Cut off, Active & Saturation Region of Transistor

In this theory, cut off region, active region and saturation region of the transistor is explained.

Active Region, Cut off Region and Saturation Region of Transistor

Cut off region of Transistor

  • When load line intersect IB = 0, it is known as cut off region of the transistor
  • As the base current is zero, only small collector leakage current flows. The base emitter junction does not remain in the forward biased because the base current is zero. 
  • The collector to emitter voltage is equal to VCC.

Saturation Region of Transistor

  • When the load line intersect IB = IB ( saturation ) , it is known as saturation region of the transistor
  • As the base current is maximum, the collector current also becomes maximum.  
  • The collector to emitter junction does not remain in the reverse biased condition therefore the transistor action is also lost. 
  • If the base current increases beyond saturation level, the collector current does not increase because of collector to base junction is not remain reverse biased.
         IC = VCC / RC
        VCE = VCE ( knee )


Active Region of Transistor

  • It is a region between active and cut off of the transistor output characteristic. 
  • The base to emitter junction remains in forward biased condition whereas the collector to emitter junction remains in the reverse biased condition.

Current During Different Region of Transistor

Active region
Collector current IC = βIB
Cut off region
Saturation region
  • The transistor behaves as a closed switch between collector and emitter. The collector – emitter path is short circuited.
  • Every transistor consists of two diodes emitter – base diode and collector to emitter diode in the common emitter configurations.

Base – emitter diode
Emitter – collector diode
Cut off

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