Study of input characteristics and output characteristics of BJT in CB configuration

Objective:-

  • To observe and draw the input and output characteristics of a transistor connected in common base configuration.
  • To find α of the given transistor. 

Theory:

A transistor is a three-terminal active device. The terminals are emitter, base, collector. In CB configuration, the base is common to both input (emitter) and output (collector). For normal operation, the E-B junction is forward biased and C-B junction is reverse biased. In CB configuration, IE is +ve, IC is –ve and IB is –ve. So, VEB=f1 (VCB,IE) and IC=f2 (VCB,IB) With an increasing the reverse collector voltage, the space-charge width at the output junction increases and the effective base width ‘W’ decreases. This phenomenon is known as “Early effect”. Then, there will be less chance for recombination within the base region. With increase of charge gradient with in the base region, the current of minority carriers injected across the emitter junction increases. The current amplification factor of CB configuration is given by,

α = IC/IE

Circuit Diagram:

Procedure: 

  • Input Characteristics:
  • Connections are made as per the circuit diagram.
  • For plotting the input characteristics, the output voltage VCB is kept constant and for different values of VEB note down the values of IE.
  • Repeat the above step keeping VCB at different constant values. All the readings are tabulated.
  • A graph is drawn between VEB and IE for constant VCB.

 

  • Output Characteristics:
  • Connections are made as per the circuit diagram.
  • For plotting the output characteristics, the input IE is kept constant and for different values of VCB, note down the values of IC.
  • Repeat the above step for different constant values of IE, all the readings are tabulated.
  • A graph is drawn between VCB and Ic for constant IE.

Apparatus  / Components  required:

 Observation Table:

  • Input Characteristics:
Sl.No
VCB1 =
VCB2=
VEB(V)IE(mA)VEB(V)IE(mA)
  • Output Characteristics:

 

Sl.No
IE1 =
IE2=
VCB(V)IC(mA)VCB(V)IC(mA)

Output Characteristics:

Conclusion:

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