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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 Vbe applied in series with VBE to the base terminal. Let Ib and Ic be the small variations in the collector terminal voltage, as a result of Vbe. The small-signal equivalent-circuit model provides a relationship among these parameters. This applet first illustrates the small-signal model with transistor parasitic resistances, and then the model ignoring these resistances.