Microelectronics
Circuit Analysis and Design
Donald A. Neamen
Chapter 4
Basic FET Amplifiers
Neamen
October 15, 2012
Microelectronics
McGraw-Hill
Chapter 4-1
In this chapter, we will:
 Investigate a single-transistor circuit that can amplify a
small, time-varying input signal
 Develop small-signal models that are used in the
analysis of linear amplifiers.
 Discuss and compare the three basic transistor amplifier
configurations.
 Analyze the common-source amplifier.
 Analyze the source-follower amplifier.
 Analyze the common-gate amplifier.
 Analyze multitransistor or multistage amplifiers.
 Design a two-stage MOSFET amplifier circuit.
Neamen
October 15, 2012
Microelectronics
McGraw-Hill
Chapter 4-2
NMOS Common-Source Circuit
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October 15, 2012
Microelectronics
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Chapter 4-3
NMOS Transistor Small-Signal
Parameters
 Values depends on Q-point
gm 
iD
vGS
id

v gs
g m  2 K n (VGSQ  VTN )  2 K n I DQ
ro  (
iD 1
v DS
)
ro  [K n (VGSQ  VTN ) 2 ]1  [I DQ ]1
Neamen
October 15, 2012
Microelectronics
McGraw-Hill
Chapter 4-4
Simple NMOS Small-Signal
Equivalent Circuit
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October 15, 2012
Microelectronics
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Chapter 4-5
NMOS Common-Source Circuit
AC
Small-signal
Av  Vo Vi   g m (ro RD )
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October 15, 2012
Microelectronics
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Chapter 4-6
Problem-Solving Technique:
MOSFET AC Analysis
1. Analyze circuit with only the dc sources to
find quiescent solution. Transistor must be
biased in saturation region for linear
amplifier.
2. Replace elements with small-signal model.
3. Analyze small-signal equivalent circuit,
setting dc sources to zero, to produce the
circuit to the time-varying input signals only.
Neamen
October 15, 2012
Microelectronics
McGraw-Hill
Chapter 4-7
Common-Source Configuration
DC analysis:
Coupling capacitor is assumed
to be open.
AC analysis:
Coupling capacitor is assumed
to be a short. DC voltage
supply is set to zero volts.
Neamen
October 15, 2012
Microelectronics
McGraw-Hill
Chapter 4-8
Small-Signal Equivalent Circuit
Ri
Av  Vo Vi   g m (ro RD )(
)
Ri  RSi
Neamen
October 15, 2012
Microelectronics
McGraw-Hill
Chapter 4-9
DC Load Line
Q-point near the middle
of the saturation region
for maximum symmetrical
output voltage swing,.
Small AC input signal for
output response to be
linear.
Neamen
October 15, 2012
Microelectronics
McGraw-Hill
Chapter 4-10
Common-Source Amplifier with
Source Resistor
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October 15, 2012
Microelectronics
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Chapter 4-11
Small-Signal Equivalent Circuit
for Common-Source with Source Resistor
 g m RD
Av 
1  g m RS
Neamen
October 15, 2012
Microelectronics
McGraw-Hill
Chapter 4-12
Common-Source Amplifier with
Bypass Capacitor
Small-signal equivalent circuit
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October 15, 2012
Microelectronics
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Chapter 4-13
NMOS Source-Follower
or Common Drain Amplifier
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October 15, 2012
Microelectronics
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Chapter 4-14
Small-Signal Equivalent Circuit
for Source Follower
RS ro
Ri
Av 
(
)
1
 RS ro Ri  RSi
gm
Neamen
October 15, 2012
Microelectronics
McGraw-Hill
Chapter 4-15
Determining Output Impedance
NMOS Source Follower
1
RO 
RS ro
gm
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October 15, 2012
Microelectronics
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Chapter 4-16
Common-Gate Circuit
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October 15, 2012
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Chapter 4-17
Small-Signal Equivalent Circuit
for Common Gate
Av 
g m ( RD R L )
1  g m RSi
Neamen
October 15, 2012
IO
g m RSi
RD
Ai 
(
)(
)
Ii
RD  RL 1  g m RSi
Microelectronics
McGraw-Hill
Chapter 4-18
Comparison of 3 Basic Amplifiers
Configuration
Common
Source
Source
Follower
Common
Gate
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October 15, 2012
Voltage
Gain
Av > 1
Av ≈ 1
Av > 1
Current
Gain
Input
Output
Resistance Resistance
__
__
Ai ≈ 1
Microelectronics
McGraw-Hill
RTH
RTH
Low
Moderate
to high
Low
Moderate
to high
Chapter 4-19
NMOS Amplifier
with Enhancement Load Device
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October 15, 2012
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Chapter 4-20
Cascade Circuit
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October 15, 2012
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Chapter 4-21
Small-Signal Equivalent Circuit
for Cascade Circuit
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October 15, 2012
Microelectronics
McGraw-Hill
Chapter 4-22