Transistor as a switchTransistors are widely used in switching operations. In the Fig, NPN transistor is connected in common emitter configuration and a resistor RB is connected in series with the base. The load resistance Rc is connected in series with the collector. A pulse type waveform is applied as the input to the transistor through RB. When the input is high, base emitter junction is forward biased and current flows through RB into the base. The values of RB and RC are chosen in such a manner that 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.
Transistor circuit configurations
There are three types of circuit connections (called configurations or modes) for operating a transistor. They are (i) common base (CB) mode (ii) common emitter (CE) mode and (iii) common collector (CC) mode.