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FET CHARACTERISTICS
AIM
To plot the characteristics of FET (BFW 10) and to find the following parameters
1. Drain dynamic resistance
2. Mutual conductance
3. Amplification factor
COMPONENTS AND EQUIPMENTS REQUIRED
1. FET
2. Ammeter
3. Voltmeter
4. DC supply
CIRCUIT DIAGRAM
THEORY
FET
is the Field Effect Transistor. It is 3 terminal voltage controlled
device. It’s terminals are drain, source and gate. Gate is the
controlling terminal. Consider an n channel device. The gate (p
material) is diffused. At zero gate voltage there is no reverse voltage
at the channel. So as Vds (drain source voltage) increases current Ids
also increases linearly. As the voltage is increased, at a particular
voltage, pinch off occurs .This voltage is known as pinch off voltage.
After pinch off drain current remains stationary .If we apply a gate
voltage (negative voltage) the pinch ff occurs early.
Advantages of FET over BJT are:
a) No minority carriers
b) High input impedance
c) It is a voltage controlled device
d) Better thermal stability
PROCEDURE
1. Identify the leads of FET
2. Make the circuit as given.
3. Keeping the gate voltage at 0 V, vary the drain source voltage and observe the current values.
4. Now make the gate voltage at any value (say -1 V) .Vary the drain source voltage and observe the current values.
5. Plot the graph of static characteristics
6. Now make drain source voltage at a value (say 10 V) and take the ammeter readings by varying the gate voltage.
7. Take the readings for other drain to source voltages.
8. Plot the graph (transfer characteristics)
9. Find the drain
dynamic resistance from static characteristics and Mutual conductance
from transfer characteristics by taking the slopes
10. Find the amplification factor by multiplying the above two values
OBSERVATIONS
1. To plot the static characteristics
.Vgs=0 V Vgs= -1 V Vgs= -2V
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Vds(volts)
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Id(mA)
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Vds
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Id
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Vds
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Id
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2. To plot the transfer characteristics
Vds=10 V Vds= 20 V
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Vgs(volts)
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Id(mA)
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Vgs
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Id
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GRAPH OF STATIC CHARACTERISTICS
TRANSFER CHARACTERISTICS
CALCULATIONS
Drain resistance, rd=∆Vds/∆ Ids (take the slope of graph)
Mutual conductance, gm=∆Id/∆Vgs
Gain, μ=gm rd
RESULT
Characteristics of FET were plotted.
Drain dynamic resistance=…………..
Mutual conductance=………………..
Amplification factor=………………..
SAMPLE VIVA QUESTIONS
1. What are the advantages of FET over BJT?
a) No minority carriers
b) High input impedance
c) It is a voltage controlled device
d) Better thermal stability
3. Define pinch off voltage
See theory
