Transistor regions of operation in Transistor characteristic curve

The collector characteristic curve shows the behavior of the transistor. To get these curves we bias a BJT transistor with a variable power supply and changing the VBB and V­CC. By variating these voltages, either transistor PN junctions may be forward or reverse biased. Based on the condition of the BJT transistor junction biasing, the transistor can operate in four regions of operation.

Transistor Regions of Operation

Suppose we have an NPN transistor connected with variable power supplies as the diagram shows.

Transistor regions of operation in transistor Characteristics Curve

Cutoff region

When the base power supply VBB is set to zero, the current IB becomes zero. Both of the PN junctions of the BJT transistor become reverse bias and no collector current IC flows. In this region, the transistor behaves as an open switch. And the VCC voltage appears across the collector-emitter terminals VCE. This region is cutoff region.

Saturation region

When collector supply VCC is set to zero, and base supply is set to produce a certain base current IB. Both of the PN junctions become forward biased. The base current I­B flows through the emitter terminal because of low impedance and no current flows through collector IC=0. This Transistor region of operation is the saturation region. In this region, both junctions are forward bias. Collector current IC reaches a certain largest current independent of base current IB. In the saturation region, the voltage across collector-emitter terminals VCE is zero.

The active or Linear region

By increasing the V­CC the voltage across collector-emitter terminals V­CE may increase. As the VCE passes the 0.7v, the base-collector PN junction becomes reverse bias. And the base-emitter is forward bias. This mode of operation is active or linear regions of operation in the BJT transistor characteristic curve. By increase the V­CE beyond 0.7v, the collector current remains constant for a given value of base current IB. Increasing the VCE can cause a very slight increase in IC because of the widening of the base-collector depletion region. The following formula can calculate the active region collector current IC.

Breakdown region

By increasing the VCC, the V­CE can be increased which may increase the reverse bias of base-collector junction. The high reverse bias may cause widening of the base-collector junction and finally breakdown the junction. As the BJT transistor enters the breakdown region, the collector current will increase rapidly as the curves diagram shows. A transistor is not used in the breakdown region.

Conclusion

  • Based on transistor characteristic curves has four regions of operation
  • Cutoff region work as an open switch
  • Saturation region work as a close switch
  • IC remains constant for a certain amount of IB active region
  • The Breakdown region of a transistor is not used

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