malvino electronic principles sixth edition. chapter 13 jfets
TRANSCRIPT
MALVINO
ElectronicPRINCIPLES
SIXTH EDITION
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 (mho) or the more modern microsiemen (S).
• 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.