Electrical – Current in CE and CB configurations of a transistor when output voltage is increased

transistors

The Early Effect seems to play a role when output voltage is increased in a transistor.

1)In the Common Base config, it causes the emitter current to increase,which in turn causes the collector current to increase. What happens to the base current in such a situation and why?

2)In the Common Emitter mode, my textbook says the emitter current increases while the base current decreases. It also says the Emitter current is constant because input voltage is constant. I am not able to follow this and seem to think otherwise.

Any help would be appreciated.

Best Answer

As far as the mentioned Early effect is concerned, there is no difference between CB and CE configuration. The effect is as follows (modulation of the base width Wb) :

As the reverse bias across collector-base junction increases (due to a rising collector-emitter voltage) the width of the collector-base depletion layer increases - and the base width Wb decreases correspondingly. This effect has - in principle - the following consequences:

1) When VBE is held constant, the emitter current IE=f(VBE) remains constant but the collector current increases and the base current is reduced correspondingly (due to IE=IB+IC=const).

2) When IB=const, both IE and IC increase at the same time (B value is larger).

In reality, neither of the cases 1) or 2) will be exactly met - instead, we will have a mixture of both cases.

Remark: As can be derived from the above (and as shown in the corresponding data sheets for constant IB and VBE, respectively) the slopes of the curves Ic=f(VCE) are not equal in both cases. Hence, the derived values for the EARLY voltage VA are different.

Thus, do we have two EARLY voltages? Surprisingly, this question has not been answered in textbooks (as far as I know). Some sources define the EARLY voltage for VBE=const, some others for IB=const. and some make no statement at all.