Passivity (engineering)
[2] For a given system with a known model, it is often easier to construct a storage function satisfying the differential inequality than directly computing the available energy, as taking the supremum on a collection of trajectories might require the use of calculus of variations.
In circuit design, informally, passive components refer to ones that are not capable of power gain; this means they cannot amplify signals.
To give other terminology, systems for which the small signal model is not passive are sometimes called locally active (e.g. transistors and tunnel diodes).
Systems that can generate power about a time-variant unperturbed state are often called parametrically active (e.g. certain types of nonlinear capacitors).
[4] Formally, for a memoryless two-terminal element, this means that the current–voltage characteristic is monotonically increasing.
This only works if only one of the above definitions of passivity is used – if components from the two are mixed, the systems may be unstable under any criteria.
This is especially important in the design of large, complex control systems (e.g. stability of airplanes).
A passive filter has several advantages over an active filter: They are commonly used in speaker crossover design (due to the moderately large voltages and currents, and the lack of easy access to a power supply), filters in power distribution networks (due to the large voltages and currents), power supply bypassing (due to low cost, and in some cases, power requirements), as well as a variety of discrete and home brew circuits (for low-cost and simplicity).
Passive filters are uncommon in monolithic integrated circuit design, where active devices are inexpensive compared to resistors and capacitors, and inductors are prohibitively expensive.
When current passes through it, an energic passive circuit element converts some of the energy supplied to it into heat.
When current passes through it, a non-energic passive circuit element converts none of the energy supplied to it into heat.
