Decoupling capacitor: Difference between revisions

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==Discussion==

Active devices of an electronic system (transistors, ICs, vacuum tubes, for example) are connected to their power supplies through conductors with finite resistance and inductance. If the current drawn by an active device changes, voltage drops from power supply to device will also change due to these impedances. If several active devices share a common path to the power supply, changes in the current drawn by one element may produce voltage changes large enough to affect the operation of others - [[voltage spike]]s or [[ground bounce]], for example - so the change of state of one device is coupled to others through the common impedance to the power supply. A decoupling capacitor provides a bypass path for transient currents, instead of flowing through the common impedance. <ref name=TTL75> Don Lancaster, ''TTL Cookbook', Howard W. Sams, 1975, no ISBN, pp.23-24 </ref>

For example, if the voltage level for a device is fixed, changing power demands are manifested as changing current demand. The power supply must accommodate these variations in current draw with as little change as possible in the power supply voltage. When the current draw in a device changes, the power supply cannot respond to that change instantaneously. As a consequence, the voltage at the device changes for a brief period before the power supply responds. The [[voltage regulator]] adjusts the amount of current it is supplying to keep the output voltage constant but can only effectively maintain the output voltage for events at frequencies from DC to a few hundred kHz, depending on the regulator (some are effective at regulating in the low MHz). For [[Transient (oscillation)|transient events]] that occur at frequencies above this range, there is a time lag before the voltage regulator responds to the new current demand level. This is where the decoupling capacitor comes in. The decoupling capacitor works as the device’s local [[Electric field|energy storage]]. The capacitor is placed between power line and ground to the circuit that current is to be provided. According to capacitor equation, ''i(t) = CdV(t)/dt'', voltage drop between power line and ground results in current draw out from the capacitor to the circuit and when capacitance ''C'' is large enough, sufficient current is supplied with acceptable range of voltage drop (however, as described below, [[Parasitic element (electrical networks)|parasitic inductance]] is typically high for large capacitors, so small and large capacitors are usually placed together in parallel to fully cover circuit bandwidth). The capacitor cannot provide DC power because it stores only a small amount of energy but this energy can respond very quickly to changing current demands. The capacitors effectively maintain power-supply voltage at [[Frequency response|frequencies]] roughly from hundreds of kHz to hundreds of MHz (in the milliseconds to nanoseconds range). Decoupling capacitors are not useful for events occurring above or below this range: at the low end, they don't store enough energy and at the high end (at or above the self-resonant frequency) their parasitic inductance dominates their (high) impedance.

The decoupling capacitor works as the device’s local [[Electric field|energy storage]]. The capacitor is placed between power line and ground to the circuit that current is to be provided. According to capacitor equation, ''i(t) = CdV(t)/dt'', voltage drop between power line and ground results in current draw out from the capacitor to the circuit and when capacitance ''C'' is large enough, sufficient current is supplied with acceptable range of voltage drop. To reduce the effective series inductance, small and large capacitors are usually placed in parallel; many small capacitors may be adjacent to individual integrated circuits. The capacitor stores a small amount of energy that can compensate for the voltage drop in the power supply conductors to the capacitor.

In digital circuits, decoupling capacitors also help prevent radiation of [[electromagnetic interference]] due to rapidly changing power supply currents.

Decoupling capacitors alone may not suffice in such cases as a high-power amplifier stage with a low-level pre-amplifer coupled to it. Care must be taken in layout of circuit conductors that heavy current of one stage do not produce power supply voltage drops that affect other stages. The may require re-routing printed circuit board traces to segregate circuits, or the use of a [[ground plane]] to improve stability of power supply.

==Decoupling==