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.
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