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EE 332

DEVICES AND CIRCUITS II

Lecture 8Small-Signal Modeling and Linear

Amplification (3)

Small Signal Operation of MOSFET

( ) ( )++=+= 2222 gsvTN V GS V gsvTN V GS V n K

d i D I Di

22

=TN V GS v

n K Di

For linearity, id should be proportional to v gs

Since MOSFET can be biased with ( V GS - V TN ) equal to several volts, itcan handle much larger values of v gs than corresponding values of vbe for BJT.

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Small-Signal Model Simplification If we assume

Generally R3 >> R D and load resistor R D .

Sample Analysis of C-S Amplifier

Problem: Find voltage gain, inputand output resistances.

Given data: n = 500 A/V 2, V TN -1

Analysis: Dc equivalent circuitis constructed.

= 1V, = 0.0167 V

61051 = DS

V I

)1(410210 I D I DS V

+=

V5

2)4.0(2=

=

DS V TN V DS V

n K D I

V V2 A025= = DGS I

C-S Amplifier Input Resistance

Input resistance of C-S amplifier ismuch larger than that of corresponding C-E amplifier.

G R RG R

==

in

xixv

39

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Sample Analysis of C-S Amplifier

(contd.) Next we construct the acequivalent and simplify it.

93.7

in

in)3out(=

+==

R I

R

R R Rm g

ivov

v A

M 121in

==G

RG R R

k 2.183out

==G

R D Ror R

S41020.5)1(2 =+= DS V DS I n K m g

k 260

1

=+

=

D I

DS V

or

Small Signal Model of JFET

Since JFET is normally operatedwith gate junction reverse-

biased,

==

g r SG I G I

For small signal operation,condition on input is:

Amplification factor is given by:

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