Question

Explain the use of N-P-N Transistor as an amplifier in common emitter mode under the following heads.
(i) Labelled circuit diagram. (ii) Working.

Answer 1

(i) Labelled circuit diagram:

Labelling: $I_b$: base current

$I_e$: Emitter current

$I_c$: Collector current

X & Y: batteries for forward and reverse biasing the junctions

$R_{out}:$ load resistance.

(ii) Working: In the positive half cycle of input alternating potential, the potential of base with respect to the emitter becomes more positive due to which the emitter current $\left(I_c\right)$ increases. With the increase in emitter current the collector current $\left(I_c\right)$ also decreases. As a result the voltage across the load resistance $\left(R_{out}\right)$ also increases and the collector voltage becomes less positive. On the other hand, in the negative half cycle of input potential, the base voltage with respect to the emitter becomes less potential due to which the emitter current decreases. With the decrease in emitter current, the collector current also decreases due to which the voltage across the load resistance also decreases and the collector voltage becomes more positive.

In this way, in this amplifier the phase difference between the input and output potential is ${180}^o$.