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Common Emitter Models

Module by: Bill Wilson

Summary: Common Emitter Models

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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 a new current controlled current source between collector and emitter. There is one small caveat which we need to keep in mind however 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, it was true that

I E = I sat ⅇq V BE kT−1 I E I sat q V BE k T 1 (1)

Figure 1: Discrete model for the common emitter configuration
Figure 1 (3_10.png)

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

I C =α I E =α I sat ⅇq V BE kT−1 I C α I E α I sat q V BE k T 1 (2)
I B = I C β=α I sat βⅇq V BE kT−1 I B I C β α I sat β q V BE k T 1 (3)
So the operational equation for the diode in the base circuit still is the usual exponential function of V BE V BE , except that it now has a saturation current of α I sat β α I sat β instead of just I sat I sat .

In principle you could put this model into a circuit, and analyze it to find all of the necessary voltages and currents. However, this would not be very convenient. The base-emitter junction is connected by a diode, which as we know, has a very non-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.

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