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Introduction to Transistors

A transistor is a device with 3 terminals used to amplify electrical signals or to use a signal to switch another circuit on and off. The three leads are called the "collector", "base" and "emitter". There are many types of transistor (BJT, JFET, MOSFET, etc.) BJT or bi-polar junction transistors were the first to be mass-produced and are very common. BJTs come in two polarities: NPN and PNP.

Symbols:

Transistors can be used as a switch, to allow small signals (e.g. the output pins of a microcontroller) to switch on and off more power-hungry devices (e.g. a motor).

E.g. using an NPN transistor:

The "base" acts as a switch, or tap, controlling how much current flows from "collector" to "emitter". A small signal from base to emitter allows a larger current from the collector to the emitter to flow (turning on the motor from the 5v supply). No signal from the base and no current is allowed to flow from the collector (turning off the motor).

The collector-emitter output flow is proportional to the base-emitter input signal, so transistor can also be used to amplify small analog signals into more powerful wave-forms (e.g. an audio amplifier).

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