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Neamen Microelectronics Chapter 4-1 October 15, 2012 McGraw-Hill
Microelectronics Circuit Analysis and Design
Donald A. Neamen
Chapter 4
Basic FET Amplifiers
Neamen Microelectronics Chapter 4-2 October 15, 2012 McGraw-Hill
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 Microelectronics Chapter 4-3 October 15, 2012 McGraw-Hill
NMOS Common-Source Circuit
Neamen Microelectronics Chapter 4-4 October 15, 2012 McGraw-Hill
NMOS Transistor Small-Signal Parameters
Values depends on Q-point
112
1
][])([
)(
2)(2
DQTNGSQno
v
i
o
DQnTNGSQnm
gs
d
v
i
m
IVVKr
r
IKVVKg
v
ig
DS
D
GS
D
Neamen Microelectronics Chapter 4-5 October 15, 2012 McGraw-Hill
Simple NMOS Small-Signal Equivalent Circuit
Neamen Microelectronics Chapter 4-6 October 15, 2012 McGraw-Hill
NMOS Common-Source Circuit
AC Small-signal
)( Domiov RrgVVA
Neamen Microelectronics Chapter 4-7 October 15, 2012 McGraw-Hill
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 Microelectronics Chapter 4-8 October 15, 2012 McGraw-Hill
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 Microelectronics Chapter 4-9 October 15, 2012 McGraw-Hill
Small-Signal Equivalent Circuit
))((Sii
iDomiov
RR
RRrgVVA
Neamen Microelectronics Chapter 4-10 October 15, 2012 McGraw-Hill
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 Microelectronics Chapter 4-11 October 15, 2012 McGraw-Hill
Common-Source Amplifier with Source Resistor
Neamen Microelectronics Chapter 4-12 October 15, 2012 McGraw-Hill
Small-Signal Equivalent Circuit for Common-Source with Source Resistor
Sm
Dmv
Rg
RgA
1
Neamen Microelectronics Chapter 4-13 October 15, 2012 McGraw-Hill
Common-Source Amplifier with Bypass Capacitor
Small-signal equivalent circuit
Neamen Microelectronics Chapter 4-14 October 15, 2012 McGraw-Hill
NMOS Source-Follower or Common Drain Amplifier
Neamen Microelectronics Chapter 4-15 October 15, 2012 McGraw-Hill
Small-Signal Equivalent Circuit for Source Follower
)(1
Sii
i
oS
m
oS
vRR
R
rRg
rRA
Neamen Microelectronics Chapter 4-16 October 15, 2012 McGraw-Hill
Determining Output Impedance NMOS Source Follower
oS
m
O rRg
R1
Neamen Microelectronics Chapter 4-17 October 15, 2012 McGraw-Hill
Common-Gate Circuit
Neamen Microelectronics Chapter 4-18 October 15, 2012 McGraw-Hill
Small-Signal Equivalent Circuit for Common Gate
Sim
LDm
vRg
RRgA
1
)()
1)((
Sim
Sim
LD
D
i
Oi
Rg
Rg
RR
R
I
IA
Neamen Microelectronics Chapter 4-19 October 15, 2012 McGraw-Hill
Comparison of 3 Basic Amplifiers
Configuration Voltage Gain
Current Gain
Input Resistance
Output Resistance
Common Source
Av > 1 __
RTH
Moderate to high
Source Follower
Av ≈ 1
__
RTH
Low
Common Gate
Av > 1
Ai ≈ 1
Low Moderate to high
Neamen Microelectronics Chapter 4-20 October 15, 2012 McGraw-Hill
NMOS Amplifier with Enhancement Load Device
Neamen Microelectronics Chapter 4-21 October 15, 2012 McGraw-Hill
Cascade Circuit
Neamen Microelectronics Chapter 4-22 October 15, 2012 McGraw-Hill
Small-Signal Equivalent Circuit for Cascade Circuit