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1
Fundamentals of Microelectronics
CH1 Why Microelectronics?
CH2 Basic Physics of Semiconductors
CH3 Diode Circuits
CH4 Physics of Bipolar Transistors
CH5 Bipolar Amplifiers
CH6 Physics of MOS Transistors
CH7 CMOS Amplifiers
CH8 Operational Amplifier As A Black Box
CH16 Digital CMOS Circuits
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2
Chapter 7 CMOS Amplifiers
7.1 General Considerations
7.2 Common-Source Stage
7.3 Common-Gate Stage
7.4 Source Follower
7.5 Summary and Additional Examples
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CH7 CMOS Amplifiers 3
Chapter Outline
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CH7 CMOS Amplifiers 4
MOS Biasing
Voltage at X is determined by VDD, R1, and R2.
VGS can be found using the equation above, and ID can be found by using the NMOS current equation.
Soxn
THDD
THGS
THGSoxnDSDGSDD
RL
WC
V
VRR
VRVVVVV
VVL
WCIRIV
RR
VR
1
2
2
1 &&
1
21
21
2
11
2
21
2
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CH7 CMOS Amplifiers 5
Self-Biased MOS Stage
The circuit above is analyzed by noting M1 is in saturation and no potential drop appears across RG.
DDDSGSDDVIRVRI
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CH7 CMOS Amplifiers 6
Current Sources
When in saturation region, a MOSFET behaves as a current source.
NMOS draws current from a point to ground (sinks current), whereas PMOS draws current from VDD to a point (sources current).
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CH7 CMOS Amplifiers 7
Common-Source Stage
DDoxnv
Dmv
RIL
WCA
RgA
2
0
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CH7 CMOS Amplifiers 8
Operation in Saturation
In order to maintain operation in saturation, Vout cannot fall below Vin by more than one threshold voltage.
The condition above ensures operation in saturation.
THGSDDDD
THDDDDGS
THoutin
VVVIR
VIRVV
VVV
)(
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Example 7.4
9
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10
CS Stage with =0
Lout
in
Lmv
RR
R
RgA
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CH7 CMOS Amplifiers 11
CS Stage with 0
However, Early effect and channel length modulation affect
CE and CS stages in a similar manner.
OLout
in
OLmv
rRR
R
rRgA
||
||
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CH7 CMOS Amplifiers 12
CS Gain Variation with Channel Length
Since is inversely proportional to L, the voltage gain actually becomes proportional to the square root of L.
D
oxn
D
oxn
vI
WLC
I
L
WC
A
2
2
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CH7 CMOS Amplifiers 13
CS Stage with Current-Source Load
To alleviate the headroom problem, an active current-source load is used.
This is advantageous because a current-source has a high output resistance and can tolerate a small voltage drop across it.
21
211
||
||
OOout
OOmv
rrR
rrgA
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CH7 CMOS Amplifiers 14
PMOS CS Stage with NMOS as Load
Similarly, with PMOS as input stage and NMOS as the load,
the voltage gain is the same as before.
)||(212 OOmv
rrgA
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CH7 CMOS Amplifiers 15
CS Stage with Diode-Connected Load
Lower gain, but less dependent on process parameters.
12
2
1
2
1
2
1
||||1
/
/1
OO
m
mv
m
mv
rrg
gA
LW
LW
ggA
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16
CS Stage with Diode-Connected PMOS Device
Note that PMOS circuit symbol is usually drawn with the
source on top of the drain.
21
1
2||||
1oo
m
mvrr
ggA
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CH7 CMOS Amplifiers 17
CS Stage with Degeneration
Similar to bipolar counterpart, when a CS stage is
degenerated, its gain, I/O impedances, and linearity change.
0
11
0
Dv
S DS
m
RA
R RR
g r
0
1
S
m
D
v
Rg
RA
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CH7 CMOS Amplifiers 18
Example of CS Stage with Degeneration
A diode-connected device degenerates a CS stage.
21
11
mm
Dv
gg
RA
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CH7 CMOS Amplifiers 19
CS Stage with Gate Resistance
Since at low frequencies, the gate conducts no current, gate resistance does not affect the gain or I/O impedances.
0G
RV
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CH7 CMOS Amplifiers 20
Output Impedance of CS Stage with Degeneration
Similar to the bipolar counterpart, degeneration boosts
output impedance.
OSOmoutrRrgr
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CH7 CMOS Amplifiers 21
Output Impedance Example (I)
When 1/gm is parallel with rO2, we often just consider 1/gm.
22
11
111
mm
mOoutgg
grR
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CH7 CMOS Amplifiers 22
Output Impedance Example (II)
In this example, the impedance that degenerates the CS
stage is rO, instead of 1/gm in the previous example.
1211 OOOmoutrrrgR
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CH7 CMOS Amplifiers 23
CS Core with Biasing
Degeneration is used to stabilize bias point, and a bypass
capacitor can be used to obtain a larger small-signal
voltage gain at the frequency of interest.
Dm
G
v
S
m
D
G
vRg
RRR
RRA
Rg
R
RRR
RRA
21
21
21
21
||
||,
1||
||
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CH7 CMOS Amplifiers 24
Common-Gate Stage
Common-gate stage is similar to common-base stage: a
rise in input causes a rise in output. So the gain is positive.
DmvRgA
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CH7 CMOS Amplifiers 25
Signal Levels in CG Stage
In order to maintain M1 in saturation, the signal swing at Vout
cannot fall below Vb-VTH.
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Example 7.12
26
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CH7 CMOS Amplifiers 27
I/O Impedances of CG Stage
The input and output impedances of CG stage are similar to those of CB stage.
DoutRR
m
ing
R1
0
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CH7 CMOS Amplifiers 28
CG Stage with Source Resistance
When a source resistance is present, the voltage gain is equal to that of a CS stage with degeneration, only positive.
S
m
D
v
Rg
RA
1
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CH7 CMOS Amplifiers 29
Generalized CG Behavior
When a gate resistance is present it does not affect the gain and I/O impedances since there is no potential drop across it ( at low frequencies).
The output impedance of a CG stage with source resistance is identical to that of CS stage with degeneration.
OSOmout
rRrgR 1
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CH7 CMOS Amplifiers 30
Example of CG Stage
Diode-connected M2 acts as a resistor to provide the bias current.
DOS
m
OmoutRrR
grgR ||||
11
2
11
Smm
Dm
in
out
Rgg
Rg
v
v
21
1
1
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CH7 CMOS Amplifiers 31
CG Stage with Biasing
R1 and R2 provide gate bias voltage, and R3 provides a path
for DC bias current of M1 to flow to ground.
Dm
Sm
m
in
out RgRgR
gR
v
v
/1||
/1||
3
3
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CH7 CMOS Amplifiers 32
Source Follower Stage
1v
A
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CH7 CMOS Amplifiers 33
Source Follower Core
Similar to the emitter follower, the source follower can be analyzed as a resistor divider.
LO
m
LO
in
out
Rrg
Rr
v
v
||1
||
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Example 7.17
34
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CH7 CMOS Amplifiers 35
Source Follower Example
In this example, M2 acts as a current source.
21
1
21
||1
||
OO
m
OOv
rrg
rrA
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CH7 CMOS Amplifiers 36
Output Resistance of Source Follower
The output impedance of a source follower is relatively low, whereas the input impedance is infinite ( at low frequencies); thus, a good candidate as a buffer.
L
m
LO
m
outR
gRr
gR ||
1||||
1
1|| if 0
1
Do O D
m m O
RR r R
g g r
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CH7 CMOS Amplifiers 37
Source Follower with Biasing
RG sets the gate voltage to VDD, whereas RS sets the drain
current.
The quadratic equation above can be solved for ID.
2
2
1THSDDDoxnD
VRIVL
WCI
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CH7 CMOS Amplifiers 38
Supply-Independent Biasing
If Rs is replaced by a current source, drain current IDbecomes independent of supply voltage.
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CH7 CMOS Amplifiers 39
Example of a CS Stage (I)
M1 acts as the input device and M2, M3 as the load.
321
3
321
3
1
||||||1
||||||1
OOO
m
out
OOO
m
mv
rrrg
R
rrrg
gA
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CH7 CMOS Amplifiers 40
Example of a CS Stage (II)
M1 acts as the input device, M3 as the source resistance,
and M2 as the load.
3
31
2
||11
O
mm
Ov
rgg
rA
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CH7 CMOS Amplifiers 41
Examples of CS and CG Stages
With the input connected to different locations, the two circuits, although identical in other aspects, behave differently.
1_
2
1O
v CG
S
m
rA
Rg
_ 1 2 2 2 1(1 ) ||v CS m m O S O OA g g r R r r
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CH7 CMOS Amplifiers 42
By replacing the left side with a Thevenin equivalent, and
recognizing the right side is actually a CG stage, the
voltage gain can be easily obtained.
Example of a Composite Stage (I)
21
11
mm
Dv
gg
RA
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CH7 CMOS Amplifiers 43
Example of a Composite Stage (II)
This example shows that by probing different places in a circuit, different types of output can be obtained.
Vout1 is a result of M1 acting as a source follower whereas Vout2 is a result of M1 acting as a CS stage with degeneration.
1
2
2
43
32
1||
1
||||1
m
O
m
OO
m
in
out
gr
g
rrg
v
v