Basic of Bipolar Junction Transistors : Mycircuitstudy

Transistor Circuits

The circuits we have encountered so far are passive and dissipate power. Even a transformer that is capable of giving a voltage gain to a circuit is not an active element. Active elements in a circuit increase the power by controlling or modulating the flow of energy or power from an additional power supply into the circuit.

Transistors are active circuit elements and are typically made from silicon or germanium

and come in two types. The bipolar transistor controls the current by varying the number

of charge carriers. The field effect transistor (FET) varies the current by varying the shape of the conducting volume.

Before starting we will define some notation. The voltages that are with respect to

ground are indicated by a single subscript. Voltages with repeated letters are power supply voltages. And voltages between two terminals are indicated by a double subscript.

1. Bipolar Junction Transistors

By placing two PN junctions together we can create a bipolar transistor. In a PNP transistor the majority charge carriers are holes and germanium is favoured for these devices. Silicon is best for NPN transistors where the majority charge carriers are electrons. The thin and lightly doped central region is known as the base (B) and has majority charge carriers of opposite polarity to those in the surrounding material. The two outer regions are known as the emitter (E) and the collector (C). Under the proper operating conditions the emitter will emit or inject majority charge carriers into the base region, and because the base is very thin, most will ultimately reach the collector.

The emitter is highly doped to reduce resistance. The collector is lightly doped to reduce the junction capacitance of the collector-base junction.

The schematic circuit symbols for bipolar transistors . The arrows on the schematic symbols indicate the direction of both IB and IC. The collector is usually

at a higher voltage than the emitter. The emitter-base junction is forward biased while the collector-base junction is reversed biased.