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ANALOGELECTRONICS II
BEE2233
Week 5 Chapter II - FET DC BIASING- E-MOSFETFET DC BIASING- E-MOSFET
Mohd Shawal JadinMohd Shawal Jadin
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Enhancement-Type MOSFET
The transfer characteristic for the enhancement-type MOSFET is very different from that
of a simple JFET or the depletion-type MOSFET.
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Feedback Biasing Q-Point
Plot the line using VGS = VDD, ID = 0 and ID = VDD / RD and VGS = 0 Plot the transfer curve using VGSTh , ID = 0 and VGS(on), ID(on); all given in the
specification sheet.
Where the line and the transfer curve intersect is the Q-Point.
Using the value of ID at the Q-point, solve for the other variables in the bias
circuit.
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DC analysis step for Feedback Biasing
Enhancement type MOSFET
Find k using the datasheet or specification given;Find k using the datasheet or specification given;
ex: Vex: VGS(ON)GS(ON),V,VGS(TH)GS(TH)
Plot transfer characteristics using the formulaPlot transfer characteristics using the formulaIIDD=k(V=k(VGSGS V VTT))
22..
Three point already defined that is IThree point already defined that is ID(ON)D(ON), V, VGS(ON)GS(ON)
and Vand VGS(TH)GS(TH) Plot a point that is slightly greater than VPlot a point that is slightly greater than VGSGS
Plot the linear characteristics (network bias line)Plot the linear characteristics (network bias line)
The intersection defines the Q-pointThe intersection defines the Q-point
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Voltage-Divider Biasing
Again plot the line and the transfer curve to find the Q-
point.
Using the following equations:
21
DD2G
RR
VRV
+
=
)( DSDDDDS
SDGGS
RRIVV
RIVV
+=
=
Input loop :
Output loop :
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Voltage-Divider Bias Q-Point
Plot the line using VGS = VG = (R2VDD)/(R1 + R2), ID = 0 and ID =
VG/RS and VGS = 0
Find k
Plot the transfer curve using VGSTh, ID = 0 and VGS(on), ID(on); all
given in the specification sheet.
Where the line and the transfer curve intersect is the Q-Point.
Using the value of ID at the Q-point, solve for the other variables
in the bias circuit.
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Example
8
Determine the valueof :
IDQ
VGSQ
VDSQ VD VS
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Solution
9
For graphical method:
Find k (answ: 0.556 mA/V2)
Plot the transfer curve by using equation, ID=k(VGS
VT)2.
Plot the dc line by using dc equivalent circuit determine Q point
21
DD2G
RR
VR
V +=
)( DSDDDDS
SDGGS
RRIVV
RIVV
+=
=
Input loop :
Output loop :
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Solution
10
IDQ = 5.5 mA and VGSQ = 6.2 V
VDSQ
VD VS
VRIV
VRIVV
VRRIVV
SDS
DDQDDD
DSDDDDSQ
13.4
9.11
93.14)(
==
==
=+=
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Discussion 1
11
Determine the value
of VDSQ
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Discussion 2
12
What are the
voltages across
RD and RS?
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=-==
= -
= -= -
= +-== - +( )
== -
=
=+-
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Design
For the design, we need toFor the design, we need toconsider at least 4 things;consider at least 4 things;
Area of application
Level of amplification
Signal strength
Operating condition
Design using the middle valuesDesign using the middle valuesof transfer characteristics isof transfer characteristics ispreferablepreferable
Examples
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P-Channel FETs
For p-channel FETs the same calculations and graphs are used, except that the voltage
polarities and current directions are the opposite. The graphs will be mirrors of the n-channel
graphs.
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Practical Applications
Voltage-Controlled Resistor
JFET Voltmeter
Timer Network
Fiber Optic Circuitry
MOSFET Relay Driver
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Thank You
End of Chapter 2
18
Test 1:
Tuesday Night 10 Feb 2009
Chapter 1 & 2 only
Lab 1:
1st week Theory, simulation &
calculation
2nd week (now) hardware experiment3rd - hardware experiment
4th week writing final report &
submission