6.002 CIRCUITS AND ELECTRONICS MOSFET Amplifier Large Signal Analysis http://electrical.globalautomation.info
6.002 CIRCUITS ANDELECTRONICS
MOSFET AmplifierLarge Signal Analysis
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Amp constructed using dependent source
Superposition with dependent sources:one way leave all dependent sources in;solve for one independent source at atime [section 3.5.1 of the text]Next, quick review of amp …
Reading: Chapter 7.3–7.7
+ –+
–a′
av
b′
b)(vfi =
a′a
b′bcontrol
port DS outputport
Dependent source in a circuit
Review
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Amp review
LDSO RiVv −=
( )2I 1v2K
−
for vI ≥ 1V= 0 otherwise
SV
Ov
LR
+–
( )2ID 1v2Ki −=
Iv
VCCS
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Key device Needed:
Let’s look at our old friend, the MOSFET …
A
Bv
C
( )vfi =voltage controlledcurrent source
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Key device Needed:Our old friend, the MOSFET …
First, we sort of lied. The on-state behavior of the MOSFET is quite a bit more complex than either the ideal switch or the resistor model would have you believe.
D
S
G
D
S
TGS Vv <G
TGS Vv ≥?
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Graphically
DSv
DSi
TGS Vv ≥
TGS Vv <
TGS Vv ≥
1GSvSaturationregion
Trio
de r
egio
n
S MODELDSv
DSi
SR MODELDSv
DSi
Demo
TGS Vv <
TGS Vv <
2GSv
3GSv
+–DSv
+–
DSiGSv
...
TGSDS Vvv −=
Cutoffregion
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Graphically
DSv
DSi
TGS Vv ≥
TGS Vv <
TGS Vv ≥
1GSvSaturationregion
Trio
de r
egio
n
S MODELDSv
DSi
SR MODELDSv
DSi
TGS Vv <
TGS Vv <
2GSv
3GSv
+–DSv
+–
DSiGSv
...
TGSDS Vvv −=
Notice thatMOSFET behaves like a current source
whenTGSDS Vvv −≥
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MOSFET SCS Model
D
S
G
D
S
TGS Vv <G
( )22 TGS VvK −=
whenTGSDS Vvv −≥
( )GSDS vfi =
TGS Vv ≥
D
S
G
When
the MOSFET is in its saturation region, and the switch current source (SCS) model of the MOSFET is more accurate than the S or SR model
TGSDS Vvv −≥
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Reconciling the models…
TGSDS Vvv −≥TGSDS Vvv −<
use SCS modeluse SR model
Note: alternatively (in more advanced courses)
or, use SU Model (Section 7.8 of A&L)
S MODEL SR MODEL SCS MODEL
for fun! for digitaldesigns
for analogdesigns
When to use each model in 6.002?
DSv
DSi
TGS Vv ≥
TGS Vv <
TGS Vv ≥
1GSvSaturationregion
Trio
de r
egio
n
DSv
DSi
DSv
DSi
TGS Vv <
TGS Vv <
2GSv
3GSv
...
TGSDS Vvv −=
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Back to Amplifier
in saturationregion
Iv OvAMP
SV
SV
LR
IvOv
G DS
( )22 TIDS VvKi −=
To ensure the MOSFET operates as a VCCS, we must operate it in its saturation region only. To do so, we promise to adhere to the
“saturation discipline”
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MOSFET Amplifier
in saturationregion
SV
LR
IvOv
G DS
( )22 TIDS VvKi −=
To ensure the MOSFET operates as a VCCS, we must operate it in its saturation region only. We promise to adhere to the
“saturation discipline.”
In other words, we will operate the amp circuit such that
vGS ≥ VT and vDS ≥ vGS – VT at all times.vO ≥ vI – vT
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Let’s analyze the circuitFirst, replace the MOSFET with itsSCS model.
for TIO Vvv −≥IGS vv =
G
Iv+
–
+–
SV
OvLR
D
S
A( )22 TIDS VvKi −=
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Let’s analyze the circuit
for TIO Vvv −≥IGS vv =
G
Iv+
–+–
SV
OvLR
D
S
A( )22 TIDS VvKi −=
or ( ) LTISO RVvKVv 2
2−−= for TI Vv ≥
TIO Vvv −≥
SO Vv = for TI Vv <(MOSFET turns off)
LDSSO RiVv −= B1 Analytical method: IO vvsv
(vO = vDSDS in our examplein our example))
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2 Graphical method
:B
From A ( ) ,2
2TIDS VvKi −=:
2ODS
DSO
TIO
v2Ki
Ki2v
Vvv
≤
⇓
≥
⇓
−≥
IO vvsv
for
L
0
L
SDS R
vRVi −=
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2 Graphical method
:B
Constraints and must be metA B
A ( ) ,2
2TIDS VvKi −=:
SV
DSi
Ov
2
2 ODS vKi ≤
L
S
RV
Load lineB
for
GSv=Iv
A
2
2 ODS vKi ≤
L
O
L
SDS R
vRVi −=
IO vvsv
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2 Graphical method
Constraints and must be met.Then, given VI, we can find VO, IDS .
A B
SV
DSi
Ov
L
S
RV
BIv
A
2
2 ODS vKi ≤
IO vvsv
IVDSI
OV
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Large Signal Analysisof Amplifier(under “saturation discipline”)
1 vO versus vI
2 Valid input operating range andvalid output operating range
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Large Signal Analysis
1 vO versus vI
Iv
SV( ) LTIS RVvKV 2
2−−
Ov
TV
gets intotriode region
TIO Vvv −=
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2 What are valid operating rangesunder the saturation discipline?
DSi
Ov
2
2 ODS vKi ≤
SV
L
S
RV
L
O
L
SDS R
vRVi −=
Large Signal Analysis
TIO
TI
VvvVv
−≥≥
2
2 ODS vKi ≤Our
Constraints
= TI
0=DSi= SO VvVv
and
?
Iv( )2
2 TIDS VvKi −=
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= TI
0=DSi= SO VvVv
and
2 What are valid operating rangesunder the saturation discipline?
DSi
Ov
2
2 ODS vKi ≤
L
O
L
SDS R
vRVi −=
Large Signal Analysis
Iv( )2
2 TIDS VvKi −=
L
SLTI KR
VKRVv
211 ++−+=
L
SLO KR
VKRv
211 ++−=
L
O
L
SDS R
vRVi −=
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Valid input range:
L
SLT KR
VKRV211 ++−
+vI : VT to
corresponding output range:
L
SL
KRVKR211 ++−vO : VS to
2 Valid operating ranges under the saturation discipline?
Large Signal Analysis Summary
1 vO versus vI
( ) L2
TISO RVv2KVv −−=
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