We have seen above that transistor can act in one of the three states : cut-off, saturated and active. The junction between base and collector may be called collector diode. The junction between base and emitter may be called emitter diode. A transistor has two pn junctions i.e., it is like two diodes. The reader may find the detailed discussion on transistor biasing in the next chapter. We provide biasing to the transistor to ensure that it operates in the active region. Consequently, the transistor will function normally in this region. In the active region, collector-base junction remains reverse biased while base-emitter junction remains forward biased. The region between cut off and saturation is known as active r egion. If base current is greater than I B( sat), then collector current cannot increase because collector-base junction is no longer reverse-biased. At saturation, collector-base junction no longer remains reverse biased and normal transistor action is lost. At this point, the base current is maximum and so is the collector current. The point where the load line intersects the I B = I B( sat) curve is called sa turation. The collector-emitter voltage is nearly equal to V C C i.e. At cut off, the base-emitter junction no longer remains forward biased and normal transistor action is lost. At this point, I B = 0 and only small collector current ( i.e.
The point where the load line intersects the I B = 0 curve is known as cut off. ( i) shows CE transistor circuit while Fig.( ii) shows the output characteristcs along with the d.c.
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