EDC UNIT IV- Transistor and FET Characteristics Devices... · 2008 EDC Lesson 19- " , Raj...

2008 EDC Lesson 19- " , Raj Kamal, 1

EDC UNIT IV- Transistor and FET Characteristics

LessonLesson--19: 19: JFET Characteristics JFET Characteristics Quantitative Discussion Quantitative Discussion

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1. Revision1. Revision

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Output CharacteristicsOutput Characteristics

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Output characteristics of an n-channel JFET with the gate short-circuited to the source VGS = 0

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Quantitative DiscussionQuantitative Discussion

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Output characteristics of an nOutput characteristics of an n--channel JFET channel JFET with the gate shortwith the gate short--circuited to the sourcecircuited to the source

The initial rise in ID is related to the buildup of the depletion layer as VDS increases.

The curve approaches the level of the limiting current IDSS when ID begins to be pinched off.

The physical meaning of this term leads to one definition of pinch-off voltage, VP , which is the value of VDS at which the maximum IDSS flows.

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2008 EDC Lesson 19- " , Raj Kamal, 8

Increasing VDS increases the widths of depletion layers, which penetrate more into channel and hence result in more channel narrowing toward the drain.

The resistance of the n-channel, RABtherefore increases with VDS.

The drain current: IDS = VDS/RAB

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2008 EDC Lesson 19- " , Raj Kamal, 10

JFET Transfer CharacteristicsJFET Transfer Characteristics

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There is a convenient relationship between IDSand VGS.

Beyond pinch-off

Where IDSS is drain current when VGS= 0 and VGS(off) is defined as –VP, that is gate-source voltage that just pinches off the channel.

The pinch off voltage VP here is a +ve quantity because it was introduced through VDS(sat).

VGS(off) however is negative, -VP.

2

)(

1

offGS

GSDSSDS V

VII

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Transconductance drain current as a Transconductance drain current as a function of Voltage between gatefunction of Voltage between gate--sourcesource

• Conductance is reciprocal of resistance• D.C. Transconductance Gm = ID ÷

VGS

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The transconductance curveThe transconductance curve The process for plotting

transconductance curve for a given JFET:

Plot a point that corresponds to value of VGS(off).

Plot a point that corresponds to value of IDSS.

Select 3 or more values of VGSbetween 0 V and VGS(off). For value of VGS, determine the corresponding value of ID from equation for ID

Plot the point from (3) and connect all the plotted point with a smooth curve.

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2008 EDC Lesson 19- " , Raj Kamal, 15

JFET Biasing CircuitsJFET Biasing Circuits

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2008 EDC Lesson 19- " , Raj Kamal, 17

Example:Example: Plot the dc bias line for the voltagePlot the dc bias line for the voltage--drivers biasing drivers biasing circuitcircuit

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SummarySummary

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We learnt We learnt

The resistance of the n-channel, RABincreases with VDS.

The drain current: IDS = VDS/RAB

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We learntWe learnt

• Transconductance Gm = ID ÷ VGS• Quantitative Relation

2

)(

1

offGS

GSDSSDS V

VII

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We learnt• When VGS = 0, ID changes nearly

linearly till pinch-off voltage. After pinch-off voltage the drain-source current remains constant at IDSS.

• When VGS is –ve (for n-JFET) at certain value of VGS, ID does not increase and is 0, transconductance = 0

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End of Lesson 19End of Lesson 19