Static Versus Dynamic Resistance
by Dennis L. Feucht
Innovatia Laboratories
The electronic circuits literature, beginning with textbooks, does not usually distinguish clearly between static and dynamic resistance. This article explains the difference, why it matters in design, and then offers some circuits with input resistances that at first might seem hard to believe.
What adds to the confusion is another related pair of yet different quantities: total-variable (or large-signal) versus incremental (or small-signal). This distinction arises because of nonlinear circuit components, primarily semiconductors. Nonlinear circuits are generally difficult to analyze. The most common approach is to approximate their nonlinear functions with linear functions in the region of operation. This can be done by either deriving the nonlinear equations for the entire nonlinear circuit, then linearizing the result around the operating point, or (as is usually done) by linearizing the individual components of the circuit and then doing a linear circuit analysis. The linear approximations for nonlinear components -- mainly semiconductors -- depend on where in the domain of the characteristic device function the linearization occurs. The accuracy of the analysis then depends on maintaining minor, or incremental, variations around this operating point.
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