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1. Unipolar device i. e. operation depends on only one type of charge carriers (h or e)
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 applications
5. 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. Very small in size, occupies very small space in ICs9. Low voltage low current operation is possible in MOSFETS 10. Zero temperature drift is possible
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Few important advantages of FET over conventional Transistors
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JFET
MOSFET (IGFET)
October, 16
n-Channel JFET
p-Channel JFET
n-Channel EMOSFET
p-Channel EMOSFET
Enhancement MOSFET
Depletion MOSFET
n-Channel DMOSFET
p-Channel DMOSFET
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 JFETOffset-gate symbol
Gate
Drain
Source
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 +
-
DC Voltage Source
+
-+
-
N
N
Operation of a JFET
Gate
Drain
Source
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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
2DS
DSPGSP
DSSDS
VVVV
V
II
2
2 PGSP
DSSDS
VVV
II
2
1 and
P
GSDSSDS V
VII
Where, IDSS is the short circuit drain current, VP is 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 vDG exceeds 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-ID Characteristics 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
GSDSSDS V
VII
IDSS
VGS (off)=VP
Transfer (Mutual) Characteristics of n-Channel JFET
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Fixed bias circuit Self bias circuit Potential Divider bias circuit
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2
1
P
GSDSSDS V
VII
0, GGSGSGGGG IFixedVVRIV
DDSDDDS
P
GSDSSDS
RIVV
V
VII
and
12
S
GSDS
SDSGS
R
VI
RIV
0
October, 16
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
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RG is present to limit current in case VGG is connected with wrong polarityThis would forward bias the gate-source junction causing high currents, which would destroy the transistor
Since GS is reverse biased, IG = 0 and VRG = 0
GS
GG G G
GG
GS
G
V + I R + V = 0
Sin
V = -
ce I =
V
0
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2GS
D DSSP
VI = I 1 -
V
GS GGV = - VDDSDDDS
P
GSDSSDS
RIVV
V
VII
and
12
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October, 16
or Fixed Bias Ckt.
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DC Equivalent Circuit
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VRG = 0
IG = 0
S
GSDS
SDSGS
R
VI
RIV
0
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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
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G GS D S
GS G D S
-V + V + I R = 0
V = V - I R
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Input Output
2G DD
1 2
R V = V
R + R DS DD D D SV = V - I (R + R )
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The Potential (Voltage) Divider Bias
01
2
S
GSG
P
GSDSS R
VV
V
VI
DSGSI V gives equation quadratic this Solving and
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A Simple CS Amplifier and Variation in IDS with Vgs
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FET Mid-frequency Analysis:
g
s
rd gmv vi = v
ii io
vo
d
s
+ +
_ _
mid-frequency CE amplifier circuit
RD RL RTh vs
+
_
is
' 'o o ivi m L L d D L vs vi
i s s i
ii Th Th 1 2
i
Analysis of the CS mid-frequency circuit above yields:
v v ZA = = -g R , where R = r R R A = = A
v v R + Z
vZ = = R , where R = R R
i
L
o iI vi
i L
o oo d D P vi I
o iseen by R
i Z A = = A
i R
v pZ = = r R A = = A A
i p
A common source (CS) amplifier is shown
to the right.
Rs Ci
RL
Co
CSS vi
vo
+
+
vs
+
_ _
_
io
ii
D
S
G
VDD
VDD
R1
RSS
RD
R2
The mid-frequency circuit is drawn as follows:
• the coupling capacitors (Ci and Co) and the
bypass capacitor (CSS) are short circuits
• short the DC supply voltage (superposition)• replace the FET with the hybrid- model
The resulting mid-frequency circuit is shown below.
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FET Mid-frequency Analysis:
g
s
rd gmv vi = v
ii io
vo
d
s
+ +
_ _
mid-frequency CE amplifier circuit
RD RL RTh vs
+
_
is
' 'o o ivi m L L d D L vs vi
i s s i
ii Th Th 1 2
i
Analysis of the CS mid-frequency circuit above yields:
v v ZA = = -g R , where R = r R R A = = A
v v R + Z
vZ = = R , where R = R R
i
L
o iI vi
i L
o oo d D P vi I
o iseen by R
i Z A = = A
i R
v pZ = = r R A = = A A
i p
A common source (CS) amplifier is shown
to the right.
Rs Ci
RL
Co
CSS vi
vo
+
+
vs
+
_ _
_
io
ii
D
S
G
VDD
VDD
R1
RSS
RD
R2
The mid-frequency circuit is drawn as follows:
• the coupling capacitors (Ci and Co) and the
bypass capacitor (CSS) are short circuits
• short the DC supply voltage (superposition)• replace the FET with the hybrid- model
The resulting mid-frequency circuit is shown below.
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Procedure: Analysis of an FET amplifier at mid-frequency:
1) Find the DC Q-point. This will insure that the FET is operating in the saturation
region and these values are needed for the next step.
2) Find gm. If gm is not specified, calculate it using the DC values of VGS as follows:
3) Calculate the required values (typically Avi, Avs, AI, AP, Zi, and Zo. Use the formulas for
the appropriate amplifier configuration (CS, CG, CD, etc).
DSSDm GS P2
GS P
Dm GS T
GS
GS
2IIg = = V - V (for JFET's and DM MOSFET's)
V V
Ig = = V - V (for EM MOSFET's)
V
(Note: Uses DC value of V )
K
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PE-Electrical Review Course - Class 4 (Transistors)
Example 7:
Find the mid-frequency values for Avi, Avs, AI, AP, Zi,
and Zo for the amplifier shown below. Assume that
Ci, Co, and CSS are large.
Note that this is the same biasing circuit used in Ex. 2, so VGS = -0.178 V.The JFET has the following specifications:
DSS = 4 mA, VP = -1.46 V, rd = 50 k
10 k Ci 8 k
Co
CSS vi
vo
+
+
vs
+
_ _
_
io
ii
D
S
G
18 V 18 V
800 k
2 k
500
400 k
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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
Rs C i
vi
+
vs
+
_
_
ii G
VDD
VDD
R1
RSS
R2
Common Drain (CD) Amplifier (also called “source follower”)
RL
C o
vo
+
_
io
D
S
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Figure: Circuit symbol for an enhancement-mode n-channel MOSFET.
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Figure: n-Channel Enhancement MOSFET showing channel length L and channel width W.
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Figure: For vGS < Vto the pn junction between drain and body is reverse biased and iD=0.
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Figure: For vGS >Vto a channel of n-type material is induced in the region under the gate. As vGS increases, the channel becomes thicker. For small values of vDS ,iD is proportional to vDS.
The device behaves as a resistor whose value depends on vGS.
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Figure: As vDS increases, the channel pinches down at the drain end and iD increases more slowly. Finally for vDS> vGS -Vto, iD becomes constant.
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October, 16
Locus of points where
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Figure: Drain characteristics
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Figure: This circuit can be used to plot drain characteristics.
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Figure: Diodes protect the oxide layer from destruction by static electric charge.
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Figure: Simple NMOS amplifier circuit and Characteristics with load line.
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Figure: Drain characteristics and load line
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Figure vDS versus time for the circuit of Figure 5.13.
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Figure Fixed- plus self-bias circuit.
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Figure Graphical solution of Equations (5.17) and (5.18).
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Figure Fixed- plus self-biased circuit of Example 5.3.
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Figure The more nearly horizontal bias line results in less change in the Q-point.
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Figure Small-signal equivalent circuit for FETs.
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Figure FET small-signal equivalent circuit that accounts for the dependence of iD on vDS.
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Figure Determination of gm and rd. See Example 5.5.
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Figure Common-source amplifier.
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For drawing an a c equivalent circuit of Amp.
•Assume all Capacitors C1, C2, Cs as short circuit elements for ac signal
•Short circuit the d c supply
•Replace the FET by its small signal model
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LgsmLoo
gs
ov
RvgRiv
v
vA
gain, Voltage
dDLLmgs
ov
rRRRgv
vA ,
Dd
DdDdo Rr
RrRrZ
imp., put Out 21
imp., Input RRRZGin
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A C Equivalent Circuit
Simplified A C Equivalent Circuit
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)R||(rgAv Ddm
DR10r D,m
dRgAv
)R||(rgAv Ddm
October, 16
This is a CS amplifier configuration therefore the input is on the gate and the output is on the drain. 21 R||RZi
Dd R||rZo
DdD 10RrRZo
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Figure vo(t) and vin(t) versus time for the common-source amplifier of Figure 5.28.
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Figure Common-source amplifier.
An Amplifier Circuit using MOSFET(CS Amp.)
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Figure Small-signal equivalent circuit for the common-source amplifier.
A small signal equivalent circuit of CS Amp.
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Figure vo(t) and vin(t) versus time for the common-source amplifier of Figure 5.28.
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Figure Gain magnitude versus frequency for the common-source amplifier of Figure 5.28.
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Figure Source follower.
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Figure Small-signal ac equivalent circuit for the source follower.
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Figure Equivalent circuit used to find the output resistance of the source follower.
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Figure Common-gate amplifier.
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Figure See Exercise 5.12.
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Figure Drain current versus drain-to-source voltage for zero gate-to-source voltage.
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Figure n-Channel depletion MOSFET.
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Figure Characteristic curves for an NMOS transistor.
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Figure Drain current versus vGS in the saturation region for n-channel devices.
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Figure p-Channel FET circuit symbols. These are the same as the circuit symbols for n-channel devices, except for the directions of the arrowheads.
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Figure Drain current versus vGS for several types of FETs. iD is referenced into the drain terminal for n-channel devices and out of the drain for p-channel devices.
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