Electronic Circuits and Diagram-Electronics Projects and Design

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

Vds(volts)

Id(mA)

Vds

Id

Vds

Id

 

 

 

 

 

 

 

 

 

 

2.      To plot the transfer characteristics

 

                           Vds=10 V                                                         Vds= 20 V

Vgs(volts)	Id(mA)	Vgs	Id

 

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