Decoupling capacitor

Active devices of an electronic system (e.g. transistors, integrated circuits, vacuum tubes) are connected to their power supplies through conductors with finite resistance and inductance.

A decoupling capacitor provides a bypass path for transient currents, instead of flowing through the common impedance.

When capacitance C is large enough, sufficient current is supplied to maintain an acceptable range of voltage drop.

To reduce undesired parasitic equivalent series inductance, small and large capacitors are often placed in parallel, adjacent to individual integrated circuits (see § Placement).

Decoupling capacitors alone may not suffice in such cases as a high-power amplifier stage with a low-level pre-amplifier coupled to it.

A bypass capacitor is often used to decouple a subcircuit from AC signals or voltage spikes on a power supply or other line.

A bypass capacitor can shunt energy from those signals, or transients, past the subcircuit to be decoupled, right to the return path.

Typical power supply lines show inherent inductance, which results in a slower response to changes in current.

The best way to reduce switching noise is to design a PCB as a giant capacitor by sandwiching the power and ground planes across a dielectric material.

The parasitic inductance in every (decoupling) capacitor may limit the suitable capacity and influence the appropriate type if switching occurs very fast.