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Working of transistor

There are four possible ways of biasing the two Pā€“N junctions (emitter junction and collector junction) of transistor.
  1. Active mode: Also known as linear mode operation.
  2. Saturation mode: Maximum collector current flows and transistor acts as a closed switch from collector to emitter terminals.
  3. Cut-off mode: Denotes operation like an open switch where only leakage current flows.
  4. Inverse mode: The emitter and collector are inter changed.
A transistor is mostly used in the active region of operation, i.e., emitter base junction is forward biased and collector base junction is reverse biased. From the operation of junction transistor, it is found that when the current in emitter circuit changes, there is corresponding change in collector current.
 
In each state of the transistor, there is an input port and an output port. In general each electrical quantity (V or I) obtained at the output is controlled by the input. Table shows the working of PNP/NPN transistor.
 
Table Circuit Diagram of PNP/NPN Transistors

NPN transistor
 
108660.png
108685.png
Fig. 25
  • 5% emitter electron combine with the holes in the base region resulting in small base current. Remaining 95% electrons enter the collector region.
  • Ie > Ic and Ie = IbIc.
PNP transistor
108674.png
108692.png
Fig. 26
  • 5% emitter holes combine with the electrons in the base region resulting in small base current. Remaining 95% holes enter the collector region.
  • Ie > Ic and Ie = IbIc.




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