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In this chapter, we will:
Microelectronics
Circuit Analysis and Design
Study and understand the operation and characteristics of
the various types of MOSFETs.
Donald A. Neamen
Understand and become familiar with the dc analysis and
design techniques of MOSFET circuits.
Examine three applications of MOSFET circuits.
Chapter 3
Investigate current source biasing of MOSFET circuits, such
as those used in integrated circuits.
The Field Effect Transistor
Analyze the dc biasing of multistage or multitransistor
circuits.
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 3-1
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 3-2
MOS Capacitor Under Bias:
Electric Field and Charge
Basic Structure of MOS Capacitor
Parallel plate capacitor
Negative gate bias:
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Microelectronics, 4e
McGraw-Hill
Chapter 3-3
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Microelectronics, 4e
McGraw-Hill
Holes attracted to gate
Chapter 3-4
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Basic Transistor Operation
Schematic of n-Channel
Enhancement Mode MOSFET
Before electron
inversion layer is
formed
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 3-5
Current Versus Voltage Characteristics:
Enhancement-Mode nMOSFET
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Microelectronics, 4e
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Chapter 3-7
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Microelectronics, 4e
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After electron
inversion layer is
formed
Chapter 3-6
Family of iD Versus vDS Curves:
Enhancement-Mode nMOSFET
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Microelectronics, 4e
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Chapter 3-8
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p-Channel Enhancement-Mode
MOSFET
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Microelectronics, 4e
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Chapter 3-9
Symbols for p-Channel
Enhancement-Mode MOSFET
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Microelectronics, 4e
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Chapter 3-11
Symbols for n-Channel
Enhancement-Mode MOSFET
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Microelectronics, 4e
McGraw-Hill
Chapter 3-10
n-Channel Depletion-Mode MOSFET
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Microelectronics, 4e
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Chapter 3-12
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Family of iD Versus vDS Curves:
Depletion-Mode nMOSFET
p-Channel DepletionMode MOSFET
Symbols
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Microelectronics, 4e
McGraw-Hill
Symbols
Neamen
Chapter 3-13
Summary of I-V Relationships
Region
NMOS
Nonsaturation
vDS<vDS(sat)
Conduction Parameters
vSD<vSD(sat)
2
SD
iD = Kn[2(vGS −VTN)vDS −v ] iD = Kp[2(vSG +VTP)vSD −v ]
vDS>vDS(sat)
i D = K n [vGS − VTN ]2
NMOSFET
Kn =
PMOSFET
Kp =
vSD>vSD(sat)
iD = K p [vSG + VTP ]2
Transition Pt.
vDS(sat) = vGS - VTN
vSD(sat) = vSG + VTP
Enhancement
Mode
VTN > 0V
VTP < 0V
Depletion
Mode
VTN < 0V
VTP > 0V
where:
Neamen
Chapter 3-14
PMOS
2
DS
Saturation
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Microelectronics, 4e
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Chapter 3-15
Neamen
Wµ n Cox
W
= k n'
2L
2L
Wµ p Cox
2L
= k p'
W
2L
Cox = ε o tox
Microelectronics, 4e
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Chapter 3-16
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Problem-Solving Technique:
NMOSFET DC Analysis
MOS Circuits
1. Assume the transistor is in saturation.
a. VGS > VTN, ID > 0, & VDS ≥ VDS(sat)
2. Analyze circuit using saturation I-V relations.
3. Evaluate resulting bias condition of transistor.
a. If VGS < VTN, transistor is likely in cutoff
b. If VDS < VDS(sat), transistor is likely in
nonsaturation region
4. If initial assumption is proven incorrect, make
new assumption and repeat Steps 2 and 3.
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Microelectronics, 4e
McGraw-Hill
Chapter 3-17
NMOS Common-Source Circuit
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Chapter 3-19
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Chapter 3-18
PMOS Common-Source Circuit
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Chapter 3-20
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Load Line and Modes of Operation:
NMOS Common-Source Circuit
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Microelectronics, 4e
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Chapter 3-21
MOS Small-Signal Amplifier
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Microelectronics, 4e
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Chapter 3-22
NMOS Common-Source Circuit
Microelectronics
Circuit Analysis and Design
Donald A. Neamen
Chapter 4
Basic FET Amplifiers
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Microelectronics, 4e
McGraw-Hill
Chapter 3-23
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Microelectronics, 4e
McGraw-Hill
Chapter 3-24
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Simple NMOS Small-Signal
Equivalent Circuit
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 = ( ∂∂viDSD ) −1
ro = [λK n (VGSQ − VTN ) 2 ]−1 ≅ [λI DQ ]−1
Neamen
Microelectronics, 4e
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Chapter 3-25
Channel Length Modulation:
Early Voltage
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Microelectronics, 4e
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Chapter 3-26
NMOS Common-Source Circuit
Small-signal
AC
Av = Vo Vi = − g m (ro RD )
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Microelectronics, 4e
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Chapter 3-27
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Microelectronics, 4e
McGraw-Hill
Chapter 3-28
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Problem-Solving Technique:
MOSFET AC Analysis
Common-Source Configuration
DC analysis:
Coupling capacitor is assumed
to be open.
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, 4e
McGraw-Hill
Chapter 3-29
AC analysis:
Coupling capacitor is assumed
to be a short. DC voltage
supply is set to zero volts.
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 3-30
DC Load Line
Small-Signal Equivalent Circuit
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.
Ri
Av = Vo Vi = − g m (ro RD )(
)
Ri + RSi
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 3-31
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Microelectronics, 4e
McGraw-Hill
Chapter 3-32
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Common-Source Amplifier with
Source Resistor
Small-Signal Equivalent Circuit
for Common-Source with Source Resistor
Av =
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Microelectronics, 4e
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Chapter 3-33
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Common-Source Amplifier with
Bypass Capacitor
Microelectronics, 4e
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− g m RD
1 + g m RS
Chapter 3-34
NMOS Source-Follower
or Common Drain Amplifier
Small-signal equivalent circuit
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Chapter 3-35
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Microelectronics, 4e
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Chapter 3-36
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Small-Signal Equivalent Circuit
for Source Follower
Av =
Neamen
RS ro
Determining Output Impedance
NMOS Source Follower
Ri
)
1
+ RS ro Ri + RSi
gm
(
Microelectronics, 4e
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RO =
Chapter 3-37
Neamen
Microelectronics, 4e
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1
RS ro
gm
Chapter 3-38
Comparison of 3 Basic Amplifiers
Configuration
Voltage
Gain
Common
Source
Av > 1
Source
Follower
Av ≈ 1
Common
Gate
Av > 1
Neamen
Current
Gain
__
__
Ai ≈ 1
Microelectronics, 4e
McGraw-Hill
Input
Output
Resistance Resistance
RTH
RTH
Low
Moderate
to high
Low
Moderate
to high
Chapter 3-39
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