2.3 fet amplifiers, Lecture notes of Basic Electronics

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FET Amplifiers

• For the output voltage to be a linear function

of the input voltage, the transistor must be

biased in the saturation region.

NMOS common-source circuit with time-varying signal source in series with gate dc source

Small-signal equivalent circuit of common-source circuit with NMOS transistor

n^ GSQ TN ^ n DQ gs

d

m K V V K I

v

i

g   2   2

D

A

DQ

o

I

V

I

r



Common-source circuit

with PMOS transistor

The Common-Source Amplifier

Common-source circuit with voltage divider biasing and coupling capacitor

vO  gmvgsr (^) O || RD

i i Si

i gs (^) R R v

R

v (^)  

  (^)  

  

 

   i Si

i m O D i

O v (^) R R

R g r R v

v A ||

Ri  R 1 || R 2 RO  RD || rO

Common-source Circuit

with Source Bypass Capacitor

• A source bypass capacitor

added to the common-

source circuit with a source

resistor will minimize the

loss in the small-signal

voltage gain, while

maintaining the Q -point

stability. The Q -point

stability can be further

increased by replacing the

source resistor with a

constant-current source.

vo  g (^) mvgs RS || ro

vin  vgs  vo  vgs  gmvgs^ RS || ro

m^ S o

in gs g R r

v v 1  ||



v where R R 1 || R 2 R R

R v (^) i i i Si

i in (^)   

  

 



    

i Si

i m S o

m S o i

o v (^) R R

R

g R r

g R r v

v A 1 ||

Input and Output Impedance

S o m

o R r g

R || ||

1 

vo  g^ mvgs RD || RL 

vi  IiRSi vgs where Ii  gmvgs

m Si

i gs g R

v v 

  1

m Si

m D L

i

o v

g R

g R R

v

v

A

m

i

g

R

Ro  RD

Input and Output Impedance

The Three Basic Amplifiers:

Summary and Comparison

Configuration Voltage Gain Input Resistance Output Resistance

Common Source > 1 Moderate High

Source Follower 1 High Low

Common Gate > 1 Low High

