Electronics Paper 2, Nov/Dec. 2014  
Questions: 1 2 3 4 5 6 7 Main
General Comments

Question 4
  1. Define positive feedback in amplifiers.
  1.  State:

(i)      three  advantages of negative feedback.
(ii)      one  advantage of positive feedback.
               (c) A two stage power amplifier has gain of 30 in each stage. Calculate the total gain of the amplifier in decibel.

     The expected answers were:

(a)        Positive feedback in amplifiers is the process where part of the output signal is fed back in phase with the input signal.

(b)        (i)         Advantages of negative feedback
                                                (i)         Gain  stability
                                                (ii)        Increase  bandwidth                          
                                                (iii)       Less harmonic distortion
                                                (iv)       Reduced noise

                                    (ii)        One advantage of positive feeback
                                                (i)         Increased gain
(c)        Gain of each stage  = 30
            No. of stages            = 2

            Power gain of each stage in dB  =  10log1030
                                                                =  14.77
            Total power gain =  14.77 + 14.77
                                         =  29.54 dB
            Total power gain = 30 x 30 = 900
            Power gain in [dB] = 10 log10900
                                           = 29.54 dB           


The question was on Amplifiers. It was unpopular with the candidates. As reported, not many of the candidates that attempted it understood the requirements of the question.

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