Transistors are essential electronic components in many circuits and are sometimes used to amplify a signal.

Transistors can be regarded as a type of switch. The diagram on the right shows the symbol of an NPN transistor.

There are hundreds of transistors which work at different voltages but all of them fall into two categories: NPN and PNP. Most circuits use NPN - that is why most examination boards specify that they are to be used in questions.

Transistors are manufactured in different shapes but they all have three connectors(legs). You have to be sure that you identify the legs correctly when you connect a transistor into a circuit.

In a bipolar transistor:

• The BASE - which is responsible for activating the transistor.
• The COLLECTOR - which is the positive connection - or 'lead'.
• The EMITTER - which is the negative connection (it emits electrons!). On cylinder shaped ones a tag is above this.

A MOSFET (Metal Oxide Semiconductor Field Effect Transistor) is a 3-lead device, the lead names being source, drain and gate.

Example Circuit:

• Before the switch is closed the voltage at point A is 0V.
• After closure of the switch the voltage at point A rises to 9V.

A current then passes through resistor R_B into the base of the transistor. This lowers the resistance of the transistor line from emitter to collector allowing a current to flow from B to C, and the LED lights up.

The transistor has a high resistance from collector to emitter when not activated. It has to have a threshold potential difference across the base-emitter before the transistor is of low enough resistance to conduct; until this happens the LED does not light. As soon as the voltage is across the base-emittter is high enough the transistor is activated and it then has a low resistance from emitter to collector and strand BC of the circuit 'works'.

It therefore works like a switch - instead of being manually pressed like or 'normal' switch - the transistor 'switch' requires a minimum voltage across the base emitter.

The resistor R_E is present to protect the transistor as transistors can be easily damaged by too high a current flow.

Transistors are often used in conjunction with transducers. The resistance of the transducer changes according to physical conditions. Therefore a change in temperature or light intensity can produce that shift in voltage distribution that will activate the transistor.

In the diagram on the right the LDR and resistor form a potential divider. there is no manual switch to close.

Under bright light the resistance of the LDR is very low - when it is in the dark it is high.

Therefore the LDR will get most of the voltage, across the strand DE of the circuit, when the LDR is in the dark. This voltage will then 'switch on' the transistor and make the LED light up.

In bright light the LED would be off. This could be used in an automatic lighting circuit. For example: lighting up EXIT signs in a cinema when the lights are low. This would have the advantage of saving energy when the main lights are on by automatically switching the sign lighting off.