MOS Amplifier Review
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1
MOS Amplifiers
How to do the DC biasing?
Common-Source Stage
Common-Gate Stage
Source Follower
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CH7 CMOS Amplifiers 2
Example
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.
12
2
4332
1||1
||||1
mO
m
OOm
in
out
gr
g
rrg
vv
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CH7 CMOS Amplifiers 3
Example: Self-Biased MOS Stage
The circuit above is analyzed by noting M1 is in saturation and no potential drop appears across RG.
DDDSGSDD VIRVRI
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CH7 CMOS Amplifiers 4
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 5
Common-Source Stage
DDoxnv
Dmv
RILWCA
RgA
2
0
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6
CS Stage with =0
Lout
in
Lmv
RRR
RgA
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CH7 CMOS Amplifiers 7
CS Stage with 0
However, Early effect and channel length modulation affect CE and CS stages in a similar manner.
OLout
in
OLmv
rRRR
rRgA
||
||
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CH7 CMOS Amplifiers 8
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
v IWLC
ILWC
A
22
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CH7 CMOS Amplifiers 9
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.
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211
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OOout
OOmv
rrRrrgA
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CH7 CMOS Amplifiers 10
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 11
CS Stage with Degeneration
Similar to bipolar counterpart, when a CS stage is degenerated, its gain, I/O impedances, and linearity change.
0
1
Sm
Dv
Rg
RA
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CH7 CMOS Amplifiers 12
Example of CS Stage with Degeneration
A diode-connected device degenerates a CS stage.
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11mm
Dv
gg
RA
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CH7 CMOS Amplifiers 13
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.
0GRV
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CH7 CMOS Amplifiers 14
Output Impedance of CS Stage with Degeneration
Similar to the bipolar counterpart, degeneration boosts output impedance.
OSOmout rRrgr
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CH7 CMOS Amplifiers 15
Output Impedance Example (I)
When 1/gm is parallel with rO2, we often just consider 1/gm.
2211
111mm
mOout gggrR
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CH7 CMOS Amplifiers 16
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 OOOmout rrrgR
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CH7 CMOS Amplifiers 17
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.
Dmv RgA
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CH7 CMOS Amplifiers 18
I/O Impedances of CG Stage
The input and output impedances of CG stage are similar to those of CB stage.
Dout RR m
in gR 1
0
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CH7 CMOS Amplifiers 19
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.
Sm
Dv
Rg
RA
1
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CH7 CMOS Amplifiers 20
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 21
Example of CG Stage
Diode-connected M2 acts as a resistor to provide the bias current.
DOSm
Omout RrRg
rgR ||||11
211
Smm
Dm
in
out
RggRg
vv
21
1
1
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CH7 CMOS Amplifiers 22
Source Follower Stage
1vA
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CH7 CMOS Amplifiers 23
Source Follower Core
Similar to the emitter follower, the source follower can be analyzed as a resistor divider.
LOm
LO
in
out
Rrg
Rrvv
||1||
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CH7 CMOS Amplifiers 24
Source Follower Example
In this example, M2 acts as a current source.
211
21
||1||
OOm
OOv
rrg
rrA
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CH7 CMOS Amplifiers 25
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.
Lm
LOm
out Rg
Rrg
R ||1||||1
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CH7 CMOS Amplifiers 26
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.
221
THSDDDoxnD VRIVLWCI