The transistors can be used as amplifiers where the amplitude of the input signal is increased. A transistor connected in common emitter mode is biased in such a way that its base terminal is given an input signal and the output is developed at the collector terminal.
For any transistor operating in active mode, the base emitter junction is forward biased, thus having a low resistance. The base collector region in reverse biased, having high resistance. The current flowing from the collector terminal is β times more than the current flowing into the base terminal. Β is the current gain for the transistor
The Transistor as a Switch:
The transistor acts as a switch when it is operated in saturated region. As the transistor is switched ON in the saturation region, the emitter and collector terminals get short circuited and the current flows from collector to emitter in an NPN transistor. Actually maximum amount of base current is given which results in a maximum amount of collector current.
The voltage at the collector emitter junction is such low such that it reduces the depletion region. This causes the current to flow from collector to emitter and they appear to be shorted. When transistor is biased in cut off region, both the input base current and the output current are zero. The reverse voltage applied to the collector emitter junction is at its maximum level. This causes the depletion region at that junction to increase such that no current flows through the transistor. Thus the transistor is switched OFF.
Here we have a load that we wanted to turn ON and OFF with a switch. When the ON / OFF switch is in closed state, current flows in the base terminal of the transistor. The transistor gets biased such that the collector and emitter terminals are shorted and connected to ground terminal. The relay coil gets energized and the contact points of the relay closes such that the load gets the supply being connected in series through this contact acting like an independent switch
