Short Notes: Basic FET Amplifiers- 1

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Basic FET Amplifiers
Field Effect T ransistor (FET) amplifiers are widely used in analog circuits due to their high input
imp edance, lo w noise, and v ersatilit y . This section co v ers the fundamen tals o f FET amplifiers, fo cusing
on their configurations, small-signal mo dels, and k ey p erformance parameters.
Key Concepts
• FET T yp es : Junction FET (JFET) and Metal-Oxide-Semiconductor FET (MOSFET). MOS-
FET s include enhancemen t and depletion t yp es.
• Amplifier Configurations : Common Source (CS), Common Drain (CD), and Common Gate
(CG).
• Small-Signal Analysis : Used to analyze amplifier b eha vior under small input signals, assum ing
linear op eration.
FET Amplifier Configurations
1. Common Source (CS) Amplifier
• Characteristics : High v oltage gain, mo derate input imp edance, phase in v ersion.
• Small-Signal V oltage Gain :
A
v
=-g
m
R
D
where g
m
is the transconductance and R
D
is the drain resistor.
• Input Imp edance : Appro ximately R
G
(gate resistor).
• Output Imp edance : Appro ximately R
D
.
2. Common Drain (CD) Amplifier
• Characteristics : Unit y v oltage gain, high input imp edance, lo w output imp edance, no phase
in v ersion (also called source follo w er).
• Small-Signal V oltage Gain :
A
v
=
g
m
R
S
1+g
m
R
S
where R
S
is the sourc e resistor.
• Input Imp edance : Appro ximately R
G
.
• Output Imp edance :
R
out
˜
1
g
m
3. Common Gate (CG) Amplifier
• Characteristics : Lo w input imp edance, high v oltage gain, no phase in v ersion.
• Small-Signal V oltage Gain :
A
v
=g
m
R
D
• Input Imp edance :
R
in
˜
1
g
m
• Output Imp edance : Appro ximately R
D
.
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Page 2


Basic FET Amplifiers
Field Effect T ransistor (FET) amplifiers are widely used in analog circuits due to their high input
imp edance, lo w noise, and v ersatilit y . This section co v ers the fundamen tals o f FET amplifiers, fo cusing
on their configurations, small-signal mo dels, and k ey p erformance parameters.
Key Concepts
• FET T yp es : Junction FET (JFET) and Metal-Oxide-Semiconductor FET (MOSFET). MOS-
FET s include enhancemen t and depletion t yp es.
• Amplifier Configurations : Common Source (CS), Common Drain (CD), and Common Gate
(CG).
• Small-Signal Analysis : Used to analyze amplifier b eha vior under small input signals, assum ing
linear op eration.
FET Amplifier Configurations
1. Common Source (CS) Amplifier
• Characteristics : High v oltage gain, mo derate input imp edance, phase in v ersion.
• Small-Signal V oltage Gain :
A
v
=-g
m
R
D
where g
m
is the transconductance and R
D
is the drain resistor.
• Input Imp edance : Appro ximately R
G
(gate resistor).
• Output Imp edance : Appro ximately R
D
.
2. Common Drain (CD) Amplifier
• Characteristics : Unit y v oltage gain, high input imp edance, lo w output imp edance, no phase
in v ersion (also called source follo w er).
• Small-Signal V oltage Gain :
A
v
=
g
m
R
S
1+g
m
R
S
where R
S
is the sourc e resistor.
• Input Imp edance : Appro ximately R
G
.
• Output Imp edance :
R
out
˜
1
g
m
3. Common Gate (CG) Amplifier
• Characteristics : Lo w input imp edance, high v oltage gain, no phase in v ersion.
• Small-Signal V oltage Gain :
A
v
=g
m
R
D
• Input Imp edance :
R
in
˜
1
g
m
• Output Imp edance : Appro ximately R
D
.
1
Small-Signal Mo del of FET
The small-signal mo del for an FET (simplified for lo w frequencies) includes:
• T ransconductance: g
m
=
?I
D
?V
GS
, t ypically giv en b y:
g
m
=
2I
D
V
GS
-V
TH
(for MOSFET in saturation)
• Drain-Source Resistance: r
ds
=
1
?I
D
, often large and neglected in basic analysis.
Biasing in FET Amplifiers
Prop er biasing ensures the FET op erates in the desired region (e.g., saturation for MOSFET s). Common
biasing metho ds:
• Self-Bias : Uses a source resistor to set the gate-source v oltage.
• V oltage Divider Bias : Pro vides stable bias u sing a resistor net w ork at the gate.
Key P arameters
• T ransconductance (g
m
) : Measures the FET’s abilit y to con v ert gate v oltage c hanges to drain
curren t c hanges.
• V oltage Gain (A
v
) : Ratio of output v oltage to input v oltage.
• Input/Output Imp edance : Determines the ease of in terfacing with other c ircuit stages.
Applications
• CS: High-gain amplifiers in audio and RF circuits.
• CD: Buffers for imp edance matc hing.
• CG: High-frequency amplifiers, curren t buffers.
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