OPERATING MODES OF TRANSISTOR
Commonly a Transistor has two junctions (Emitter-Base and Collector-Base Junctions), and each of these two junctions may be either Forward Biased or Reverse Biased.
Therefore, there are Four Possible configurations can be made to bias these Junctions.
Accordingly transistor may operate in different conditions.
It is easy to understand the various configurations of Transistors and the region of operation by take a look on the Tubular Column.
|sl.no.||Biasing Condition||Emitter Base Junction||Collector Base Junction||Operation Region|
|1||Forward-Reverse||Forward Biased||Reverse Biased||Active|
|2||Forward-Forward||Forward Biased||Forward Biased||Saturation|
|3||Reverse-Reverse||Reverse Biased||Reverse Biased||Cut-off|
|4||Reverse-Forward||Reverse Biased||Forward Biased||Inverted|
1. Forward-Reverse Biasing
It is the normal biasing of a transistor in which the EMITTER-BASE Junction is Forward Biased and the COLLECTOR-BASE Junction is Reverse Biased.
This configuration is commonly called as FR biasing.
In this mode the transistor operates in Active region
2. Forward-Forward Biasing
In this biasing, both the emitter and collector junctions are Forward Biased.
In this mode the transistor operates in Saturation region
3. Reverse-Reverse Biasing.
As the name denotes both the junctions are in Reverse Biased condition.
In this biasing, the transistor has practically Zero current because the emitter doesnot emit charge carriers into the base and no charge carriers are collected by the collector expect a few thermally generated minority carriers.
Thus the transistor acts like an open Switch.
4. Reverse-Forward Biasing
In this biasing, the Emitter Junction is Reverse biased while the collector Junction is Reverse biased.
The collector is not doped to the extent as the Emitter is doped, Therefore it cannot inject the majority charge carriers to the base.
So in this region, the action of transistor is poor.
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Comparison Between Three Common Transistor Configurations