Question

For a transistor to work as an amplifier:
(A) Its emitter junction is in reverse bias and collector junction is in forward bias
(B) The transistor must have breakdown region
(C) Its emitter junction is in forward bias and collector junction is in reverse bias
(D) Its emitter and collector junction are in forward bias

Answer 1

For voltage gain amplifier, the output resistance should be greater than the input resistance. In a forward biased condition, the diode offers very small resistance and in case of reverse biasing, the resistance offered is very high.

Complete step by step answer:
For a common emitter transistor to work as an amplifier, the input (base-emitter) circuit is forward-biased by a low-voltage battery ${{V}_{BE}}$ so that the resistance of the input circuit is small. The output (collector-emitter) circuit is reverse biased by means of a high voltage battery ${{V}_{CC}}$ so that the resistance of the output circuit is high. The load resistance is connected in the collector-emitter output circuit. The weak input A.C. signal is applied across the base-emitter circuit and the amplified signal is obtained across the collector-emitter circuit.As the emitter Its emitter junction is in forward bias and collector junction is in reverse bias, so option C is the correct answer.
Additional information:
In a common emitter circuit, the change in collector current due to change in base current, causes a change in collector-emitter voltage and the voltage drop across the load resistor, because collector supply voltage is fixed.In a transistor connected in CE mode input current is ${{I}_{B}}$ and output current is ${{I}_{C}}$.The d.c. current gain depends on the collector current and temperature.The ratio of collector current to emitter current is called transistor alpha $\alpha$. Usually in god transistors $\alpha$ ranges from $0.98$ to $0.988$.

Note: If emitter junction is reversed biased no current will flow through the circuit and hence the voltage will not be amplified.Similarly if both emitter and collector junction are forward biased the output voltage will be same as the input and no amplification will happen.