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Electronics Paper 2, May/June 2008  
Questions: 1 2 3 4 5, 6 &7 8 9 10 Main
General Comments

Question 1
 (a) Explain the term thermionic emission.
 (b) Sketch and label the circuit symbol of a pentode valve.
  (c) State two advantages of the indirectly heated cathode over the directly heated cathode.



The expected answers were:
  (a)            Thermionic emission is the emission of electrons from the surface of metals                                       when sufficiently supplied with heat energy from external sources.
Electron emission will occur when heat energy transferred to the electron exceeds the work function of the metal.
(b)           The pentode tube

  (c) Advantages of indirectly heated cathode:
      -              Eliminates the unwanted a.c. noise associated with the directly heated cathode on                                 account of varying voltage drop along filament length.
       -              Better electron emission efficiency due to improved surface area characteristics.

This question was on electron emission and thermionic devices. Most of the candidates that responded to this question gave correct answers. However, very few indeed could identify the grids, though majority knew the anode (plate) and cathode.


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