chapter-8 fet amplifiers

8/3/2019 Chapter-8 FET Amplifiers

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Chapter 8:

FET Amplifiers


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IntroductionIntroduction

Excellent voltage gain g nput mpe ance

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

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FET SmallFET Small--Si nal ModelSi nal Model

TransconductanceTransconductance

e re a ons p o a c ange n D o e correspon ng c ange n

VGS is called transconductancetransconductance

ranscon uc ance s eno e gm an g ven y:

DI=GSV




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Gra hical Determination ofGra hical Determination of




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Mathematical Definitions ofMathematical Definitions of

GS

Dm

V

Ig =

P

GS

P

DSSm

V

V1

V

2Ig

P

DSSm0

V

2Ig =

Where VGS =0V

P

GSm0mV

V1gg

DSS

D

P

GS

I

I

V

V1 =

Where

DSS

Dm0

P

GSm0m

Ig

V1gg =




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FET Im edanceFET Im edance

Input impedance:Input impedance:

iZ

do1

rZ =Output Impedance:Output Impedance:

os

where:

constantVD

DSd GSI

r =

yos= admittance parameter listed on FET specification sheets.




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FET AC Equivalent CircuitFET AC Equivalent Circuit




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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 shiftbetween input and output




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Gi RZ =Out ut im edance:Out ut im edance:

dDo r||RZ =

10RrDo Dd

)R||(rgV

V

A Ddmo

v

Dd 10RrDm

i

ov Rg

V

VA




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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 betweeninput and output




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CalculationsCalculations


Gi =Out ut im edance:Out ut im edance:

Ddo R||rZ =

10RrDo Dd

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

Dd 10RrDmv RgA




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

Removing Cs affects

the gain of the circuit.




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RZ =Input impedance:Input impedance:

Output impedance:Output impedance:

10RrDo Dd

RZ

Dmo RgV Voltage gain:Voltage gain:

d

SDSm

iv

r

RRRg1

V ++=

RV)R(R01r

Smiv SDdRg1V

++




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

This is a common-source

amplifier configuration, so the

in ut is on the ate and theoutput is on the drain.




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ImpedancesImpedances


21i R||RZ =

Ddo R||rZ =

10RrDo DdRZ

)R||(rgA Ddmv

Dd 10RrDmv RgA




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--

-

configuration, the input is on the

gate, but the output is from the

.

There is no phase shift between

npu an ou pu .




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ImpedancesImpedances


Gi =

Output impedance:Output impedance:

mSdo

g

1||R||rZ =

Sd 10Rrm

So g||RZ

)R||(rg1

)R||(rg

V

V

A Sdm

Sdm

i

o

v +

10rSm

Sm

i

ov dRg1

Rg

V

VA +




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CommonCommon--Gate CG CircuitGate CG Circuit

and the output is on the

drain.

There is no phase shift

between input and output.




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++=

dm

DdSi

rg1

Rr||RZ

Dd 10Rrm

Sig

||RZ

dDo r||RZ =10rDo d

RZ

+ DDm rR

Rg

+=

d

Div

r

R1

VA 10RrDmv Dd




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DD--T e MOSFET AC E uivalentT e MOSFET AC E uivalent




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--

gm and rd can be found inthe specification sheet for

the FET.




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-- --

There is a 180 phase shiftbetween input and output.




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)R||(rg1

R||rRZ

Ddm

DdFi +

+=FR

Out ut im edanceOut ut im edance::

DdDdF r,rDmRg1

>+

DdFo ||Rr||RZ =10RrR||rRDo RZ >

Voltage gainVoltage gain::

)R||r||(RgA DdFmv Dmv D10Rd,rD||RdrFRRgA >




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-- --




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Input impedanceInput impedance::

21i R||RZ =

Ddo R||rZ =u pu mpe ance:u pu mpe ance:

10rDod

)R||(rgA Ddmv

Dd 10RrDmvg




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Summar TableSummar Table




more2626


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Summar TableSummar Table




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Check 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:

,

stage




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Practical A licationsPractical A lications

ThreeThree--Channel Audio MixerChannel Audio Mixer

Silent SwitchingSilent Switching

Phase Shift NetworksPhase Shift Networks

Motion Detection S stemMotion Detection S stem




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