Chapter 8: FET Amplifierssite.iugaza.edu.ps/ahdrouss/files/2010/02/lect-2-B.pdf · Robert L....

Chapter 8:FET Amplifiers

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

Introduction

FETs provide:

• Excellent voltage gain• High input impedance• Low-power consumption• Good frequency range

2



FET Small-Signal Model

Transconductance

The relationship of a change in ID to the corresponding change in VGS is called transconductance

Transconductance is denoted gm and given by:

GS

Dm V

Ig

∆

∆=

3



Graphical Determination of gm

4



Mathematical Definitions of gm

GS

Dm V

Ig

∆∆

=

−=

P

GS

P

DSSm V

V1

V

2Ig

P

DSSm0 V

2Ig =

−=

P

GSm0m V

V1gg

DSS

D

P

GSII

V

V1 =−

DSS

Dm0

P

GSm0m I

Ig

V

V1gg =

−=

Where VGS =0V

Where

5



Ω∞=iZ

osdo y

1rZ ==

constant VD

DSd GSI

Vr ==

∆∆

Input impedance:

FET Impedance

Output Impedance:

where:

yos= admittance parameter listed on FET specification sheets.

6



FET AC Equivalent Circuit

7



Common-Source (CS) Fixed-Bias Circuit

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

8

There is a 180° phase shift between input and output



Calculations

Gi RZ =

dDo r||RZ =

10RrDo

DdRZ

≥≅

Input impedance:

Output impedance:

9

)R||(rgV

VA Ddm

i

ov −==

Dd 10RrDmi

ov Rg

V

VA

≥−==

Voltage gain:



Common-Source (CS) Self-Bias Circuit

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

10

There is a 180° phase shift between input and output



Calculations

Gi RZ =

Ddo R||rZ =

10RrDo

DdRZ

≥≅

Input impedance:

Output impedance:

11

)R||(rgA Ddmv −=

Dd 10RrDmv RgA≥

−=

Voltage gain:



Common-Source (CS) Self-Bias CircuitRemoving Cs affects the gain of the circuit.

12



Calculations

Gi RZ =

10RrDo

DdRZ

≥≅

Input impedance:

Output impedance:

13

d

SDSm

Dm

i

ov

r

RRRg1

RgV

VA

+++

−==

)R(R01rSm

Dm

i

ov SDdRg1

RgV

VA +≥+

−==

Voltage gain:



Common-Source (CS) Voltage-Divider Bias

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

14



Impedances

21i R||RZ =

Ddo R||rZ =

10RrDo

DdRZ

≥≅

Input impedance:

Output impedance:

15

)R||(rgA Ddmv −=

Dd 10RrDmv RgA≥

−=

Voltage gain:



Source Follower (Common-Drain) Circuit

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

16

There is no phase shift between input and output.



Impedances

mSdo g

1||R||rZ =

Sd 10Rrm

So g1

||RZ ≥≅

Gi RZ =

Input impedance:

Output impedance:

17

)R||(rg1

)R||(rg

V

VA

Sdm

Sdm

i

ov +

==

10rSm

Sm

i

ov dRg1

Rg

V

VA ≥+

==

Voltage gain:



Common-Gate (CG) Circuit

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

18

There is no phase shift between input and output.



Calculations

Input impedance:

Output impedance:

++

=dm

DdSi rg1

Rr||RZ

Dd 10Rrm

Si g1

||RZ ≥≅

dDo r||RZ =

10rDo dRZ ≥≅

19

+

+

==

d

D

d

DDm

i

ov

rR

1

rR

Rg

V

VA 10RrDmv Dd

RgA ≥=

Voltage gain:



D-Type MOSFET AC Equivalent

20



E-Type MOSFET AC Equivalent

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

21



Common-Source Drain-Feedback

22

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



CalculationsInput impedance:

Output impedance:

)R||(rg1

R||rRZ

Ddm

DdFi +

+=

DdDdF 10Rr,R||rRDm

Fi Rg1

RZ ≥>>+

≅

DdFo ||Rr||RZ =

DdDdF 10Rr,R||rRDo RZ ≥>>≅

23

)R||r||(RgA DdFmv −= Dmv D10Rd,rD||RdrFRRgA ≥>>−≅

Voltage gain:



Common-Source Voltage-Divider Bias

24



Calculations

21i R||RZ =

Ddo R||rZ =

Input impedance:

Output impedance:

10rDo dRZ ≥≅

25

)R||(rgA Ddmv −=

Dd 10RrDmv RgA ≥−≅

Voltage gain:



Summary Table

more…

26



Summary Table

27



Troubleshooting

.

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:

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

28



Practical Applications

Three-Channel Audio MixerSilent Switching

Phase Shift NetworksMotion Detection System

29

Chapter 8: FET Amplifierssite.iugaza.edu.ps/ahdrouss/files/2010/02/lect-2-B.pdf · Robert L....

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Transcript of Chapter 8: FET Amplifierssite.iugaza.edu.ps/ahdrouss/files/2010/02/lect-2-B.pdf · Robert L....