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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 3

(a)  Compare Class B push-pull amplifiers with Class A amplifiers with reference to
                  (i)   Operation;
                  (ii)  Output power;
                  (iii) Efficiency;
(b)  State how distortion is reduced in amplifiers.



The expected answers were:
(a) The difference between class B push-pull amplifiers and Class A amplifiers in terms of the                            three specified parameters are as follows:
                               -   Operations: Transistors utilized for Class B push-pull amplifiers are biased to operate                                      during only half of the input cycle; for class A amplifiers, the transistors are biased to                                      operate over complete input cycle.
                               -   Output power: Output power obtainable from class B push-pull amplifier is about                                        twice that available from a comparable Class A amplifier for the same power input.
                                -    Efficiency: Class B push-pull amplifiers are much more efficient (maximum efficiency                                      of about 78.5%) than class A amplifiers, for which maximum possible efficiency is                                             25%.

  (b) Most amplifier distortion types can be reduced through negative feedback, whilst others will                     improve with choice of transistor operating input.

This question was on amplifiers. Many candidates could not answers recall question correctly.

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