How a BJT works

Bipolar junction transistor (BJT) was the first solid state amplifier, which boosted up the solid state electronics revolution. It is a type of a transistor that is used as an amplifying or switching device. They are called bipolar since there are two types of charge carriers, electrons and holes.

A bipolar junction transistor constitutes PN junction diodes that are connected back-to-back and regulate the amount of current that can pass through. A smaller controlling current regulates the flow.

A bipolar junction transistor has three terminals namely collector, emitter and base. The middle layer is called base and the outer layers are called collector and emitter. The standard combination of P-N-P or N-P-N type is used. The current that is to be controlled (main current) flows from the collector to the emitter (or the other way). The controlling current (base current) flows from the base to the emitter (or the other way).

Specifications for a BJT

All the electronics/electrical components are susceptible to damages when the specified voltage or current ratings are overridden. Some of the factors to consider when selecting a BJT are listed below.

§ Power dissipation: The power dissipated by a transistor is equal to the product (multiplication) of collector current and collector-emitter voltage. It is advisable to have a BJT with less power dissipation.

§ Reverse voltages: Maximum permissible reverse bias voltages across the PN junctions are specified for BJTs. This rating is of particular importance when using a bipolar transistor as a switch.

§ Collector current: Every BJT will be supplied with a maximum collector current. This parameter should be considered as it greatly affects the power dissipation rating of the transistor.

§ Saturation voltages: Ideally, a saturated transistor acts as a closed switch contact between collector and emitter, dropping zero voltage at full collector current.

§ Beta: It is a fundamental parameter that defines the amplifying capacity of the bipolar junction transistor. Beta is the ratio of collector current to base current and is highest for medium collector currents, decreasing for very low and very high collector currents.

BJT applications

Bipolar junction transistors remain important devices for ultra-high-speed discrete logic circuits such as emitter coupled logic (ECL), power-switching applications and in microwave power amplifiers. BJTs are universally used in electrical circuits where current needs to be controlled. Some of the areas are: switching elements to control DC power to a load, amplifiers for analog signals, 3D bipolar simulation, NPN device, AC frequency response, emitter-coupled logic element simulation, 3-phase AC motors.

Common emitter Common base Common collector

Darlington