v
v
g
r
g
rg
v
v
Chapter Summary
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The impedances looking into the gate, drain, and source of a
MOS are equal to , rO and 1/gmrespectively (under proper
conditions).
The transistor has to be properly biased before small-signal
can be applied.
Resistive path between the supply rails establishes the gate
bias voltage.
Only three amplifiers topologies are possible.
CS stage provides moderate AV, high Rinand moderate Rout.
Source degeneration improves linearity but lower AV.
Source degeneration raises the Rout of CS stage considerably.
CG stage provides moderate AV, low Rin and moderate Rout.
AVfor CS and CG stages are similar but for a sign.
Source follower provides AVless than 1, high Rinand low Rout,
serving as a good voltage buffer