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In this chapter, we will:
Microelectronics
Investigate a single-transistor circuit that can amplify a
small, time-varying input signal
Circuit Analysis and Design
Develop small-signal models that are used in the
analysis of linear amplifiers.
Donald A. Neamen
Discuss and compare the three basic transistor amplifier
configurations.
Chapter 4
Analyze the common-source amplifier.
Basic FET Amplifiers
Analyze the source-follower amplifier.
Analyze the common-gate amplifier.
Analyze multitransistor or multistage amplifiers.
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 4-1
NMOS Common-Source Circuit
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 4-3
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 4-2
NMOS Common-Source Circuit
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 4-4
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NMOS Common-Source Circuit
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 4-5
NMOS Common-Source Circuit
Neamen
NMOS CS Circuit
Microelectronics, 4e
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Chapter 4-6
Simple NMOS Small-Signal
Equivalent Circuit
phasor components in parentheses
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Microelectronics, 4e
McGraw-Hill
Chapter 4-7
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 4-8
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Simple NMOS Small-Signal
Equivalent Circuit
Recall the slope in the MOSFET characteristic curve!
The effect is that of drain to source resistance r.
NMOS Transistor Small-Signal
Parameters
Values depends on Q-point
∂iD
∂vGS
id
v gs
ro = ( ∂∂viDSD ) −1
gm =
ro = [λK n (VGSQ − VTN ) 2 ]−1 ≅ [λI DQ ]−1
g m = 2 K n (VGSQ − VTN ) = 2 K n I DQ
=
ro = ( ∂∂viDSD ) −1
ro = [λK n (VGSQ − VTN ) 2 ]−1 ≅ [λI DQ ]−1
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 4-9
Neamen
Small-signal
Chapter 4-10
Problem-Solving Technique:
MOSFET AC Analysis
NMOS Common-Source Circuit
AC
Microelectronics, 4e
McGraw-Hill
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.
Av = Vo Vi = − g m (ro RD )
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 4-11
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 4-12
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Common-Source Configuration
Small-Signal Equivalent Circuit
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, 4e
McGraw-Hill
Chapter 4-13
DC Load Line
Av = Vo Vi = − g m (ro RD )(
Neamen
Microelectronics, 4e
McGraw-Hill
Ri
)
Ri + RSi
Chapter 4-14
Common-Source Amplifier with
Source Resistor
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, 4e
McGraw-Hill
Chapter 4-15
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 4-16
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Small-Signal Equivalent Circuit
for Common-Source with Source Resistor
Common-Source Amplifier with
Bypass Capacitor
Small-signal equivalent circuit
Av =
Neamen
Microelectronics, 4e
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− g m RD
1 + g m RS
Chapter 4-17
NMOS Source-Follower
or Common Drain Amplifier
Neamen
Microelectronics, 4e
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Small-Signal Equivalent Circuit
for Source Follower
Av =
Neamen
Microelectronics, 4e
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Chapter 4-19
Chapter 4-18
Neamen
RS ro
Ri
(
)
1
+ RS ro Ri + RSi
gm
Microelectronics, 4e
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Chapter 4-20
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Determining Output Impedance
NMOS Source Follower
RO =
Neamen
Microelectronics, 4e
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Common-Gate Circuit
1
RS ro
gm
Chapter 4-21
Small-Signal Equivalent Circuit
for Common Gate
Neamen
Av =
1 + g m RSi
Neamen
Ai =
IO
RD
g R
=(
)( m Si )
Ii
RD + RL 1 + g m RSi
Microelectronics, 4e
McGraw-Hill
Chapter 4-23
Chapter 4-22
Comparison of 3 Basic Amplifiers
Configuration
g m ( RD RL )
Microelectronics, 4e
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Voltage
Gain
Common
Source
Av > 1
Source
Follower
Av ≈ 1
Common
Gate
Av > 1
Neamen
Current
Gain
__
__
Ai ≈ 1
Microelectronics, 4e
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Input
Output
Resistance Resistance
RTH
RTH
Low
Moderate
to high
Low
Moderate
to high
Chapter 4-24
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NMOS Amplifier
with Enhancement Load Device
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 4-25
CMOS Common-Source Amplifier
Neamen
Small-Signal Equivalent Circuit
for CMOS Common Source
Microelectronics, 4e
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Chapter 4-26
CMOS Common Gate
Av = − g mn (ron rop )
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Microelectronics, 4e
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Chapter 4-27
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 4-28
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Cascade Circuit
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 4-29
PMOS Common-Source Circuit
Small-Signal Equivalent Circuit
for Cascade Circuit
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 4-30
PMOS Common-Source Circuit
Av = Vo Vi = − g m (ro RD )
Neamen
Microelectronics, 4e
McGraw-Hill
Av = Vo Vi = − g m (ro RD )
Chapter 4-31
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 4-32
8