Transistors

Transistors

Transistors amplify current, they can be used to amplify the small output current from a logic IC so that it can operate a lamp, relay or other high current devices. In many circuits a resistor is used to convert the changing current to a changing voltage, so the transistor is being used to amplify voltage. A transistor may be used as a switch (either fully on with maximum current, or fully off with no current) and as an amplifier (always partly on). The amount of current amplification is called the current gain, symbol hFE.

There are two types of the standard transistor, NPN and PNP, with different circuit symbols. The letters refer to the layers of semiconductor material used to make the transistor. Most transistors used today are NPN because this is the easiest type to make from silicon. The transistor leads are normally labelled base (B), collector (C) and emitter (E).
The labelled terms refer to the internal operation of a transistor but they are not much help in understanding how a transistor is used, so just treat them as labelled  A Darlington pair is two transistors connected together to give a very high current gain. 
Transistors have three leads which must be connected the correct way round. Wrongly connecting a transistor will cause damage instantly when switching the device on.