) at constant collector-emitter voltage ( V CE ) is called forward current gain (β). β = ∆ ∆ C V CE ( . ) Its value is very high and it generally ranges from to . ∆ I C I B I B ∆ I C (mA) ( A) Figure .
Current transfer characteristics . . Relation between α and β There is a relation between current gain in the common base configuration α and current gain in the common emitter configuration β which is given below. α β β = + (or) β α α = − ( .
) The collector current is independent of the collector- emitter voltage in the active region. Note EXAMPLE . In the circuit shown in the figure, the input voltage V i is V, V BE = V and V CE = V. What are the values of I B , I C , β?
ii) Cut-off region A small collector current exists even after the base current is reduced to zero. This region below the curve for I B = is called cut-off region because the main collector current is cut-off. iii) Active region The central region of the curves is called active region. In this region, the base-emitter junction remains in the forward biased condition and the collector-emitter junction remains in the reverse biased condition.
The transistor in this region can be used for voltage, current and power amplification. iv) Breakdown region If the collector-emitter voltage is increased beyond the rated value given by the manufacturer, the collector current increases enormously leading to the junction breakdown of the transistor. This avalanche breakdown can damage the transistor. Output impedance The ratio of the change in the collector- emitter voltage ∆ V CE