LESSON 11 · Components
Transistors (BJT)
A small signal controlling a much larger one — the building block of amplification and switching.
A Bipolar Junction Transistor has three legs — base, collector, and emitter. A small current into the base allows a much larger current to flow between collector and emitter, acting as an electronic switch or amplifier.
NPN TRANSISTOR SYMBOLBase on the left, collector and emitter branching off the vertical bar (the arrow on the emitter shows current direction).
- NPN transistors turn on when the base is more positive than the emitter; PNP work the opposite way.
- A tiny base current (milliamps) can switch a much larger collector current — the basis of amplifiers and simple digital logic.
- Common beginner use: switching a motor or relay on and off using a microcontroller pin that can't supply enough current on its own.