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.


Transistors are manufactured in different shapes but they have three leads (legs).
The BASE - which is the lead responsible for activating the transistor.
The COLLECTOR - which is the positive lead.
The EMITTER - which is the negative lead.
The diagram below shows the symbol of an NPN transistor. They are not always set out as shown in the diagrams to the left and right, although the ‘tab’ on the type shown to the left is usually next to

the ‘emitter’. How they are related to each other ??? Since the Transistor is a Current device, any signal Voltage must first be Converted to a Current.   Voltage to Current Converter First, you must convert the input voltage to a current by using a Voltage to Current Convertor--a resistor. Since the Transistor is a Current in/Current out device, any Current Output is Converted to a Voltage Drop by the Current flowing thru a Load Resistor. Current to Voltage Converter: Next, you convert the output current into a voltage by using a Current to Voltage Converter in the collector circuit--you guessed it--a resistor.