NPN Transistor Operation

 

A transistor isn’t controlling voltage directly—it’s controlling current. An NPN transistor uses a small base current to control a much larger current flowing from collector to emitter. When about 0.7 V is applied between base and emitter, it turns ON and current starts flowing. No base current means it stays OFF, just like an open switch.

There are three working modes. Cut-off: no base current, so no output current. Active: output current follows the base input—this is where amplification happens. Saturation: fully ON, acting like a closed switch with maximum current.

The graph shows how collector current changes with voltage for different base currents. More base current shifts the curve higher, meaning more output current. This is why transistors are used everywhere—from switching circuits to signal amplification.