MOSFET Amplifier Large Signal Analysis

6.002 CIRCUITS ANDELECTRONICS

MOSFET AmplifierLarge Signal Analysis

http://electrical.globalautomation.info

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


Amp review

LDSO RiVv −=

( )2I 1v2K

−

for vI ≥ 1V= 0 otherwise

SV

Ov

LR

+–

( )2ID 1v2Ki −=

Iv

VCCS


Key device Needed:

Let’s look at our old friend, the MOSFET …

A

Bv

C

( )vfi =voltage controlledcurrent source


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 ≥?


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


Graphically

DSv

DSi

TGS Vv ≥

TGS Vv <

TGS Vv ≥


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 −≥


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 −≥


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 ≥


Trio

de r

egio

n

DSv

DSi

DSv

DSi

TGS Vv <

TGS Vv <

2GSv

3GSv

...

TGSDS Vvv −=


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”


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


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 −=


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))


2 Graphical method

:B

From A ( ) ,2

2TIDS VvKi −=:

2ODS

DSO

TIO

v2Ki

Ki2v

Vvv

≤

⇓

≥

⇓

−≥

IO vvsv

for

L

0

L

SDS R

vRVi −=


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


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


Large Signal Analysisof Amplifier(under “saturation discipline”)

1 vO versus vI

2 Valid input operating range andvalid output operating range


Large Signal Analysis

1 vO versus vI

Iv

SV( ) LTIS RVvKV 2

2−−

Ov

TV

gets intotriode region

TIO Vvv −=


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 −=


TIO

TI

VvvVv

−≥≥

2

2 ODS vKi ≤Our

Constraints

= TI

0=DSi= SO VvVv

and

?

Iv( )2

2 TIDS VvKi −=


= 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 −=


Iv( )2

2 TIDS VvKi −=

L

SLTI KR

VKRVv

211 ++−+=

L

SLO KR

VKRv

211 ++−=

L

O

L

SDS R

vRVi −=


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 −−=


MOSFET Amplifier Large Signal Analysis

Documents

Transcript of MOSFET Amplifier Large Signal Analysis