JFETs
Chapter 13
Junction field effect transistor (JFET)
D
Drain
G
n
Gate
p
p
S
VDD
D
VGG
Source
G
S
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
Drain family of curves
10
0V
8
VGS
6
-1 V
ID in mA
4
-2 V
2
0
5
10
15
VDS in Volts
20
-3 V
-4 V
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
IDSS
10
0V
8
VP = 4 volts
Constant current region
6
VGS
-1 V
ID in mA
4
-2 V
2
0
5
10
15
VDS in Volts
20
-3 V
-4 V
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
10
0V
8
Ohmic region
VGS
6
-1 V
ID in mA
4
-2 V
2
0
5
10
15
VDS in Volts
20
-3 V
-4 V
Transconductance curve
10
ID = IDSS
(
VGS
1VGS(off)
8
2
)
6
ID in mA
4
2
-4
-2
-3
VGS in volts
-1
0
Gate bias is suitable for the ohmic region.
+VDD
+VDD
RD
VDD
ID(sat) =
RD
RD
RDS
RG
-VGG
Use 0 volts for VGS
and ID(sat) << IDSS.
Equivalent
circuit
Q point in the ohmic region
10
0V
8
ID(sat)
6
VGS
Q
-1 V
ID in mA
4
-2 V
2
0
ID RDS
5
10
VDS in Volts
15
20
VDD
-3 V
-4 V
Voltage-divider bias
+VDD
R1
Gate bias is not suitable
for the active region.
RD
VDD
ID(sat) =
RD + RS
VS = VG - VGS
R2
RS
VG - VGS
IDQ =
RS
Q point in the active region
10
0V
8
VGS
ID(sat)
6
-1 V
ID in mA
4
Q
-2 V
2
0
5
10
VDS in Volts
15
20
VDD
-3 V
-4 V
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.
Transconductance curve
Max. slope
gm0
8
gm = gm0
(
VGS
1VGS(off)
)
6
Larger
slope
4
ID in mA
2
-4
-3
Smaller
slope
-2
-1
VGS in volts
0
Common-source amplifier
+VDD
rd = RD RL
R1
RD
A = gmrd
RL vout
vin
R2
RS
Source follower
+VDD
R1
RD
rs = RS RL
A=
vin
gmrs
1 + gmrs
R2
RS
RL vout
Shunt analog switch
vout
vin
RD
vin < 100 mV
VGS
RD >> RDS
Series analog switch
vout
vin
RD
VGS
Better on-off ratio
than the shunt switch
Multiplexer
vin1
vin2
vout
vin3
RD
V1
V2
V3
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.