Chapter 8: FET Amplifiers

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Chapter 8:

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Introduction Introduction

FETs provide:

• Excellent voltage gain

Hi h i i d

• High input impedance

• Low-power consumption

• Good frequency range

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

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FET Small

FET Small--Signal Model Signal Model g g

Transconductance Transconductance

Th l ti hi f h i I t th di h i

The relationship of a change in I D to the corresponding change in V GS is called transconductance transconductance

Transconductance is denoted g and given by:

Transconductance is denoted g m and given by:

m D

g Δ I

=

GS

m V

g Δ

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Graphical Determination of g Graphical Determination of g p p g g m m m m

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Mathematical Definitions of g Mathematical Definitions of g g g m m m m

GS m D

V g I

Δ

= Δ

⎥ ⎦

⎢ ⎤

⎣

⎡ −

=

P GS P

m DSS

V 1 V V

g 2I

P m0 DSS

V g = 2I

⎤

⎡ Where V GS =0V

⎥ ⎦

⎢ ⎤

⎣

⎡ −

=

P m0 GS

m V

1 V g

g

DSS D P

GS

I I V

1 − V =

I V ⎞

⎛

Where

DSS m0 D

P m0 GS

m I

g I V

1 V g

g ⎟⎟ ⎠ =

⎜⎜ ⎞

⎝

⎛ −

=

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FET Impedance FET Impedance

Input impedance:

p p

Ω

∞

i = Z

d

o y

r 1 Z = = Output Impedance:

Output Impedance:

y os

Δ V

where:

constant V

D

d DS GS

I

r = V =

Δ Δ

y os = admittance parameter listed on FET specification sheets.

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FET AC Equivalent Circuit

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Common

Common--Source (CS) Fixed Source (CS) Fixed--Bias Circuit ( ( ) ) Bias Circuit

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

There is a 180° phase shift between input and output

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Calculations Calculations Calculations Calculations

Input impedance:

G i R Z =

Output impedance:

d D

o R || r Z =

R Z

p p

10R D r

o d D

R

Z ≅ ≥

Voltage gain:

) R

||

(r V g

A V m d D

i

v = o = − Voltage gain:

Voltage gain:

i

D d

10R D r

m i

v o g R

V A V

− ≥

=

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Common

Common--Source (CS) Self Source (CS) Self--Bias Circuit ( ( ) ) Bias 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

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Calculations Calculations

R Z

Input impedance:

G i R Z =

Output impedance:

D d

o r || R Z =

R Z

p p

10R D r

o d D

R

Z ≅ ≥

) R

||

(r g

A v = − m d D Voltage gain:

Voltage gain:

D d 10R D r

m

v g R

A = − ≥

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Common

Common--Source (CS) Self Source (CS) Self--Bias Circuit Bias Circuit

Removing C s affects the gain of the circuit. g

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Calculations Calculations

G i R Z =

Input impedance:

G i

Output impedance:

10R D r

o d D

R

Z ≅ ≥

D

o g m R

A = V = − Voltage gain:

Voltage gain:

d S S D

i m v

r R R R

g V 1

A +

+ +

=

D m

o g R

A V r 1 0 (R R )

S m

D m i

v o d D S

R g 1

g

A V ≥ +

− +

=

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Common

Common--Source (CS) Voltage Source (CS) Voltage--Divider Bias Divider Bias

This is a common-source

amplifier configuration, so the input is on the gate and the p g output is on the drain.

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Impedances Impedances

Input impedance:

2 1

i R || R Z =

Output impedance:

D d

o r || R Z =

Output impedance:

10R D r

o d D

R

Z ≅ ≥

Voltage gain:

) R

||

(r g

A v = − m d D

Voltage gain:

D d 10R D r

m

v g R

A = − ≥

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Source Follower (Common

Source Follower (Common--Drain) Circuit Drain) Circuit Source Follower (Common

Source Follower (Common--Drain) Circuit Drain) Circuit

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

source.

There is no phase shift between

i t d t t

input and output.

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Impedances Impedances

R Z

Input impedance:

G i R Z =

Output impedance:

m S

d

o g

|| 1 R

||

r Z =

1

S d 10R r

m S

o g

|| 1 R

Z ≅ ≥

Voltage gain:

) R

||

(r g 1

) R

||

(r g V

A V

S d

m

S d

m i

v = o = + Voltage gain:

Voltage gain:

10 S r

m

v o d

R g 1

R g V

A V ≥

= +

=

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Common

Common--Gate (CG) Circuit Gate (CG) Circuit ( ( ) )

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

There is no phase shift between input and output.

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Calculations Calculations

Input impedance:

⎥ ⎦

⎢ ⎤

⎣

⎡ +

= +

d m

D S d

i 1 g r

R

|| r R Z

1

Output impedance:

D d

10R r

m S

i g

|| 1 R

Z ≅ ≥

Output impedance:

d D

o R || r Z =

Voltage gain:

10 r D

o R d

Z ≅ ≥

⎥ ⎦

⎢ ⎤

⎣

⎡ +

d D D

m

o r

R R

V g A g R

Voltage gain:

⎥ ⎦

⎢ ⎤

⎣

⎡ +

⎦

= ⎣

=

D d i

v o

r 1 R V

A V A v = g m R D r d ≥ 10R D

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D

D--Type MOSFET AC Equivalent Type MOSFET AC Equivalent yp yp q q

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E

E--Type MOSFET AC Equivalent Type MOSFET AC Equivalent E

E--Type MOSFET AC Equivalent Type MOSFET AC Equivalent

g m and r d can be found in

the specification sheet for

the FET.

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Common

Common--Source Drain Source Drain--Feedback Feedback Common

Common Source Drain Source Drain Feedback Feedback

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

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Calculations Calculations

Input impedance:

) R

||

(r g 1

R

||

r Z R

D d

m

D d

i + F

= +

10R R

||

F R i

Z ≅ R ≥

Output impedance Output impedance::

D d

F

r || R , r 10R R

D m i 1 g R

Z >> ≥

≅ +

p p

D d

F

o R || r ||R Z =

D d

F r || R , r 10R R

D o R

Z ≅ >> ≥

D d

F || ,

Voltage gain Voltage gain::

) R

||

r

||

(R g

A v = − m F d D

D m

v g R R r ||R ,r 10R

A ≅ − >> ≥

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Common

Common--Source Voltage Source Voltage--Divider Bias Divider Bias Common

Common--Source Voltage Source Voltage--Divider Bias Divider Bias

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Calculations Calculations

Input impedance Input impedance::

2 1

i R || R Z =

O t t i d O t t i d

D d

o r || R Z =

Output impedance:

R

Z o D r 10 R d

Z ≅ ≥

Voltage gain:

) R

||

(r g

A v = − m d D

R g

A ≅

Voltage gain:

D d 10R r

D m

v g R

A ≅ − ≥

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Summary Table Summary Table y y

more…

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Summary Table

Summary Table y y

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

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Practical Applications Practical Applications pp pp

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