Transistors are circuit elements designed to function either as amplifiers or as switches. At transistor has three parts: base, collector and emitter. The base is the controlling agent for a large supply of voltage, the collector is this large voltage supply and the emitter is the output for the transistor. A good analogy to use when explaining transistors as amplifiers is that of a tap. The gate is the faucet that controls the flow of water, the collector is the water supply and the emitter is the mouth of the tap from which the water comes out. Functioning as a switch allows the transistor to control current traveling through it and it can either allow current through it (On) or not (Off).
The name NPN transistors is based on the way they are created, that is, by placing two P-N junctions in parallel. A P-N junction is formed by joining a p-type and an n-type semiconductor together. The p and n distinction is based on the type of charges that form a majority in the semiconductor, positive or negative charges. The NPN configuration for transistors is used most commonly today.
Use as a Switch
A common application for NPN transistors is to use then as switches in circuits. In high-power devices such as motors and solenoids, the NPN transistor can be made to operate in two modes, ON and OFF. In doing so, the transistor is usually made to run in saturation mode when ON and in cutoff mode when OFF.
Use as an Amplifier
Another common application for NPN transistors is to use them as an amplifier, in which a small increase in the input voltage induces a large change in the output voltage. NPN transistors are used for this purpose in almost all phones electronic devices in which sound amplification or reproduction is required.
Use in Darlington Pair
A Darlington pair is a commonly used circuit configuration to amplify weak signals. The pair consists of two NPN transistors arranged so that the emitter of the first transistor feeds the base of the second transistor.