Chapter 13

JFETs

Junction field effect transistor (JFET)

n

pp

Source

Gate

Drain

VDD

VGG

G

S

D

G

S

D

JFET

• Unipolar device (one polarity of charge carrier)

• Voltage controlled (gate voltage controls drain current)

• High input impedance• No minority carrier storage• Source and drain are interchangeable in

most low-frequency applications

0

VDS in Volts

ID in mA

-4 V

0 V

-1 V

-2 V

-3 V

VGS

5 10 15 20

2

4

6

8

10

Drain family of curves

Drain curves

• With VGS = 0 the drain current is maximum at IDSS

• VP = the pinchoff voltage

• When VDS = VP the depletion layers almost touch

• With VDS > VP the JFET acts as a current source

• VGS(off) = -VP

0

VDS in Volts

ID in mA

-4 V

0 V

-1 V

-2 V

-3 V

VGS

5 10 15 20

2

4

6

8

10

VP = 4 volts

IDSS

Constant current region

VGS(off)

Ohmic region

• VP separates the active region from the ohmic region.

• The ohmic region is the almost vertical part of the drain curve.

• In this region, a JFET acts as a resistor.• RDS = VP/IDSS

0

VDS in Volts

ID in mA

-4 V

0 V

-1 V

-2 V

-3 V

VGS

5 10 15 20

2

4

6

8

10

Ohmic region

0

2

4

6

8

10

-4 -3 -2 -1VGS in volts

ID in mA

Transconductance curve

ID = IDSS 1-VGS

VGS(off)( )

2

RD

RG

+VDD

-VGG

Gate bias is suitable for the ohmic region.

ID(sat) =VDD

RD

Use 0 volts for VGS

and ID(sat) << IDSS.

+VDD

RD

RDS

Equivalentcircuit

0

VDS in Volts

ID in mA

-4 V

0 V

-1 V

-2 V

-3 V

VGS

5 10 15 20

2

4

6

8

10

VDD

ID(sat)

ID RDS

Q

Q point in the ohmic region

RD

R2

+VDD

R1

RS

Voltage-divider bias

Gate bias is not suitablefor the active region.

VS = VG - VGS

ID(sat) =VDD

RD + RS

VG - VGS

RS

IDQ =

0

VDS in Volts

ID in mA

-4 V

0 V

-1 V

-2 V

-3 V

VGS

5 10 15 20

2

4

6

8

10

VDD

ID(sat)

Q

Q point in the active region

Transconductance• Tells how effective the gate voltage is in

controlling the drain current.• gm = id/vgs

• Common units for JFETs are the micromho (mmho) or the more modern microsiemen (mS).

• gm is the slope of the transconductance curve.

• gm0 is the maximum value and occurs at VGS = 0.

0

2

4

6

8

-4 -3 -2 -1

VGS in volts

ID in mA

Transconductance curve

gm = gm0 1-VGS

VGS(off)( ) Larger

slope

Max. slopegm0

Smallerslope

RD

R2

+VDD

R1

RS

vin

RLvout

Common-source amplifier

rd = RD RL

A = gmrd

RD

R2

+VDD

R1

RS

vin

RLvout

Source follower

rs = RS RL

A =gmrs

1 + gmrs

voutvin

VGS

Shunt analog switch

Series analog switch

RD

voutvin

VGS

RD

RD >> RDS

vin < 100 mV

Better on-off ratiothan the shunt switch

voutvin3

V3

RD

vin2

V2

vin1

V1

Multiplexer

Voltage-controlled resistance

• Operates in the ohmic region with VGS values between 0 and cutoff.

• Works well for ac signals of 200 mVPP or less.

• Small-signal resistance: rds = VDS/ID

• As VGS becomes more negative, rds increases.

• Both series and shunt operation can be used.