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JFET

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AGENDA

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JFETOverviewJFET vs BJTPhysical structureWorkingInput parametersDC formulaOutput parametersTransconductanceAc parametersSimulatioin parameters micro cap

JFETFixed biasSelf biasVoltage divider biasMOSTFETComponentDCac

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J-FET

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FET =FieldEffectTransistorN-channelorP-channel

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J-FET, OVERVIEW

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JFET vs BJT

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J-FET vs BJT

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Physical structure

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There is one PN junction from gate to channel used in REVERSE

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JFET working

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JFET working

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JFET input parameters

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Vp and IdssIn data sheet p = pinch-off dss = drain source shorted or saturated= maximum current

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JFET DC formula

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2

1 gsds dss

p

UI I

U

ID

0–VGS

10 mA

8.0

6.0

4.0

2.0

(mA)

1 2 3 4

10dssI mA

4pU Volt

Solve with abc formula

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JFET Output parameters

Bollen Uds < Up = ohmic region, Uds > Up = saturated region

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JFET

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JFET

is not often used in the ohmic (triode) regionIf so it is used as an variable resistor

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JFET output

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JFET output

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JFET transconductance

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JFET transconductance

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JFET ac parameters

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1

p

gsdssds U

UII

VmA

UIggs

dsm

dss

ds

p

dssm I

IUIg 2

3,33mg mS or mmho or m

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JFET simulation parameters micro cap

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pto VV

2p

dss

UIbeta

VTO Threshold voltageBETA Transconductance parameter

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JFET fixed bias

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Steady Ugs, no variation possible,so we make the same effect with resistors. U = I . RNow we can vary around the bias point

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JFET self bias

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Ug = 0 volt, NO gate currentUgs = Ug – Us = 0 – Ids.RsRin = 1 Meg Ohm

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1

p

gsdssds U

UII

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JFET self bias

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Varying Rs = varying the Q-point= operating point

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JFET voltage divider bias

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Ug 0 voltUgs = Ug – Us = Vg – Ids.Rs,bias is made at GATE and SOURCE

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MOSFET

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Metal Oxide

SiliciumFieldEffectTransistor

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MOSFET

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2p

ds gs TH

K WI V VL

gs

th

W with m

L length m

V gate source voltage

V treshold voltage

2tan /

ds

p

I FET drain current mA

K transconduc ce parameter mA V

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MOSFET

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2

2p

ds gs TH

K WI V VL

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MOSFET dc

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2

2p

ds gs TH

K WI V VL

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MOSFET ac

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m p gs THWg K V VL