A transistor comes in variety of forms tailored for example switching application, power amplifier etc. A circuit diagram is shown below that turns on a light bulb which has a voltage rating of 6.3V. The battery or the actual voltage source is only 5V but with the help of the transistor we can achieve higher voltage at the output of the transistor(collector) and turn on the light bulb.

When the switch is turned on the voltage and current from the battery appears at the base of the transistor(2N3771) and gets amplified and drives the light bulb. This circuit illustrates transistor as a switch.

This circuit was drawn in Proteus professional circuit simulator. We can see the light bulb glow when we turn on the switch.


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