Chapter 8: FET Amplifiers - dartec.comdartec.com/MIC4120/CHAP8a.pdf · FET Amplifiers. Introduction...

Chapter 8:FET Amplifiers

IntroductionIntroduction

FETs provide:FETs provide:FETs provide: FETs provide:

• Excellent voltage gainHi h i i d• High input impedance

• Low-power consumption• Good frequency range

Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.

Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky

22

FET SmallFET Small--Signal ModelSignal Modelgg

TransconductanceTransconductance

Th l ti hi f h i I t th di h iThe relationship of a change in ID to the corresponding change in VGS is called transconductancetransconductance

Transconductance is denoted g and given by:Transconductance is denoted gm and given by:

Dm

Ig

Δ=

GSm V

gΔ



33

Graphical Determination of gGraphical Determination of gmmp gp gmm



44

Mathematical Definitions of gMathematical Definitions of gmmggmm

GS

Dm V

Ig

ΔΔ

=

⎥⎦

⎤⎢⎣

⎡−=

P

GS

P

DSSm V

V1

V2I

g

P

DSSm0 V

2Ig =

⎤⎡

Where VGS =0V

⎥⎦

⎤⎢⎣

⎡−=

P

GSm0m V

V1gg

DSS

D

P

GSII

VV

1 =−

IV ⎞⎛

Where

DSS

Dm0

P

GSm0m I

Ig

VV

1gg =⎟⎟⎠

⎞⎜⎜⎝

⎛−=



55

FET ImpedanceFET Impedance

Input impedance:Input impedance:

pp

Ω∞=iZ

do y1rZ ==

Output Impedance:Output Impedance:

osy

VΔwhere:

constant VD

DSd GSI

Vr ==

ΔΔ

yos= admittance parameter listed on FET specification sheets.



66

FET AC Equivalent CircuitFET AC Equivalent Circuit



77

CommonCommon--Source (CS) FixedSource (CS) Fixed--Bias CircuitBias Circuit( )( )

The input is on the gate and the output is on the drain

There is a 180° phase shift between input and output



88

CalculationsCalculationsCalculationsCalculationsInput impedance:Input impedance:

Gi RZ =

Output impedance:Output impedance:

dDo r||RZ =

RZ

p pp p

10RrDo

DdRZ

≥≅

Voltage gain:Voltage gain:

)R||(rgVV

A Ddmi

ov −==


i

Dd 10RrDmi

ov Rg

VV

A≥

−==



99

CommonCommon--Source (CS) SelfSource (CS) Self--Bias CircuitBias Circuit( )( )

This is a common-source amplifier configuration, so the input is on the gate and the output is on the drain

There is a 180° phase shift between input and output



1010

CalculationsCalculations

RZ


Gi RZ =


Ddo R||rZ =

RZ

p pp p

10RrDo

DdRZ

≥≅

)R||(rgA Ddmv −=


Dd 10RrDmv RgA≥

−=



1111

CommonCommon--Source (CS) SelfSource (CS) Self--Bias CircuitBias CircuitRemoving Cs affects the gain of the circuit.g



1212


Gi RZ =


Gi


10RrDo

DdRZ

≥≅

Dmo RgVA −==


d

SDSm

iv

rRR

Rg1VA

+++

==

Dmo RgVA )R(R01r

Sm

Dm

i

ov SDdRg1

gV

A +≥+−==



1313

CommonCommon--Source (CS) VoltageSource (CS) Voltage--Divider BiasDivider Bias

This is a common-source amplifier configuration, so the input is on the gate and the p goutput is on the drain.



1414

ImpedancesImpedances


21i R||RZ =


Ddo R||rZ =


10RrDo

DdRZ

≥≅

Voltage gain:Voltage gain:)R||(rgA Ddmv −=


Dd 10RrDmv RgA≥

−=



1515

Source Follower (CommonSource Follower (Common--Drain) CircuitDrain) CircuitSource Follower (CommonSource Follower (Common--Drain) CircuitDrain) Circuit

In a common-drain amplifierIn a common drain amplifier configuration, the input is on the gate, but the output is from the sourcesource.

There is no phase shift between i t d t tinput and output.



1616

ImpedancesImpedances

RZ


Gi RZ =


mSdo g

1||R||rZ =

1Sd 10Rr

mSo

g1||RZ ≥≅


)R||(rg1)R||(rg

VV

ASdm

Sdm

i

ov +

==


10rSm

Sm

i

ov dRg1

RgVV

A ≥+==



1717

CommonCommon--Gate (CG) CircuitGate (CG) Circuit( )( )

The input is on the sourceThe input is on the source and the output is on the drain.

There is no phase shift between input and output.



1818



⎥⎦

⎤⎢⎣

⎡++

=dm

DdSi rg1

Rr||RZ

1


Dd 10Rrm

Si g1||RZ ≥≅


dDo r||RZ =Voltage gain:Voltage gain:

10rDo dRZ ≥≅

⎥⎦

⎤⎢⎣

⎡+

d

DDm

o rR

RgV RgA


⎥⎦

⎤⎢⎣

⎡+

⎦⎣==

d

D

d

i

ov

rR

1VV

A 10RrDmv DdRgA ≥=



1919

DD--Type MOSFET AC EquivalentType MOSFET AC Equivalentyp qyp q



2020

EE--Type MOSFET AC EquivalentType MOSFET AC EquivalentEE--Type MOSFET AC Equivalent Type MOSFET AC Equivalent

gm and rd can be found in the specification sheet for the FET.



2121

CommonCommon--Source DrainSource Drain--FeedbackFeedbackCommonCommon Source DrainSource Drain FeedbackFeedback

There is a 180° phase shift between input and output.



2222

CalculationsCalculationsInput impedance:Input impedance:

)R||(rg1R||rR

ZDdm

DdFi +

+=

10RR||RF

iR

Z ≥≅

Output impedanceOutput impedance::

DdDdF 10Rr,R||rRDm

i Rg1Z ≥>>+

≅

p pp p

DdFo ||Rr||RZ =

DdDdF 10Rr,R||rRDo RZ ≥>>≅DdDdF ,||

Voltage gainVoltage gain::

)R||r||(RgA DdFmv −=

Dmv D10Rd,rD||RdrFRRgA ≥>>−≅



2323

CommonCommon--Source VoltageSource Voltage--Divider BiasDivider BiasCommonCommon--Source VoltageSource Voltage--Divider BiasDivider Bias



2424


Input impedanceInput impedance::

21i R||RZ =

O t t i dO t t i d

Ddo R||rZ =


RZ 10rDo dRZ ≥≅


)R||(rgA Ddmv −=

RgA ≅


Dd 10RrDmv RgA ≥−≅



2525

Summary TableSummary Tableyy



more…

2626

Summary TableSummary Tableyy



2727

TroubleshootingTroubleshootingTroubleshootingTroubleshootingCheck the DC bias voltages:Check the DC bias voltages:

If not correct check power supply, resistors, FET. Also check to ensure that the coupling capacitor between amplifier stages is OK.

.Check the AC voltages:Check the AC voltages:

If not correct check FET capacitors and the loading effect of the nextIf not correct check FET, capacitors and the loading effect of the next stage



2828

Practical ApplicationsPractical Applicationspppp

ThreeThree--Channel Audio MixerChannel Audio MixerSilent SwitchingSilent Switching

Phase Shift NetworksPhase Shift NetworksMotion Detection SystemMotion Detection SystemMotion Detection SystemMotion Detection System



2929

Chapter 8: FET Amplifiers - dartec.comdartec.com/MIC4120/CHAP8a.pdf · FET Amplifiers. Introduction...

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