6.002 CIRCUITS ELECTRONICS...6.002 Fall 2000 Lecture 9 5Key device Needed: Our old friend, the...
Transcript of 6.002 CIRCUITS ELECTRONICS...6.002 Fall 2000 Lecture 9 5Key device Needed: Our old friend, the...
6.002 Fall 2000 Lecture 19
6.002 CIRCUITS ANDELECTRONICS
MOSFET AmplifierLarge Signal Analysis
6.002 Fall 2000 Lecture 29
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
6.002 Fall 2000 Lecture 39
Amp review
LDSO RiVv −=
( )2I 1v2K
−
for vI ≥ 1V= 0 otherwise
SV
Ov
LR
+–
( )2ID 1v2Ki −=
Iv
VCCS
6.002 Fall 2000 Lecture 49
Key device Needed:
Let’s look at our old friend, the MOSFET …
A
Bv
C
( )vfi =voltage controlledcurrent source
6.002 Fall 2000 Lecture 59
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 ≥?
6.002 Fall 2000 Lecture 69
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
6.002 Fall 2000 Lecture 79
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 −≥
6.002 Fall 2000 Lecture 89
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 −≥
6.002 Fall 2000 Lecture 99
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
nDSv
DSi
DSv
DSi
TGS Vv <
TGS Vv <
2GSv
3GSv
...
TGSDS Vvv −=
6.002 Fall 2000 Lecture 109
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”
6.002 Fall 2000 Lecture 119
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
6.002 Fall 2000 Lecture 129
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 −=
6.002 Fall 2000 Lecture 139
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))
6.002 Fall 2000 Lecture 149
2 Graphical method
:B
From A ( ) ,2
2TIDS VvKi −=:
2ODS
DSO
TIO
v2Ki
Ki2v
Vvv
≤
⇓
≥
⇓
−≥for
IO vvsv
L
0
L
SDS R
vRVi −=
6.002 Fall 2000 Lecture 159
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
6.002 Fall 2000 Lecture 169
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
6.002 Fall 2000 Lecture 179
Large Signal Analysisof Amplifier(under “saturation discipline”)
1 vO versus vI
2 Valid input operating range andvalid output operating range
6.002 Fall 2000 Lecture 189
Large Signal Analysis
1 vO versus vI
Iv
SV( ) LTIS RVvKV 2
2−−
Ov
TV
gets intotriode region
TIO Vvv −=
6.002 Fall 2000 Lecture 199
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 −=
6.002 Fall 2000 Lecture 209
= 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 −=
6.002 Fall 2000 Lecture 219
Valid input range:
L
SLT KR
VKRV
211 ++−+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 −−=