BJT-Bipolar Junction Transistor PNP & NPN

BJT (Bipolar Junction Transistor):

Bipolar junction transistor is a solid state device and in BJTs current flows in two terminals, they are emitter and collector and the amount of current is controlled by the third terminal i.e. base terminal. It is different from the other type of transistor i.e. field effect transistor, in which the output current is controlled by the input voltage.

Types: Bipolar Junction Transistor PNP & NPN

• PNP Junction Transistors

• NPN Junction Transistors

PNP Junction Transistor

In PNP transistor, the emitter is more positive with the base and also with respect to the collector. Generally, the collector terminal is connected to the positive terminal and the emitter to a negative supply with a resistor, either the emitter or collector circuit.

NPN Junction Transistor

The NPN transistor is exactly opposite to the PNP transistor. Generally, the positive supply is provided to the collector terminal and the negative supply to the emitter terminal with a resistor, either the emitter, collector or emitter circuit.

BJT Working Principle

The BE junction is a forward bias junction and the CB is a reverse bias junction. The width of the depletion region of the CB junction is larger than that of the BE junction. Forward polarization at the BE junction decreases the barrier potential and produces electron flow.

 Using KCL we can observe the mathematical equation.

IE = IB + IC

The base current is very less compared to the emitter and collector current.

That is ~ IC

BJT Applications

1.Current Control

2.Amplification