FET Amplifiers Chapter 8 Boylestad Electronic Devices and Circuit Theory.

FET AmplifiersFET Amplifiers

Chapter 8

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Electronic Devices and Circuit TheoryElectronic Devices and Circuit Theory

Electronic Devices and Circuit TheoryBoylestad

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Ch.8 Summary

Introduction

FETs provide:

• Excellent voltage gain

• High input impedance

• Low-power consumption

• Good frequency response



Ch.8 Summary

FET Small-Signal Model

Transconductance: The ratio of a change in ID to the corresponding change in VGS

• Transconductance is denoted gm and given by:

GS

Dm ΔV

ΔIg



Ch.8 Summary

Geographical Determination of gm



Ch.8 Summary

Mathematical Definitions of gm

GS

Dm ΔV

ΔIg

P

GS

P

DSSm V

V

V

Ig 1

2

P

DSSm V

Ig

20

DSS

Dm

P

GSmm I

Ig

V

Vgg 00 1

For VGS = 0 V



Ch.8 Summary

FET Impedence

Ω iZ

Input impedance:

Output Impedance:

osdo y

rZ1

constant VGS

D

DSd ΔI

ΔVrwhere

yos= admittance parameter listed on FET spec sheets



Ch.8 Summary

FET AC Equivalent Circuit



Ch.8 Summary

Common-Source (CS) Fixed-Bias

The input is applied to the gate and the output is taken from the drain

There is a 180 phase shift between the circuit input and output



Ch.8 Summary

Calculations

Gi RZ

dDo ||rRZ

10

RrDo

Dd

RZ

Input impedance:

Output impedance:

)( Ddmi

ov ||Rrg

V

VA

Dd RrDmi

ov Rg

V

VA

10

Voltage gain:



Ch.8 Summary

Common-Source (CS) Self-Bias

This is a common-source amplifier configuration, so the input is applied to the gate and the output is taken from the drain.

There is a 180 phase shift between input and output.



Ch.8 Summary

Calculations

Gi RZ

Ddo ||RrZ

RrDo

Dd

RZ10

Input impedance:

Output impedance:

)( Ddmv ||RrgA

Dd RrDmv RgA10

Voltage gain:



Ch.8 Summary

Common-Source (CS) Self-Bias

Removing Cs affects the gain of the circuit.



RrDo

Dd

RZ10

Ch.8 Summary

Calculations

Gi RZ

Output impedance:

d

SDSm

Dm

i

ov

rRR

Rg

Rg

V

VA

1

)(101 SDd RRrSm

Dm

i

ov Rg

Rg

V

VA

Voltage gain:

Input impedance:



Ch.8 Summary

Common-Source (CS) Voltage-Divider Bias

This is a common-source amplifier configuration, so the input is applied to the gate and the output is taken from the drain.



Ch.8 Summary

Impedances

2 1||RRZi

Ddo ||RrZ

RrDo

Dd

RZ10

Input impedance:

Output impedance:

)( Ddmv ||RrgA

Dd RrDmv RgA10

Voltage gain:



Ch.8 Summary

Source Follower (Common-Drain)

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

There is no phase shift between input and output.



Ch.8 Summary

Impedances

mSdo g

||||RrZ1

Sd Rrm

So g

||RZ 10

1

Gi RZ

Input impedance:

Output impedance:

)(1

)(

Sdm

Sdm

i

ov ||Rrg

||Rrg

V

VA

101

dr

Sm

Sm

i

ov Rg

Rg

V

VA

Voltage gain:



Ch.8 Summary

Common-Gate (CG) Circuit

The input is applied to the source and the output is taken from the drain.

There is no phase shift between input and output.



Ch.8 Summary

CalculationsInput impedance:

Output impedance:

dm

DdSi rg

Rr||RZ

1

Dd Rrm

Si g||RZ 10

1

dDo ||rRZ

10drDo RZ

d

D

d

DDm

i

ov

rR

rR

Rg

V

VA

1

RrDmv DdRgA 10Voltage gain:



Ch.8 Summary

D-Type MOSFET AC Equivalent



Ch.8 Summary

E-Type MOSFET AC Equivalent

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



Ch.8 Summary

Common-Source Drain-Feedback

There is a 180 phase shift between input and output.



Ch.8 Summary

Calculations

Input impedance:

Output impedance:

)(1

Ddm

DdFi ||Rrg

||RrRZ

DdDdF R,r||RrRDm

Fi Rg

RZ 10 1

DdFo ||R||rRZ

DdDdF R,r||RrRDo RZ 10

)( DdFmv ||R||rRgA Dmv DRdr,D||RdrFRRgA 10 Voltage gain:



Ch.8 Summary

Common-Source Voltage-Divider Bias

The input is applied to the gate and the output is taken from the drain.

There is a 180º voltage phase shift between input and output.



Ch.8 Summary

Calculations

21||RRZ i

Input impedance:

Output impedance:

Ddo ||RrZ

10drDo RZ

)( Ddmv ||RrgA Dd RrDmv RgA 10Voltage gain:



Ch.8 Summary

Summary Table



Ch.8 Summary

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



Ch.8 Summary

Practical Applications

• Three-Channel Audio Mixer

• Silent Switching

• Phase Shift Networks

• Motion Detection System

FET Amplifiers Chapter 8 Boylestad Electronic Devices and Circuit Theory.

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