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What are the operating states of saturation, active and cut off region?

Latest Updated:03/01/2007

Question:

What are the operating states of the following three operating regions of transistors: saturation region, active region, and cut-off region?

Answer:

 
Saturation region: As the base current increases, the voltage between both terminals of the load resistance (VRl = IC × Rl) becomes larger. In the figure below, a -> b on the load line. If the power supply voltage is constant, the collector current past c on the load line does not increase no matter how much the base current increases, and the voltage between the collector and the emitter is minimized.
This minimum voltage is called the collector saturation voltage VCE (sat).
The state of this region means that switch operation is on.
 
Active region: The light blue region in the figure below is the region where the change of the input signal on which the center is set at point b of the load line, which is a suitable base current, can be amplified without distortion to a large collector current change.
 
Cut-off region: Even if the base pin is left open, collector cut-off current ICEO (no general ratings exist) flows between the collector and emitter. This region is called the cut-off region (collector cut-off region).
The state of this region means that switch operation is off.




Related item: Collector saturation voltage

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