In previous chapters, we discussed the dynamic behavior of transistors from a small-signal point of view using the hybrid-pi model. In other words, the small-signal models developed so far are valid only for small variations of the voltages and currents around an operating point. We used the method of open-circuit time constants to analyze bandwidth limitations in linear amplifiers.

A further modeling technique, called the charge control model, permits us to analyze the behavior of transistor circuits during large-signal switching and during transistor saturation. Similarly to the method of open-circuit time constants, the charge control model gives results that aren t necessarily numerically accurate; rather the usefulness of the method is the design insight that we derive from the application of relatively simple models.