The reciprocal of the slope of the output characteristics of a transistor in common base configuration gives

Explanation:

The output resistance of the transistor is equal to the reciprocal of the slope of the output characteristic which is shown figure.

Input Characteristics:

• The output voltage VCE is kept constant and P.D. between the base and emitter VBE is gradually increased. The base current IB is measured.
• The curves obtained by plotting base current on the y-axis and VBE on the x-axis at different values of VCE are called input characteristics of the transistor.
• The reciprocal of the slope of the curve in the linear region gives input resistance
• Ri. For CE mode is very small.
• So input resistance is very small for BJT in CE mode of operation.