Draw the circuit diagram for studying the characteristics of a transistor in common emitter configuration. Explain briefly and show how input and output characteristics are drawn.

Explanation

Output or collector characteristics : This is a set of curves of collector current `I_(C)` against collector-to-emitter voltage `V_(CE)` for different constant base currents `I_(B)`, as shown below.

(a) For `V_(BE)` less than the threshold voltage for the junction, `I_(B)=0` and `I_(C)=0` i.e., there is no current through the transistor. Both the junctions are reverse biased and the transistor acts like an open switch. Then the transistor is said to be switched Fully-OFF or in the cut-off region or OFF-mode.
(b) The base current is set to a suitable value by varying the base-to-emitter voltage `V_(BE)` to a value greater than the threshold voltage. Then, the collector-to-emitter voltage `V_(CE)` is varied and the variation of `I_(C)` is plotted against `V_(CE)`. For very small values of `V_(CE)`, ideally zero, `I_(C)` is maximum, equal to `V_(C C)//R_(L)`. Both the junction are forward biased and the transistor acts like a closed switch. The transistor is said to be switched Fully-ON, or in the saturation region or ON-mode.
(c) But for values of `V_(CE)` above the threshold voltage, `I_(C)` is constant and `V_(CE)` has relatively little effect on it. In this region, called as the active region, `I_(C)` is determined almost entirely by `I_(B)`. In this region, the common-emitter current gain `beta` is large.