power electronics 2 mark with answer

2 Marks with answers

UNIT   - III

      FET AND ITS APPLICATIONS

1.             Sketch and explain the basic structure of an N channel junction     

      field effect  transistor.

2.               What are the advantages and disadvantages of JFET over BJT?

3. Why is FET known as a unipolar device?

4. Draw and explain the small signal low frequency model of  

            JFET.

5.  Draw and explain the transfer curve of MOSFET in

              enhancement mode.

6. Distinguish between JFET and MOSFET.

7. How do you protect the MOSFET gate?

8. Describe the various FET parameters.

9. What is a channel?

           It is a base like structure which determines the type of FET

10. What are the advantages of FET

         * Input impedance is very high. This allow high degree of              

             isolation between the input & output Circuit. 

          *Current carriers are not crossing the junctions hence noise is  

                highly reduced.

          * It has a negative temperature Co-efficient of resistance . This

             avoids the thermal runaury .

            * It has a smaller size, longer life high efficiency.

11. What are the advantage of MOSFET compared to JFET?

          The input impedance of MOSFET is higher than that of JFET

12. What are the two modes of MOSFET?

               (a) Depletion mode (b) Enhancement mode

13. Why UJT is called so?

      UJT has only one PN junction so it is called as uni junction transistor

14. What are the advantages of SCR and TRIAC?
     SCR performs rectification, inversion and regulation of power flow   

    TRIAC is a bidirectional switch and hence it can conduct in both  

     the direction.

15. Define breakdown voltage
      The applied voltage at which the thyristors conducts heavily   

       without gate voltage.

16. Define latching current

It is the minimum current required to latch the device from off to ON state

17. Define holding current
       It is defined as the minimum current required to hold the device

       into conduction.

18. Define turn - on time
        It is the time taken by the SCR to reach to its full conduction

         from the time the trigger is applied.

19. Define turn - off time
     It is the finite time taken by the SCR after application of the  

      reverse voltage to switch the device off.
20. What are the advantage of SCR?
        Switching speed is high No moving parts. So it gives noiseless

         operation at high frequency It controls large current in the load

         by means of small gate current Occupies less space

21. Give some applications of thysistor?
          Used for power control Used for speed control of a d.c shunt

           motor

22. Define finger voltage
        It is defined as the minimum voltage which is required between

         anode and the cathode of thyristor to trigger into conduction

23. What is the name for solid state equivalent of thyristor

           Thyratron.

24. Define inter-base resistance 
        It is the resistnce offered by the silicon bar

25. Define pinch-off voltage
      It is the drain source voltge above which the drain current become

     constant

26. What are the differences between JFET & BJT

         jFET                                                                   BJT
1. Unipolar device                                           1. Bipolar device
2. High input impedance              2. Low input impedance due

                                                                to forward bias

3. Voltage driven device               3. Current driven device
4. Gain is characterised

by transconductance              4. Gain is chracterised by voltge gain
5. Low noise level                  5. High noise level

27. . What is amplification factor
            It is the product of drain resistnce and transconductance   

           m=Rd x gm Rd=Drain resistnce gm=Transconductance

28. Define drain resistance 
          It is the ratio of change in drain source voltage to change in drain current at constant gate source voltage.