lecture8(20-3-11)
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7/27/2019 Lecture8(20-3-11)
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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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