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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=…………