C ) and the collector to emitter voltage ( V CE ) at constant input current ( I B ) and are shown in Figure . . Initially, the base current is set to a particular value. Then collector-emitter voltage is increased in suitable steps and the corresponding collector current is recorded.
A graph is plotted with V CE along the x-axis and I C along the y-axis. This procedure is repeated for different values of I B . The four important regions in the output characteristics are: V CE Cut-off region I C I = μA I = μA I = μA I = μA I = Saturation region Active region (mA) (V) I C Δ V CE Δ Figure . Output characteristics i) Saturation region When V CE is increased above V, the I C increases rapidly and reaches a saturation value at a particular value of V CE , called knee voltage.
The initial part of the curve OA (the ohmic region) between the origin and the knee point A is called saturation region. Transistors are always operated above this knee voltage. V CE =1V V CE = 2V I B V knee (µA) V BE (V) ∆ V BE ∆ I B Figure . Input characteristics The following observations are made from the graph.
· The curve looks like the forward characteristics of an ordinary p-n junction diode. · There exists a threshold voltage or knee voltage ( V knee ) below which the base current is very small. This value is . V for silicon and .
V for germanium transistors. Beyond the knee voltage, the base current increases with the increase in base-emitter voltage. · It is also noted that the increase in the collector-emitter voltage decreases the base current. This shifts the curve outward.
This is because the increase in collector-emitter voltage increases the width of the depletion region which in turn, reduces the effective base width and thereby the base current. Input impedance The ratio of the change in base-emitter voltage ∆ V BE