CBSE Class 12 · 2015 · Delhi · SemiconductorsShort Answer

Circuit: an n-p-n transistor in CE configuration with the base-emitter junction forward biased and a collector load $R_C$, signal applied at the base.
(i) Input resistance is the ratio of a small change in base-emitter voltage to the resulting change in base current, at constant $V_{CE}$:

found from the reciprocal slope of the input characteristic.
(ii) Current amplification factor is the ratio of change in collector current to change in base current at constant $V_{CE}$:

found from the vertical spacing of the output characteristics for equal steps of $I_B$.
$\boxed{r_i={\left(\frac{\Delta V_{BE}}{\Delta I_B}\right)}_{V_{CE}},\beta ={\left(\frac{\Delta I_C}{\Delta I_B}\right)}_{V_{CE}}.}$