FET Transistor
Islamic university –Gaza
Electrical Engineering
Electronic II Lab
Fall-2011iugaza2010.blogspot.com
BJT FETCurrent Control Device Voltage Control Device
Bipolar(electrons,holes) Unipolar
Two types:NPN,
PNP
Two types:N-channel,
p-channel
Large size Small size
Low input impedance low power consumption
High input impedance
Larger voltage gain Smaller voltage gain
Application :
-Amplification(best)
-Switching
Application :
-Amplification
-Switching(best)
Ig=o
ID=IS
VBE=0.7 V
IC=IE
IC=βIB
FET Transistor
2
D DSSI =I * 1gs
p
V
V
the flow of current through the N channel is negative
in the form of electrons
the flow of current through the P channel is positive
in the form of holes.
N-channel JFET's have a greater channel conductivity
(lower resistance)
N-channel JFET's is more efficient conductor
compared to P-channel .
There are two depletion regions without current.
CHARACTERISTICS OF JFETs
IG=0
ID=IS
If VDS increase ID increase
Depletion region will be wider
At VDS=Vp ID=IDSS
: max current(at saturation)
When Vgs=Vp ID=0
The result of applying a negative bias to the gate is to
reach the saturation level at a lower level of VDS .
Ohmic or voltage-controlled resistance region
Steps:
•Connect the circuit shown
•Chang VDC from 0-25V while simultaneously monitoring
the voltage across RD(VRD=ID*RD).
•Stop raising once the VRD ceases to rise significantly.
•Calculate ID which is IDSS.
Part(1) Calculating IDSS
VRD=ID*RDVdc
1
2
3
4
5
6
7
8
9
10
11
12
15
Vdc
VRD
IDSS=…….
Note:
The maximum current IDSS occurs when VGS = 0 V and VDS ≥ |Vp|
Steps:
Replace the short from Vgs with the output of
negative (0-25)V power supply.
Adjust VDC to 15V.
Start with Vgs=0 and lower its value
( more negative) while simultaneously monitoring
VRD.
At VRD =0 then the value of Vgs=Vp (pinch-off voltage).
Part(2) Calculating Vp (pinch-off voltage)
VRD=ID*RDVgs
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
1.2
1.3
1.4
5
Vp=Vgs at VRD=0 Vp=……
Vgs
VRD
V_V1
0V 5V 10V 15V 20V
-I(R1)
0A
2.0mA
4.0mA
Vgs =Vp=….. V ID=…. Vgs =0 ID=………
Part(3) Measure Vgs,ID
Vgs=………V
ID = ……mA
Check by calculations !!!
2
D DSSI =I * 1gs
p
V
V
2
D DSS
0
(1)
(2) I =I * 1
:
........................
gs g s s s s
gs D s
gs
p
V V V V I R
V I R
V
V
complete
Part(4) AC analysis
No load state:
Av=…………
10k load
Av=…………