1
Key educational goals:
Analyze the basic principle of operation of a MOSFET and its applications.
Reading/Preparatory activities for class
i)Textbook: Chapter 12.
ii) Power-point file:MOSFET
Questions to guide your reading and to think about ahead of time.
1. What are key differences between a MOSFET and a BJT?
2. What are the two different types of MOSFET?
3. The triode and saturation modes of a MOSFET are analogous to which modes in a BJT?
4. In which mode does a MOSFET works like a resistor?.
5. In which mode does the MOSFET work as in an amplifier?
6. In what respect are MOSFET amplifiers better than transistor amplifiers?
2
Introduction
Chapter 1: Introduction and Chapter 2:Resistive circuits
The main concepts for the module
1.Contrast the working of a MOSFET with that of a BJT.
2. Categorize the different modes of operation of a MOSFET
3. Identify the application areas with the different modes of operation of a MOSFET.
4. Analyze why.
5.Formulate the basis on which the bias point in terms of the drain current and gate to
source voltage are selected out of two possibilities .
6. Compare the small signal model of a MOSFET with that of a BJT.
7.Compare the common source amplifier with a common emitter amplifier and a
an emitter follower with a source follower.
3
Summary
The knowledge gained from this module will be useful in understanding simple logic circuits
(CMOS logic gates) and amplifiers for applications where BJT amplifiers are not suitable.
For next time
We will next look into the operational amplifiers which uses amplifier configurations shown
with MOSFETs and BJTs along with some additional circuits to increase input impedance, gain
and stability.
.
Sample test/exam questions/problems to help you study:
1. For Fig. 1 in the next slide draw small signal equivalent circuit ,assuming that the capacitors are
short circuits for the ac signal.
2. For Fig.2 in the next slide find the output impdedance. Vto = 3V and K=0.5mA/V2.
4
•Fig.1
•Fig.2
MOSFET 5
MOSFET
• Introduction to MOSFET
• MOSFET structure and symbols
• Biasing N-channel MOSFET
• Conduction and output characteristics of N-channel
MOSFET
• Setting up bias point of a MOSFET
• Small signal model of a N-channel MOSFET
• Common Source and source follower amplifier analysis
• Comparison the input and output impedance of BJT and
MOSFET amplifiers
MOSFET 6
Transistors
•They are unidirectional current carrying devices like diodes with
capability to control the current flowing through them
• The switch current can be controlled by either current or voltage
• Bipolar Junction Transistors (BJT) control current by current
• Field Effect Transistors (FET) control current by voltage
•They can be used either as switches or as amplifiers
•Diodes and Transistors are the basic building blocks of the
multibillion dollar semiconductor industries
MOSFET 7
MOSFET: Metal Oxide Semiconductor
Field Effect Transistor
• A voltage controlled device
• Handles less current than a BJT
• Two types NMOS and PMOS
• Easier to manufacture
MOSFET 8
MOSFET Structure
MOSFET 9
N-Channel MOSFET Symbol
•D : Drain, S: Source, G: Gate, B: Body
•Normally Body and Source are shorted internally
•Drain current is controlled by controlling Gate to
Source Voltage
MOSFET 10
Biasing N-Channel MOSFET
MOSFET 11
Biasing N-Channel MOSFET (2)
MOSFET 12
Conduction of N-Channel MOSFET
MOSFET 13
Output Characteristics of N-Channel
MOSFET Triode region of
MOSFET is
equivalent to
saturation region of
BJT. Here the
MOSFET works like
a resistor.
Saturation region of
MOSFET is
equivalent to Active
region of BJT. Here
the MOSFET works
as in an amplifier.
MOSFET 14
Relationship between Drain Current and
Gate to Source Voltage of MOSFET
In Triode Region: iD = K [2(vGS-Vto)vDS-v2DS]
In Saturation Region: iD = K [(vGS-Vto)2]
At the Boundary between the Triode and Saturation Region:
iD = K v2DS (How?)
K =Device Constant which can vary from device to device
MOSFET 15
Fixed Plus Self Bias CircuitA Good circuit to establish stable Q point that is relatively
independent of device parameter
21
2DDTHG
RR
RVVV
21
21THG
RR
RRRR
Gate Current iG = 0
(why?)
MOSFET 16
To Find iD and VGS in a Fixed Plus
Self Bias Circuit
•The circuit operates in
Saturation Mode
•Discard the extraneous root
•iD and VGS can also be obtained
by solving the quadratic
equation involving iD or VGS
that can be
obtained using 12.12 and 12.13
(see example 12.2)
Equation 12.12
Equation 12.13
MOSFET 17
Common Source Amplifier(Analogous to Common Emitter Amplifier)
MOSFET 18
Small-Signal Equivalent Circuit Of
MOSFET
MOSFET 19
Reason for Modification of Small- Signal
Equivalent Circuit Of MOSFET
MOSFET 20
Derivation of the Small-Signal Equivalent
Circuit of MOSFET on the Greenboard
MOSFET 21
Small Signal AC Equivalent Circuit
for Common Source Amplifier
MOSFET 22
Derivation of the Av etc. of the CS
amplifier on the Greenboard
MOSFET 23
Source Follower(Analogous to Emitter Follower)
MOSFET 24
Small Signal AC Equivalent Circuit
for Source Follower
MOSFET 25
Derivation of the Av etc. of the Source
Follower on the Greenboard
MOSFET 26
Type of
Amplifier
Input
Impedance
Output
Impedance
CE Few k Few k
Emitter
Follower
Tens of k Few
CS Hundreds of
k
Few k
Source
Follower
Few M Few
Typical Input and Output Impedances of
Different types of Amplifiers
MOSFET 27
Other MOS Application-Active Load
MOS ‘resistor’ occupies a lot less space
on an IC than a resistor
MOSFET 28
Other MOS Application- CMOS Inverter
