Chapter 11
Field effect
Transistors:
Operation,
Circuit, Models,
and Applications
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Context
• 11.1 Classification of Field-Effect
Transistor
• 11.2 Overview of Enhancement-mode
Mosfet
• 11.3 Biasing Mosfet Circuit
• 11.4 Mosfet Large-Signal Amplifiers
• 11.5 Mosfet Switches
2
Classification of Field-Effect
Transistors
• This figure depicts the classification of fieldeffect transistors, as well as the more commonly
used symbols for these devices.
Classification of
field-effect
transistors
3
Overview of Enhancement-Mode
Mosfets
• This figure depicts the circuit symbol and the
approximate construction of a typical n-channel
enhancement-mode MOSFET.
The n-channel
enhancement MOSFET
construction and circuit
symbol
4
Channel formation in NMOS transistor: (a) With no external gate voltage,
the source-substrate and substrate-drain junctions are both reverse-biased,
and no conduction occurs; (b) when a gate voltage is applied, chargecarrying electrons are drawn between the source and drain regions to form a
conducting channel.
5
Operation of the n-channel
Enhancement-Mode Mosfet
Saturation region
Regions of operation of NMOS transistor
6
Drain characteristic curves for a typical NMOS transistor with VT
= 2 V and K = 1.5 mA/V2
7
EXAMPLE 11.1 Determining the
Operating State of a Mosfet
Problem
• Determine the operating state of the
MOSFET shown in the circuit of figure
11.6 for the given value of VDD and VGG if
the ammeter and voltmeter shown read the
following value:
8
b. VGG = 4V; VDD = 10V ;νDS = 2.8V; ίD
= 72mA; RD = 100Ώ
c. VGG = 3V; VDD = 10V ;νDS = 1.5V; ίD
= 13.5mA; RD = 630Ώ
9
CHECK YOUR
UNDERSTANDING
• What is the operating state of the MOSFET
of the example 11.1 for the following
conditions?
VGG = 10/3V; VDD = 10V ;νDS = 3.6V;ίD = 32mA; RD
= 200Ώ
10
11
EXAMPLE 11.2 Mosfet Q-Point
Graphical Determination Problem
• Determine the Q point for the MOSFET in
the circuit of figure 11.7.
The n-channel enhancement MOSFET circuit and drain
characteristic for Example 11.2
12
13
CHECK YOUR
UNDERSTANDING
• Determine the operating region of the
MOSFET of example 11.2 when νGS = 3.5V.
14
EXAMPLE 11.3 Mosfet Q-Point
Calculation Problem
• Determine the Q point for the MOSFET in
the circuit of figure 11.7.
15
16
CHECK YOUR
UNDERSTANDING
• Find the lowest value of RD for the
MOSFET of the example 11.3 that will
place the MOSFET in the ohmic region.
17
EXAMPLE 11.4 Mosfet Self-Bias
Circuit Problem
• Figure 11.8(a) depicts a self-bias circuit for
a MOSFET. Determine the Q point for the
MOSFET by choosing Rs such that νDSQ
=8V.
18
19
CHECK YOUR
UNDERSTANDING
• Determine the appropriate value of RS if we
wish to move the operating point of the
MOSFET of example 11.4 to νDSQ =12V.
Also find the value of νGSQ and ίDQ. Are
these values unique?
20
EXAMPLE 11.5 Analysis of
Mosfet Amplifier Problem
• Determine the gate and drain-source
voltage and the drain current for the
MOSFET amplifier of figure11.9.
21
22
Operation of the P-channel
Enhacement-Mode Mosfet
The p-channel enhancement-mode field-effect transistor
(PMOS)
23
The Resulting equations for the
three modes of operation of the
PMOS
24
Mosfet Large-Signal Amplifiers
Common-source MOSFET amplifier
25
• Thus, the load voltage, across the load
resistance, is given by the expression.
26
• Where △υ= VG-VT. We can then
solve for the load current from the
quadratic equation
27
(a) Source-follower MOSFET amplifier. (b) Drain current
response for a 100-Ω load when K = 0.018 and VT = 1.2 V
28
EXAMPLE 11.6 Using a Mosfet as
a Current Source for Battery
Charging
• Analyze the two battery charging
circuit shown in figure 11.14. use the
transistor parameters to determine the
range of require gate voltages, VG ,to
provide a variable charging current up
to a maximum of 0.1A. Assume that
the terminal voltage of a fully
dischanged battery is 9V, and of a fully
charged battery 10.5V.
29
30
31
CHECK YOUR
UNDERSTANDING
• What is the maximum power dissipation of
the MOSFET for each of the circuit in
example 11.6?
32
EXAMPLE 11.7 Mosfet DC Motor
Drive Circuit Problem
DC motor drive circuit
33
34
CHECK YOUR
UNDERSTANDING
• What is the range of duty cycle needed to
cover the current range of the Lego motor?
35
Digital Switches and Gates
CMOS inverter
CMOS inverter approximate by ideal switches:
(a) When Vin is “high,” Vout is tied to ground;
(b) when Vin is “low,” Vout is tied to VDD.
36
EXAMPLE 11.8 Mosfet Switch
Problem
• Determine the operating points of the
MOSFET switch of figure 11.18 when the
signal source output is equal to 0and 2.5V,
respectively.
37
CHECK YOUR
UNDERSTANDING
• What value of RD would ensure a drain-tosource voltage νDS of 5V in the circuit of
example 11.8?
38
EXAMPLE 11.9 COMS Gate
Problem
• Determine the logic function implemented
by the CMOS gate of figure 11.20.use the
table below to summarize the behavior of
the circuit.
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40
CHECK YOUR
UNDERSTANDING
• Analyze the CMOS gate of figure 11.23
and find the output voltage for the
following conditions: (a) ν1 = 0v, ν2 =0V (b)
ν1 = 5V, ν2 =0V (c) ν1 = 0V, ν2 =5V (d) ν1 =
5V, ν2 =5V.identify the logic function
accomplished by the circuit.
41
Analog Switches
42
Symbol for bilateral FET analog gate
MOSFET analog switch
43
44
Homework Problem
45