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JFET

Bollen

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AGENDA

Bollen

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 =

Field

Effect

Transistor

N-channel

or

P-channel

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J-FET, OVERVIEW

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JFET vs BJT

Bollen

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J-FET vs BJT

Bollen

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Physical structure

Bollen

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 Idss

In 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

Bollen

JFET

is not often used in the ohmic (triode) region

If 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

Bollen

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JFET ac parameters

Bollen

2

1

p

gsdssds U

UII

V

mA

U

Ig

gs

dsm

dss

ds

p

dssm I

I

U

Ig

2

3,33mg mS or mmho or m

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JFET simulation parameters micro cap

Bollen

pto VV

2p

dss

U

Ibeta

VTO Threshold voltage

BETA Transconductance parameter

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JFET fixed bias

Bollen

Steady Ugs,

no variation possible,

so we make the same effect with resistors. U = I . R

Now we can vary around the bias point

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JFET self bias

Bollen

Ug = 0 volt, NO gate current

Ugs = Ug – Us = 0 – Ids.Rs

Rin = 1 Meg Ohm

2

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 volt

Ugs = Ug – Us

= Vg – Ids.Rs,

bias is made at

GATE and SOURCE

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MOSFET

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Metal

Oxide

Silicium

Field

Effect

Transistor

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MOSFET

Bollen

2

2p

ds gs TH

K WI V V

L

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

Bollen

2

2p

ds gs TH

K WI V V

L

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MOSFET dc

Bollen

2

2p

ds gs TH

K WI V V

L

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MOSFET ac

Bollen

m p gs TH

Wg K V V

L