Q.What is the necessity of cascading?

The gain of a single amplifier stage is often not enough to build a signal up to a usable level. Two or more stages are then connected one after another — cascaded — with the output of each stage feeding the input of the next. Any amplifier with more than one stage is a multistage amplifier; a radio receiver, for example, may use six or more stages. The purpose of cascading is simply to increase the overall gain.

Multistage amplifiers fall into two families: cascaded amplifiers, in which every stage and every inter-stage coupling is identical, and compound amplifiers, in which the stages and couplings may differ (one stage common-emitter, another common-collector, and so on).

The key result is the overall gain. As ratios, the total voltage gain is the product of the individual stage gains:

$A_v = A_{v1} \times A_{v2} \times A_{v3} \cdots A_{vn}.$ …