Configurations of BJT

A transistor has three terminals. Any of these three terminals could be connected to form the common input and output terminal. Depending upon the terminal, which is used as common terminal, there are three types of transistor configurations:

Common-Base Configuration (CB Configuration)

In this type of configuration, the base terminal of the transistor serves as a common terminal, so it is called as common-base (CB) configuration. The input voltage is applied between base and emitter terminal. The output is taken between collector and base terminal of the device. Common-base configuration has low input resistance but a high output resistance. Current gain in common-base configuration is denoted by Greek letter α (alpha). It is defined as the ratio of collector current to the emitter current.

Mathematically, it is represented as:

α = Collector current / Emitter current

As the collector current is always smaller than emitter current, so the current gain (α) in common-emitter configuration is always less than unity.

Common-Emitter Configuration (CE Configuration)

This type of configuration is called as common-emitter (CE) configuration as the emitter terminal of the transistor is made as a common terminal. The input voltage is applied between base and emitter terminal of the transistor. The output is obtained between emitter and collector terminal. Common-emitter configuration has a low input resistance but very high output resistance. Current gain in common-emitter configuration is denoted by a Greek letter ß (beta). It is defined as the ratio of collector current to the base current.

Mathematically, it is represented as:

ß = Collector current / Base current

As the value of collector current is much larger than that of base current, so the current gain in common-emitter configuration is very large (greater than unity). The value of ß ranges between 20 to 300.

Relationship between α and ß:

Mathematically, the relationship between α and ß is represented as:

α = ß / 1+ß

OR

ß = α / 1-α

where,

α = current gain in common-base configuration

ß = current gain in common-emitter configuration

Common-Collector Configuration (CC Configuration)

This type of configuration is called as common-collector (CC) configuration as the collector terminal of the transistor is made as a common terminal. The input voltage is applied between base and collector terminal of the transistor. The output is obtained between emitter and collector terminal of the transistor. Common-collector terminal has a very high input resistance but a low output resistance. Current gain in common-collector configuration is denoted by Greek letter γ (gamma). It is defined as the ratio of emitter current to the base current.

Mathematically, it is represented as:

γ = Emitter current / Base current

Relationship between α and ß and γ:

Mathematically, the relationship of γ with α and ß is represented as:

γ = 1 / 1-α

γ = 1+ß

where,

α = current gain in common-base configuration

ß = current gain in common-emitter configuration

γ = current gain in common-collector configuration

A transistor is also called as bipolar junction transistor or simply BJT. It is called as bipolar as only two types of charge carriers, i.e. majority charge carriers and minority charge carriers are required to perform the operations on transistor. BJT is a current-controlled device as the output voltage, current or power is controlled by the input current in the transistor. As we know, BJT is a three-terminal device , so the three configurations- we will study in this article. The three main configurations are – common emitter , common base and common collector , which has its unique characteristics and applications respectively.