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 −−=