passive

In electronics, a passive component is one that cannot amplify a signal or produce energy on its own. It can only consume, store, dissipate, or redirect electrical energy flowing through it. The defining characteristic is lack of power gain: the signal leaving the component is always equal to or smaller than the signal entering it. This contrasts sharply with active components like transistors and integrated circuits, which can boost a signal using power from an external supply.

The common passive components are resistors, capacitors, inductors, and diodes. A resistor dissipates energy as heat according to Ohm's law. A capacitor stores energy in an electric field between its plates; a 100 microfarad capacitor at 12 volts holds approximately 7.2 millijoules. An inductor stores energy in a magnetic field around a coil of wire. A diode allows current to flow in one direction only, with typical forward voltage drops of 0.6 to 0.7 volts for silicon types. None of these can increase the amplitude of an incoming signal; they can only modify it by filtering, blocking, or redirecting it.

The term matters because circuit design depends on understanding what each component can and cannot do. Passive networks are inherently stable; they cannot oscillate uncontrollably or generate noise from internal feedback. They require no power supply, no bias, and no control signals. This makes them ideal for power supplies, audio coupling, RF filtering, and impedance matching. However, a chain of passive components will always attenuate a signal more than amplify it, so passive-only circuits are limited to low-noise, low-gain applications or situations where loss is acceptable.

In practice, the boundary between passive and active blurs slightly with certain components. A transformer is passive; it transfers energy from one coil to another via magnetic coupling but produces no gain. Some engineers debate whether a varactor diode, whose capacitance changes with applied voltage, is truly passive since it is voltage-controlled, but it still produces no power gain. Ferrite beads and surge suppressors are passive; they absorb or block unwanted energy rather than amplifying desired signals.

Passive components appear in every electronic device, often in large numbers. A typical amplifier circuit might contain dozens of resistors and capacitors alongside one or two transistors. In high-frequency work, passive networks can perform remarkable feats: a well-designed LC filter can reject unwanted signals 60 decibels or more below the passband. Passives are also far cheaper and more reliable than active alternatives, making them the first choice whenever the job can be done without amplification.