For a transistor in CE mode to be used as an amplifier, it must be operated in :

Solution

Physics sir · 3 steps

An amplifier needs the output to follow the input smoothly, and only the active region gives that.

1. What amplifying needs. A small change in base current must produce a proportional, larger change in collector current, Δ I_C = β Δ I_B. The output must follow the shape of the input, with no flattening.

   Region	Emitter-base junction	Collector-base junction	Collector current
   Cut-off	reverse biased	reverse biased	almost zero, stays near zero
   Active	forward biased	reverse biased	I_C = β I_B, follows the input
   Saturation	forward biased	forward biased	maximum, no longer set by I_B

2. Why the extremes fail. In cut-off the transistor is an open switch and in saturation it is a closed switch. In both, changing I_B barely changes I_C, so the signal is clipped, not amplified. These two regions are used for switching.
3. So. For CE amplification the transistor must be biased in the active region only, which is option (d).

The idea to keep. Active region means amplifier. Cut-off and saturation mean switch. Remember the biasing: forward-biased emitter-base, reverse-biased collector-base for amplification.

Answer from the NTA final answer key. Solution by PrepWiser.