2.3 Common emitter models

Let's go ahead and draw a model for the transistor in the common emitter configuration . We again have a diode connected between the base and the emitter, and anew current controlled current source between collector and emitter. There is one small caveat which we need to keep in mindhowever when drawing the common emitter circuit. The diode we see in the base circuit is not the same one as we had in the common base model. In the common base model, itwas true that

For the base however, only a small fraction of the current that goes through this "diode" actually goes inthrough the base, the rest is coming in through the collector. Thus we have to make a couple of changes

In principle you could put this model into a circuit, and analyze it to find all of the necessary voltages andcurrents. However, this would not be very convenient. The base-emitter junction is connected by a diode, which as we know, has a verynon-linear I-V relationship. It would be nice if we could come up with a linear model which, at least over some limited range of inputs, we could use with confidence.