TRANSISTOR AS AN AMPLIFIER

• The  operation  of  a transistor  as  an  amplifier  is  based  on  the fact  that  base current, I in a transistor can control the collector current, I the base current can  be  varied  by  variation  of  forward  bias  and  this  produce  corresponding variation in the collector current.

• Fig shows the basic circuit of a transistor amplifier. The weak signal is applied between   emitter-base   junction   and   output   is   taken   across   the   load   RC connected in the collector circuit.

• The  emitter-base  junction  in  a  transistor  is forward  biased  and,  as  such the input  impedance  is  low.  On  the  other  hand,  the  base-collector  is  reverse biased and hence the output impedance is very high.

• A  DC voltage VDD is applied in the input circuit in addition to the signal. This D.C voltage magnitude is such that it always keeps the input forward biased regardless of the polarity of the signal.

• Even a small change in signal voltage causes an appreciable change in emitter current, since the input circuit has low resistance.

• Due to transistor action the same change in collector current take place. The collector  current  flows  through  a  large  load  resistance  (RC)  which  in  turn produces a large voltage across it.

• Thus a weak signal applied in the input circuit appears in the amplified form in the collector circuit. In this way transistor acts as an amplifier.