Electrical Engineering ⇒ Topic : Input Characteristics

Input Characteristics

The curve between the input current IE (emitter current) and output voltage V_EB (emitter-base voltage) at constant collector-base voltage V_CB. The emitter current is taken along the Y-axis and the emitter-base voltage is taken along the X-axis. The input characteristic for a typical transistor in common-base configuration is shown in Figure (a). The N-P-N transistor is connected in CB configuration.

                           Figure (a) Input characteristic of a transistor in CB configuration.                             

The following conclusions can be drawn from Figure (a).

The emitter current (I_E) increases rapidly with a small change in emitter-base voltage (V_EB) after the cut-in voltage. This means that the dynamic input resistance of  CB configuration is very small.There is a slight increase in emitter current (I_E) with increase in V_CB because there is a change in the width of depletion region in the base region under the reverse biased condition. When reverse bias voltage V_CB increases, the width of depletion region also increases at the collector-base junction. The emitter-base junction is forward biased and hence the depletion width at the emitter-base junction is very small. This reduces the electrical base width. This effect is known as Early Effect or Base Width Modulation.

Figure (b) shows the electrical width. The width of depletion region at the emitter base junction is neglected because this junction is forward biased

                                                   Figure (b) Base width.

Figure (c) shows the change in the widths of base and depletion regions with reverse biased voltage

          Figure (c) Change in base and depletion region widths with increase in reverse biased voltage.

The early effect can be explained with the help of the following expression:

                                                        ............. (1)

where V and W are the reverse bias voltage and the width of the space-charge region/depletion region.

The depletion region width is more in base region. This effect is more in the base region, as a result, there is reduction in the electrical or effective width.