Passivity is a property of engineering systems, most commonly used in electronic engineering and control systems. A passive component, depending on field, may either refer to a component that consumes (but does not produce) energy, or to a component that is incapable of gain|power gain. A component that is not passive is called an active component. An electronic circuit consisting entirely of passive components is called a passive circuit (and has the same properties as a passive component).
Other definitions of passivity
In some very informal settings, passivity may refer to the simplicity of the device, although this definition is almost universally considered incorrect (indeed, maybe should not even be included here). Here, devices like diodes would be considered active, and only very simple devices like capacitors, inductors, and resistors are considered passive. In some cases, the term "linear element" may be a more appropriate term than "passive device." In other cases, "solid state device" may be a more appropriate term than "active device."
Passivity, in most cases, can be used to demonstrate that passive circuits will be stable under specific criteria. Note that this only works if only one of the above definitions of passivity is used -- if components from the two are mixed, the systems will, in general, not be stable under any criteria. In addition, passive circuits will not necessarily be stable under all stability criteria. For instance, a resonant series LC circuit will have unbounded voltage output for a bounded voltage input, but will be stable in the sense of Lyapunov stability|Lyapunov, and given bounded energy input will have bounded energy output.
Passivity is frequently used in control systems to design stable control systems or to show stability in control systems. Passivity is also used in some areas of circuit design, especially filter design.
A passive filter is a kind of electronic filter that is made only from passive elements -- in contrast to an active filter, it does not require an external power source (beyond the signal). Since most filters are linear, in most cases, passive filters are composed of just the four basic linear elements -- resistors, capacitors, inductors, and transformers. More complex passive filters may involve nonlinear elements, or more complex linear elements, such as transmission lines.
A passive filter has several advantages over an active filter:
- Guaranteed stability
- Passive filters scale better to large signals (tens of amps, hundreds of volts), where active devices are often impractical
- No power consumption (aside from possibly taking some power out of the signal)
- For linear filters, generally, more linear than filters including active (and therefore non-linear) elements
They are commonly used in loudspeaker|speaker crossover design (due to the moderately large voltages and currents, and the lack of easy access to power), 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 less common in integrated circuit design, where active devices are comparatively inexpensive compared to resistors and capacitors, and inductors are prohibitively expensive.