1 JFET Bollen. 2 AGENDA Bollen JFET Overview JFET vs BJT Physical structure Working Input parameters...

1

JFET

Bollen

2

AGENDA

Bollen

JFETOverviewJFET vs BJTPhysical structureWorkingInput parametersDC formulaOutput parametersTransconductanceAc parametersSimulatioin parameters micro cap

JFETFixed biasSelf biasVoltage divider biasMOSTFETComponentDCac

3

J-FET

Bollen

FET =

Field

Effect

Transistor

N-channel

or

P-channel

4

J-FET, OVERVIEW

Bollen

5

JFET vs BJT

Bollen

6

J-FET vs BJT

Bollen

7

Physical structure

Bollen

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

8

JFET working

Bollen

9

JFET working

Bollen

10

JFET input parameters

Bollen

Vp and Idss

In data sheet

p =

pinch-off

dss =

drain source shorted or saturated

= maximum current

11

JFET DC formula

Bollen

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

12

JFET Output parameters

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

13

JFET

Bollen

JFET

is not often used in the ohmic (triode) region

If so it is used as an variable resistor

14

JFET output

Bollen

15

JFET output

Bollen

16

JFET transconductance

Bollen

17

JFET transconductance

Bollen

18

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

19

JFET simulation parameters micro cap

Bollen

pto VV

2p

dss

U

Ibeta

VTO Threshold voltage

BETA Transconductance parameter

20

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

21

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

22

JFET self bias

Bollen

Varying Rs

= varying the Q-point

= operating point

23

JFET voltage divider bias

Bollen

Ug 0 volt

Ugs = Ug – Us

= Vg – Ids.Rs,

bias is made at

GATE and SOURCE

24

MOSFET

Bollen

Metal

Oxide

Silicium

Field

Effect

Transistor

25

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

26

MOSFET

Bollen

2

2p

ds gs TH

K WI V V

L

27

MOSFET dc

Bollen

2

2p

ds gs TH

K WI V V

L

28

MOSFET ac

Bollen

m p gs TH

Wg K V V

L