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Wednesday, October 5, 2011

Electronic devices and circuits lab manual FET CHARACTERISTICS


FET CHARACTERISTICS

AIM:      a). To draw the drain and transfer characteristics of a given 
         FET.
   b). To find the drain resistance (rd) amplification factor (μ) and
         Tran conductance (gm) of the given FET.

APPARATUS:            FET (BFW-11)
                        Regulated power supply
                        Voltmeter (0-20V)
                        Ammeter (0-100mA)
                         Bread board
                          Connecting wires
THEORY:
 A FET is a three terminal device, having the characteristics of high input impedance and less noise, the Gate to Source junction of the FET s always reverse biased. In response to small applied voltage from drain to source, the n-type bar acts as sample resistor, and the drain current increases linearly with VDS. With increase in ID the ohmic voltage drop between the source and the channel region reverse biases the junction and the conducting position of the channel begins to remain constant. The VDS at this instant is called “pinch of voltage”.
                     If the gate to source voltage (VGS) is applied in the direction to provide additional reverse bias, the pinch off voltage ill is decreased.
                      In amplifier application, the FET is always used in the region beyond the pinch-off.
                               FDS=IDSS(1-VGS/VP)^2



CIRCUIT DIAGRAM
     
PROCEDURE:
1.    All the connections are made as per the circuit diagram.
2.    To plot the drain characteristics, keep VGS constant at 0V.
3.    Vary the VDD  and observe the values of VDS and ID.
4.    Repeat the above steps 2, 3 for different values of VGS  at 0.1V and 0.2V.
5.    All the readings are tabulated.
6.    To plot the transfer characteristics, keep VDS constant at 1V.
7.    Vary VGG and observe the values of VGS and ID.
8.    Repeat steps 6 and 7 for different values of VDS at 1.5 V and 2V.
9.    The readings are tabulated.
10. From drain characteristics, calculate the values of dynamic resistance (rd) by using the formula
rd = ∆VDS/∆ID
11. From transfer characteristics, calculate the value of transconductace (gm)  By using the formula
Gm=∆ID/∆VDS
12. Amplification factor (μ) = dynamic resistance. Tran conductance
μ = ∆VDS/∆VGS
OBSERVATIONS:

DRAIN CHARACTERISTICS:

S.NO
VGS=0V

VGS=0.1V

VGS=0.2V


VDS(V)
ID(mA)
VDS(V)
ID(mA)
VDS(V)
ID(mA)









TRANSFER CHARACTERISTICS:            

S.NO
VDS =0.5V

VDS=1V

VDS =1.5V


VGS (V)
ID(mA)
 VGS (V)
ID(mA)
 VGS (V)
ID(mA)











MODEL GRAPH:

TRANSFER CHARACTERISTICS

DRAIN CHARACTERISTICS

     
PRECAUTIONS:

1.    The three terminals of the FET must be care fully identified
2.    Practically FET contains four terminals, which are called source, drain, Gate, substrate.
3.    Source and case should be short circuited.
4.    Voltages exceeding the ratings of the FET should not be applied.

RESULT :

1.    The drain and transfer characteristics of a given  FET are drawn
2.    The dynamic resistance (rd), amplification factor (μ) and Tran conductance (gm) of the given FET are calculated.

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