TABLE OF CONTENTS The requirement to define a large-signal model consistently was already mentioned at several points in Chapter 1. It shall be discussed in detail in this chapter, since an inconsistent formulation can either result in a misleading implementation the model behaves different than claimed and expected, or in a violation of basic physics namely, charge and energy conservation.
It is, however, not much effort to ensure consistency, if one follows some basic principles, which were presented in (1.4) (1.7), p. 16, where all large-signal and small-signal elements available in circuit simulators were introduced along with their interdependences. The inconsistency frequently results from a wrong formulation of the problem, such as when one attempts to define nonlinear resistances, capacitances, or transit times instead of currents and charges. Basically, consistency is ensured if two rather obvious principles are observed:
Resistances, capacitances, and transit times are only defined as small-signal quantities, which are derivatives of voltage-current, charge-voltage, and charge-current functions. The large-signal description has to be formulated in terms of the governing functions (i.e., the model has to be based on nonlinear current and charge sources).
Nonlinear capacitances must be formulated in such a way that they act merely as charge and energy storage elements, and do not generate or dissipate charges or energy.
The intended small-signal model must be identical with the linearized large-signal model.
Since model consistency is a common pitfall, it shall be discussed in broader detail in the following. The first subsection...
