1. MALVINO
Electronic
PRINCIPLES
SIXTH EDITION

2. JFETs
Chapter 13

3. Junction field effect transistor (JFET)
Drain G n
Gate p p S
VDD D
VGG G
Source S

4. 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

5. Drain family of curves 10 0 V 8 VGS 6 -1 V ID in mA 4 -2 V 2 -3 V -4 V5 10 15 20
VDS in Volts

6. 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

7. IDSS 10
0 V 8
VP = 4 volts
VGS
Constant current region 6
-1 V
ID in mA 4
-2 V 2
-3 V
-4 V 5 10 15 20
VDS in Volts
VGS(off)

8. 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

9. 10
0 V 8
Ohmic region
VGS 6
-1 V
ID in mA 4
-2 V 2
-3 V
-4 V 5 10 15 20
VDS in Volts

10. ( ) Transconductance curve 10 8 ID = IDSS 1- VGS VGS(off) 2 6 ID in mA4 2 -4 -3 -2 -1
VGS in volts

11. and ID(sat) << IDSS.
Gate bias is suitable for the ohmic region.
+VDD
+VDD RD
RDS
Equivalent
circuit RD
ID(sat) =
VDD RD
Use 0 volts for VGS
and ID(sat) << IDSS. RG
-VGG

12. Q point in the ohmic region10
0 V 8
VGS
ID(sat) Q 6
-1 V
ID in mA 4
-2 V 2
-3 V
-4 V 5 10 15 20
VDS in Volts
VDD
ID RDS

13. Gate bias is not suitableRD R2
+VDD R1 RS
Voltage-divider bias
Gate bias is not suitable
for the active region.
ID(sat) =
VDD
RD + RS
VS = VG - VGS
VG - VGS RS
IDQ =

14. Q point in the active region10
0 V 8
VGS
ID(sat) 6
-1 V
ID in mA 4 Q
-2 V 2
-3 V
-4 V 5 10 15 20
VDS in Volts
VDD

15. 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.

16. ( ) Transconductance curve 8 1- VGS VGS(off) gm = gm0 6 4 ID in mA 2
Max. slope
gm0 8 1-
VGS
VGS(off) ( )
gm = gm0 6
Larger
slope 4
ID in mA 2 -4 -3 -2 -1
Smaller
slope
VGS in volts

17. Common-source amplifier
+VDD
rd = RD RL
A = gmrd RD R1
vout RL R2
vin RS

18. Source follower +VDD rs = RS RL RD R1 gmrs A = 1 + gmrs R2 vin RS vout

19. vout
vin
VGS
Shunt analog switch RD
vin < 100 mV
RD >> RDS
Series analog switch
vout
vin
VGS RD
Better on-off ratio
than the shunt switch

20. Multiplexer
vin1
vin2
vin3
vout RD V1 V2 V3

21. 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.