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FET ( Field Effect Transistor)
1. Unipolar device i. e. operation depends on only one type ofcharge carriers (hor e)
2. Voltage controlled Device (gate voltage controls draincurrent)
3. Very high input impedance (109-1012 )
4. Source and drain are interchangeable in most Low-frequencyapplications
5. Low Voltage Low Current Operation is possible (Low-powerconsumption)
6. Less Noisy as Compared to BJT
7. No minority carrier storage (Turn off is faster)
8. Self limiting device
9. Very small in size, occupies very small space in ICs
10. Low voltage low current operation is possible in MOSFETS
11. Zero temperature drift of out put is possiblek
Few important advantages of FET over conventional Transisto
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Types of Field Effect Transistors(The Classification)
JFET
MOSFET(IGFET)
n-Channel JFET
p-Channel JFET
n-ChannelEMOSFET
p-ChannelEMOSFET
EnhancementMOSFET
DepletionMOSFET
n-ChannelDMOSFET
p-ChannelDMOSFET
FET
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Figure: n-Channel JFET.
The Junction Field Effect Transistor (JFET)
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Gate
Drain
Source
SYMBOLS
n-channel JFET
Gate
Drain
Source
n-channel JFET
Offset-gate symbol
Gate
Dra
S
p-channel JFET
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Figure: n-Channel JFET and Biasing Circuit.
Biasing the JFET
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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
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P P+
-
+
-
+
-
N
N
Operation of a JFET
Gate
Drain
Source
O D i (V I ) Ch i i f JFET
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Figure: Circuit for drain characteristics of the n-channel JFET and its Drain characteristics.
Non-saturation (Ohmic) Region:
The drain current is given by
2
2
2
2
DS
DSPGS
P
DSS
DS
VVVV
V
II
2
2 PGS
P
DSS
DSVV
V
I
I
2
1and
P
GS
DSSDS V
V
II
Where,IDSSis the short circuit drain current, VPis the pinch off voltage
Output or Drain (VD-ID) Characteristics of n-JFET
Saturation (or Pinchoff) Region:
PGSDSVVV
PGSDSVVV
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Figure: n-Channel FET for vGS = 0.
Simple Operation and Break down of n-Channel JFET
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Figure: If vDGexceeds the breakdown voltage VB, drain current increases rapidly.
Break Down Region
N-Channel JFET Characteristics and Breakdown
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Figure: Typical drain characteristics of an n-channel JFET.
VD-IDCharacteristics of EMOS FET
Saturation or Pinch off
Reg.
Locus of pts where PGSDS VVV
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Figure: Transfer (or Mutual) Characteristics of n-Channel JFET
2
1
P
GS
DSSDS V
V
II
IDSS
VGS (off)=VP
Transfer (Mutual) Characteristics of n-Channel JFET
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JFET Transfer CurveThis graph shows the value of IDfor a givenvalue of VGS
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Biasing Circuits used for JFET
Fixed bias circuit Self bias circuit
Potential Divider bias circuit
JFET (n channel) Biasing
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JFET (n-channel) BiasingCircuits
2
1
P
GS
DSSDS V
V
II
GSGSGGGG FixedVVRIV
DDSDDDS
P
GS
DSSDS
RIVV
V
VII
and
1
2
S
GS
DS
SDSGS
R
VI
RIV
0
For Self Bias Circuit
For Fixed Bias Circuit
Applying KVL to gate circuit we g
and
Where, Vp=VGS-off& IDSSis Short ckt
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JFET BiasingCircuits Count
or Fixed Bias Ckt.
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JFET Self (or Source) Bias Circuit
2
1and
P
GS
DSSDS V
V
II
S
GS
P
GS
DSS R
V
V
V
I
2
1
21
2
P
GS
P
GS
DSS R
V
V
V
V
V
I
This quadratic equation can be solved for VGS& I
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The Potential (Voltage) Divider Bias
01
2
S
GSG
P
GS
DSS R
VV
V
V
I
DSGS
IVgivesequationquadraticthisSolving and