Transistor Basics

Transistors can be regarded as a type of switch, as can many electronic components. They are used in a variety of circuits and you will find that it is rare that a circuit built in a school Technology Department does not contain at least one transistor. They are central to electronics and there are two main types; NPN and PNP. Most circuits tend to use NPN. There are hundreds of transistors which work at different voltages but all of them fall into these two categories.

Fig A

Fig B
Fig 'A' shows an NPN transistor which is often used as a type of switch. A small current or voltage at the base allows a larger voltage to flow through the other two leads (from the collector to the emitter).

The circuit shown in Fig B is based on an NPN transistor. When the switch is pressed a current passes through the resistor into the base of the transistor. The transistor then allows current to flow from the +9 volts to the 0vs, and the lamp comes on.

The transistor has to receive a voltage at its ‘base’ and until this happens the lamp does not light.

The resistor is present to protect the transistor as they can be damaged easily by too high a voltage / current. Transistors are an essential component in many circuits and are sometimes used to amplify a signal.