Operation Modes of BJT

Operation Modes of BJT

There are two junctions in bipolar junction transistor (B.J.T), which are separately forward or reverse biased. So, in this way, four operating modes of a B.J.T are found. (It means a B.J.T can operate in four different modes). These modes are named as:

1-   Forward active

2-   Cut Off

3-   Saturation

4-   Reverse Active

1.          Forward Active.

 In This Mode of Operation of Transistor, Emitter-base junction is reverse biased. Due to this configuration the transistor works as a controlled source in its forward active or linear mode. Due to its controlled characteristics, a B.J.T can be used as an amplifier in analogue circuits.

2.          Cut-0ff.

When Both Junctions of Transistor are reverse biased, then this mode of transistor is known as cut-off. In this situation, transistor works as an open switch (means, turned to off condition) and value of the current passing through it is almost zero.

      In this mode of operation, a B.J.T can be used for switching in computer and

      digital circuits

3.          Saturation.

In this operation mode of transistor, its both junctions are forward biased. As a result, when low voltages are applied parallel to collector-base junction, a large amount of collector current passes through transistor. Thus, in saturation mode, a transistor works as a closed switch (means turned to ON condition) in this mode of operation; a B.J.T can be used for switching in computer and digital circuits.

4.          Reverse Active.

In a transistor, if emitter-base junction is reverse biased and collector-base junction is forward biased (operation to forward-active mode) then such operating mode of transistor is known as reverse-active. This mode is also called inverted-mode. Practically, this mode is not useful for amplification due to a large flow of collector current (because a transistor can become useless due to this type of biasing).

However, Reverse-active mode is used in digital circuits and certain switching analog circuits.