PPT: Fet Amplifier | Analog and Digital Electronics - Electrical Engineering (EE) PDF Download

 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.