Basic Electronics Paper 2, WASSCE (SC), 2019

Question 3

       

(a)State three advantages of a transistor over a triode.
(b)State one reason why a triode is regarded as voltage-controlled device.
(c)Draw and label the circuit symbol of a pentode valve.

 

OBSERVATION

The Chief Examiner reported that question 3 ‘s first two sub-questions are simple recall questions; one concerning the advantages of the transistor over the vacuum tube triode, A few candidates recalled the fact that the triode is  a voltage-controlled device on account of the functioning of its control grid, but none could explain exactly why.  Only a few candidates could give a sketch of the pentode, and many of those who did, could not correctly locate the five electrodes.

 

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  The expected responses were:

(a)        Advantages of a transistor over a triode

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The expected responses were:

(a)Advantages of a transistor over a triode

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  • Small in size (compactness)
  • Cheaper
  • Less power consumption
  • Higher efficiency(faster in operation)
  • Ruggedness/Durability
  • Tighter clearance
  • Produces less heat                                                                      

(b)Why a triode is regarded as a voltage-controlled device

  • It has a very high input resistance/impedance.
  • Input voltage controls the output current.

 

(c)Labelled diagram of the circuit symbol of a pentode valve
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