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# PPT - Fet Amplifier Electrical Engineering (EE) Notes | EduRev

## Electronic Devices

Created by: Cs Toppers

## Electrical Engineering (EE) : PPT - Fet Amplifier Electrical Engineering (EE) Notes | EduRev

``` Page 1

FET Amplifier
Chapter8. FET Amplifier
? FET small-signal model
Transconductance g
m
output impedance r
d
? Model applied to some configurations
D- & E-MOSFET
JFET: Fixed biasing
Self biasing
V oltage divider
Page 2

FET Amplifier
Chapter8. FET Amplifier
? FET small-signal model
Transconductance g
m
output impedance r
d
? Model applied to some configurations
D- & E-MOSFET
JFET: Fixed biasing
Self biasing
V oltage divider
FET Amplifier
FET Transconductance
FET is voltage-controlled device.
?I
D
=g
m
?V
GS
The V
GS
controls the I
D
of an FET.
The change in V
GS
will result a change of
the I
D
in the following manner:
where the g
m
is referred to as transconductance.
It is provided as y
fs
on datasheet.
Page 3

FET Amplifier
Chapter8. FET Amplifier
? FET small-signal model
Transconductance g
m
output impedance r
d
? Model applied to some configurations
D- & E-MOSFET
JFET: Fixed biasing
Self biasing
V oltage divider
FET Amplifier
FET Transconductance
FET is voltage-controlled device.
?I
D
=g
m
?V
GS
The V
GS
controls the I
D
of an FET.
The change in V
GS
will result a change of
the I
D
in the following manner:
where the g
m
is referred to as transconductance.
It is provided as y
fs
on datasheet.
FET Amplifier
FET Input Impedance
The input impedance of all commercially
available FET is sufficiently large to assume
that the terminals approximate an open circuit.
Z
in
= ??
The typical value of Z
in
is 10
9
? for JFET and
10
12
~10
15
? for MOSFET.
Page 4

FET Amplifier
Chapter8. FET Amplifier
? FET small-signal model
Transconductance g
m
output impedance r
d
? Model applied to some configurations
D- & E-MOSFET
JFET: Fixed biasing
Self biasing
V oltage divider
FET Amplifier
FET Transconductance
FET is voltage-controlled device.
?I
D
=g
m
?V
GS
The V
GS
controls the I
D
of an FET.
The change in V
GS
will result a change of
the I
D
in the following manner:
where the g
m
is referred to as transconductance.
It is provided as y
fs
on datasheet.
FET Amplifier
FET Input Impedance
The input impedance of all commercially
available FET is sufficiently large to assume
that the terminals approximate an open circuit.
Z
in
= ??
The typical value of Z
in
is 10
9
? for JFET and
10
12
~10
15
? for MOSFET.
FET Amplifier
FET Output Impedance
The output impedance of FET is typically
from 20k ? to 100k ?.
Z
out
=r
d
= 1/y
os
It is defined by the reciprocal of the slop of the
output characteristics, I
D
~V
DS
.
On datasheet, output impedance is provided as
y
os
with the units of ?S.
Page 5

FET Amplifier
Chapter8. FET Amplifier
? FET small-signal model
Transconductance g
m
output impedance r
d
? Model applied to some configurations
D- & E-MOSFET
JFET: Fixed biasing
Self biasing
V oltage divider
FET Amplifier
FET Transconductance
FET is voltage-controlled device.
?I
D
=g
m
?V
GS
The V
GS
controls the I
D
of an FET.
The change in V
GS
will result a change of
the I
D
in the following manner:
where the g
m
is referred to as transconductance.
It is provided as y
fs
on datasheet.
FET Amplifier
FET Input Impedance
The input impedance of all commercially
available FET is sufficiently large to assume
that the terminals approximate an open circuit.
Z
in
= ??
The typical value of Z
in
is 10
9
? for JFET and
10
12
~10
15
? for MOSFET.
FET Amplifier
FET Output Impedance
The output impedance of FET is typically
from 20k ? to 100k ?.
Z
out
=r
d
= 1/y
os
It is defined by the reciprocal of the slop of the
output characteristics, I
D
~V
DS
.
On datasheet, output impedance is provided as
y
os
with the units of ?S.
FET Amplifier
FET AC Equivalent Circuit
Shown in the figure is the  FET AC equivalent
circuit.
The control of I
d
by V
gs
is included as a
controlled current source.
The input impedance is represented by the
open circuit between G & S.
The output impedance is represented by the
resistor r
d
between D & S.
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