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Transistor configurations

A transistor can be connected in a circuit in the following three different configurations: common base (CB), common emitter (CE), and common collector (CC) configurations:

CB configurations

Base is common to both emitter and collector.
  • Input current = Ie
  • Input voltage = VEB
  • Output voltage = VCB
  • Output current = IC
Fig. 27
With small increase in emitter-base voltage VEB, the emitter current Ie increases rapidly due to small input resistance.
  • Input characteristics: If VCB = constant, curve between Ie and VEB is known as input characteristics. It is also known as emitter characteristics (Fig. 28).
Fig. 28
The input characteristics of NPN transistor are also similar to Fig. 28, but Ie and VEB both are negative and VCB is positive.
Dynamic input resistance of a transistor is given by 107102.png [Ri is of the order of 100 ]
  • Output characteristics: Taking the emitter current ie constant, the curve drawn between IC and VCB are known as output characteristics of CB configuration (Fig. 29).
Fig. 29
Dynamic output resistance, 107108.png

CE configurations

Emitter is common to both base and collector.
The graphs between voltages and currents when emitter of a transistor is common to input and output circuits are known as CE characteristics of a transistor (Fig. 30).
Fig. 30
  • Input characteristics: Input characteristic curve is drawn between base current Ib and emitter base voltage VEB, at constant collector emitter voltage VCE (Fig. 31).
Fig. 31
Dynamic input resistance, 107114.png
  • Output characteristics: Variation of collector current IC with VCE can be noticed for VCE between 0 to 1 V only. The value of VCE up to which the IC changes with VCE is called knee voltage. The transistor are operated in the region above knee voltage.
Fig. 32
Dynamic output resistance, 107120.png

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