NPN Transistor

A three-terminal semiconductor that uses a small base current to control a much larger collector-emitter current.

Symbol: IEC 60617 (circle + arrow out)

How it works

An NPN bipolar junction transistor has three terminals: base, collector, and emitter. A small current flowing into the base allows a much larger current to flow from collector to emitter — the ratio between them is the transistor's current gain (hFE or β), typically 50-300 for small-signal parts.

This makes an NPN transistor useful as a current amplifier (driving a relay or motor from a microcontroller pin) or as a switch (fully on or fully off, controlled by the base).

CircuitForge models the base-emitter junction with the diode equation and applies the gain parameter to derive collector current — the same core approach as a real small-signal transistor datasheet's DC characteristics.

Key Formulas

Current Gain (Active Region)Ic = hFE × Ib
Emitter CurrentIe = Ic + Ib

Common Mistakes

Forgetting the base resistor. The base-emitter junction acts exactly like a diode (dropping ~0.7V). Connecting the base directly to a 5V logic pin without a resistor will short the pin and destroy both the transistor and the microcontroller.

Common Variants

2N2222 Transistor

A widely used general-purpose NPN switching transistor, common in beginner motor-driver and relay-driver circuits.

BC547 Transistor

A small-signal NPN transistor extremely common in Indian and European hobbyist circuits, functionally similar to the 2N2222 in a smaller package.