Question

Explain the function of a transistor as a switch.

Answer 1

Transistor as a Switch:
Transistors are widely used in switching operations.
In the figure, NPN transistor is connected in common emitter configuration and a resistor $R_B$ is connected in series with the base. The load resistance $R_C$ is connected in series with the collector. A pulse type waveform is applied as the input to the transistor through $R_B$. When the input is high, base-emitter junction is forward biased and current flows through $R_B$ into the base. The values of $R_B$ and $R_C$ are chosen in such a manner that the base current flowing, is enough to saturate the transistor. When the transistor is saturated, it is said to be ON (maximum current). When the input is low (i.e) at $0$V, the base-emitter junction is not forward biased. So, no base current flows. Hence the transistor is said to be OFF.