FET ( Field Effect Transistor)
description
Transcript of FET ( Field Effect Transistor)
FET ( Field Effect Transistor)
1. Unipolar device i. e. operation depends on only one type of charge carriers (holes or electrons)
2. Voltage controlled Device (gate voltage controls drain current)3. Very high input impedance (109-1012 )4. Source and drain are interchangeable in most Low-frequency applications5. Low Voltage Low Current Operation is possible (Low-power consumption)6. Less Noisy as Compared to BJT7. No minority carrier storage (Turn off is faster) 8. Self limiting device9. Very small in size, occupies very small space in ICs10. Low voltage low current operation is possible in MOSFETS 11. Zero temperature drift of out put is possible.
Few important advantages of FET over conventional Transistors
JFET
MOSFET (IGFET)
n-Channel JFET
p-Channel JFET
Types of Field Effect Transistors (Classifications)
n-Channel EMOSFET
p-Channel EMOSFET
Enhancement MOSFET
Depletion MOSFET
n-Channel DMOSFET
p-Channel DMOSFET
FET
Figure: n-Channel JFET.
The Junction Field Effect Transistor (JFET)
Gate
Drain
Source
JFET SYMBOLS
n-channel JFET
Gate
Drain
Source
n-channel JFET(Offset-gate symbol)
Gate
Drain
Source
p-channel JFET
Figure: n-Channel JFET and Biasing Circuit.
Biasing the JFET
Figure: The nonconductive depletion region becomes broader with increased reverse bias.
(Note: The two gate regions of each FET are connected to each other.)
Operation of JFET at Various Gate Bias Potentials
P P +
-
DC Voltage Source
+
-+
-
N
N
Operation of a JFET
Gate
Drain
Source
Figure: n-Channel FET for vGS = 0.
Simple Operation and Break down of n-Channel JFET
Figure: If vDG exceeds the breakdown voltage VB, drain current increases rapidly.
Break Down Region
N-Channel JFET Characteristics and Breakdown
Figure: Transfer (or Mutual) Characteristics of n-Channel JFET
2
1
P
GSDSSDS V
VII
IDSS
VGS (off)=VP
Transfer (Mutual) Characteristics of n-Channel JFET
Biasing Circuits used for JFET
Fixed bias circuit Self bias circuit Potential Divider bias circuit
JFET (n-channel) Biasing Circuits
2
1
P
GSDSSDS V
VII
0, GGSGSGGGG IFixedVVRIV
DDSDDDS
P
GSDSSDS
RIVV
V
VII
and
12
S
GSDS
SDSGS
R
VI
RIV
0
For Self Bias Circuit
For Fixed Bias Circuit
Applying KVL to gate circuit we get
and
Where, Vp=VGS-off & IDSS is Short ckt. IDS
JFET Biasing Circuits Count…or Fixed Bias Ckt.
JFET Self (or Source) Bias Circuit
2
1 and
P
GSDSSDS V
VII
S
GS
P
GSDSS R
V
V
VI
2
1
021
2
S
GS
P
GS
P
GSDSS R
V
V
V
V
VI
This quadratic equation can be solved for VGS & IDS
The Potential (Voltage) Divider Bias
01
2
S
GSG
P
GSDSS R
VV
V
VI
DSGSI V gives equation quadratic this Solving and
A Simple CS Amplifier and Variation in IDS with Vgs
FET Amplifier Configurations
and Relationships:
'' ' m L
vi m L m L 'm L
'L d D L d D L SS L
i Th SS Thm
o d D d D SSm
i i ivs vi vi vi
s i s i s i
i i iI vi vi vi
L L L
P vi I vi I
CS CG CD
g RA -g R g R
1 g R
R r R R r R R R R
1Z R R R
g
1Z r R r R R
g
Z Z ZA A A A
R + Z R + Z R + Z
Z Z ZA A A A
R R R
A A A A A
vi I
Th 1 2
A A
where R = R R
VCC
RD
S
R2
RSS
Rs Ci
RL
Co
C2
vi vo
+
+
vs
+
_
_ _
io ii
Common Gate (CG) Amplifier
R1
D
G
Note: The biasing circuit is the same for each amp.
Rs Ci
RL
Co
CSS vi
vo
+
+
vs
+
_ _
_
io
ii
D
S
G
VDD
VDD
R1
RSS
RD
R2
Common Source (CS) Amplifier