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*Introduction*

It is frequently necessary to represent the transistor by a simple
equivalent circuit in order to calculate the small-signal circuit response. The most
widely used equivalent circuit is the *hybrid-pi model*,
shown in this applet. It represents a transistor operated in the normal active mode in the
common-emitter configuration. A very important feature of the hybrid-pi model is that all
the parameters are related to the physical processes in the transistor.

Basically, the hybrid-pi model represents the dynamic incremental
variation of the stored charge in the base upon the incremental change of the applied
emitter voltage. Consider a small-signal input voltage *V _{be}* applied in
series with