Transistor types and configuration

Transistor types and configuration 

Types of transistor:- there are two types of transistors

1 PNP transistor:- inward arrow direction on emitter  lead.
2 NPN transistor:- outward arrow direction on emitter  lead.

Transistors has three lead known as base emitter and collector. There are two junction in a transistor, first is base emitter junction and second is base collector junction. As there are three point in a transistor, one lead is made common for input and output, in this way transistor can be connected in three ways. First as a common base circuit, second as common collector circuit and third as common emitter circuit.
Common base circuit is used for impedance matching it has gain slightly less than one. Low input and high output impedance.
 

Common collector circuit is used as a buffer stage also known as emitter follower circuit, its gain is slightly more than one.

The common emitter circuit is used for amplification purpose its gain is very high in the range of 100-500 as per design. In any transistor circuit base emitter junction is always given a forward bias and base collector junction is given reversed bias. So for PNP transistor emitter is connected to positive side, collector is connected to negative side trough a load resistance, and base is given a negative biasing voltage with respect to emitter. This make the dc biasing circuit completed. now signal voltage is applied to base through a coupling capacitor and out put is taken from collector circuit with a capacitor. capacitor is used to pass ac signal but to block dc voltage from different stage.

Similarly in NPN transistor biasing is done. the emitter is connected to negative side, collector is connected to positive side through load resistance and base is given a positive biasing voltage with respect to emitter. A single transistor makes a single stage amplifier whose gain is less so multi stage amplification is used to achieve high gain and high power amplification.