Download FET Part 2

GOVERMENT
ENGINEERING COLLEGE
EC Department
Sem 3
Sub - EDC
FIELD-EFFECT
TRANSISTORS (FET)
PART 2
PRESANTED BY :
BHATT YASHASVI HIRENBHAI
(130210111007)
FET AS A VOLTAGE VARIABLE
RESISTOR (VVR)
• FET is a device that is usually operated in the
•
•
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constant current portion of its output
characteristics for the linear application.
JFET in the region just before pitch off. In the
region Vds is small but Id is large.
This region is marked as ohmic region in the
output characteristics of a JFET.
The drain to source resistance rds in this region
is dependent on the gate to source voltage.
• Small signal FET drain resistance rd varies with
applied gate voltage Vgs and FET act like a
VERIABLE PASSIVE RESISTOR.
Application of VVR AGC
• Automatic gain control (AGC) is used in most
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•
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modern receiver.
The FET used as VVR finds applications in many
areas where property can be used.
The VVR for example can be used to vary the
voltage gain of a multistage amplifier “A” as the
signal level is increased. This called automatic
gain control.
The gain of a CE amplifier is approximately
equal to –RC/RE.
METAL INSULATOR
SEMICONDUCTOR FIELD EFFECT
TRANSISTOR (MISFETs)
• Metal insulator semiconductor field effect
•
•
transistor is one of the most widely used
electronic devices.
Generally as a insulated gate field effect
transistor (IGFET).
Most of such devices are made using silicon for
the semiconductor,Sio2 for the insulator, and
metal or heavily doped polysilicon for the gate
electrode.
• The term metal oxide semiconductor field effect
•
•
transistor (MOSFET) or simply MOS is commonly
used.
The operation of MOSFET is similar to that of
JFET.
All equations apply equally well to the MOSFET
and JFET in amplifier connection.
• MOSFETs are of two types namely
1) depletion type MOSFET or DE-MOSFET
2) enhancement type MOSFET or E-MOSFET
STURCTURE OF MOSFET TYPES
•
DE-MOSFET
•
E-MOSFET
1) DEPLETION TYPE OF MOSFET
• Construction of the
•
depletion type
MOSFET is as shown
in fig…
A p type of
semiconductor silicon
is used as a
substrate.
Operation
The operation can be explained with three
different operating conditions
1) operation with Vgs =0
2) operation with negative Vgs
3) operation with positive Vgs
• The level of free electrons has been
“enhanced” due to the application of
positive gate voltage. Therefore the region
of operation corresponding to the positive
gate current is called as ‘enhancement’
region of operation and the region
between cutoff and saturation is referred
to as depletion region this is shown in fig
Characteristics
• The drain characteristics of a N channel
depletion MOSFET. note that this
characteristics is same as that of a JFET
except for the positive part of Vgs.
Schematic Symbol
2) ENHANCEMENT MOSFET
• The basic
•
construction of an n
channel enhancement
type of MOSFET is as
shown in fig..
A slab of p type
semiconductor is used
as substrate.
Operation
The operation can be explained with two
different operating conditions
1)operation with Vgs=0 volt
2)operation with Vgs is positive
Characteristics
• The drain Characteristics and transfer
characteristics of a n channel
enhancement MOSFET are as shown in
fig..
Schematic Symbol
MOSFET RESISTOR
• The MOSFET can be employed as a
resistor.
• DE MOSFET can be employed as a resistor
by connecting the gate to the source as
shown in fig..
• E MOSFET may also be employed as a
resistor by connecting the gate to the
drain as shown in fig..
• So that gate-to-source voltage Vgs
becomes equal to drain-to-source voltage
Vds.
COMPARISON
• N channel FETs
• P channel FETs
• Current carriers –
• Current carriers –
•
•
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Electrons
Mobility – Large
Noise - Less
Tranceconductance Larger
holes
Mobility – Poor
Noise –More
Tranceconductance Lesser
COMPARISON
• JFET
• MOSFET
• JFET are of two types
• MOSFETs can be of
•
•
,p-channel and nchannel JFETs.
JFETs do not have the
insulated gate.
• Input impedance is
lower than that of the
MOSFETs.
•
depletion type or
enhancement type.
MOSFETs have the
insulated gate
structure.
Input impedance is
higher than that of
JFET due to the
insulated gate
structure.
• Drain resistance is
• Drain resistance is
•
•
•
lower than that of a
MOSFET.
The input resistance
rgs and the feedback
resistance rgd are
very much smaller
than that of MOSFET.
Biasing methods used
are self bias and
voltage divided bias.
•
higher than that of a
JFET.
rgs and rgd are very
much larger than that
of a JFET.
Biasing circuits of
depletion MOSFET are
same as those used
for JFET but biasing
circuit for
enhancement
MOSFET are feedback
biasing and voltage
divider biasing.
THANK YOU