Analog Electronics II Week 5 Chp 2 (Dc Biasing) Cont'

8/14/2019 Analog Electronics II Week 5 Chp 2 (Dc Biasing) Cont'

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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


2/18

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.


3/18


4/18

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.


5/18

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 :


7/18

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.


8/18

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 :


10/18

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


12/18

Discussion 2

12

What are the

voltages across

RD and RS?


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=-==

= -

= -= -

= +-== - +( )

== -

=

=+-


15/18

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


16/18

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


18/18

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

Analog Electronics II Week 5 Chp 2 (Dc Biasing) Cont'

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