Draw the typical input and output characteristic of an npn transistor in CE configuration. Show how these characteristics can be used to determine (a) the input resistance (ri), and (b) current amplification factor (β).

(a) The input resistance of the transistor is defined as the ratio of small change in base - emitter voltage to the corresponding change in base current at a given VCE.∴ Input impedance,ri=(ΔVBEΔIB)VCEThe input impedance of the transistor in CE mode is very high (b) The current amplification factor or current gain is defined as the ratio of a small change in the collector current to the corresponding change in the base current at a constant VCE.Current gain,β=(ΔICΔIB)VCE