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Capacitors are passive devices used in electronic circuits to store energy in the form of an electric field. They are the compliment of inductors, which store energy in the form of a magnetic field. An ideal capacitor is the equivalent of an open circuit (infinite ohms) for direct currents (DC), and presents an impedance (reactance) to alternating currents (AC) that depends on the frequency of the current (or voltage). The reactance (opposition to current flow) of a capacitor is inversely proportional to the frequency of the of the signal acting on it. Capacitors were originally referred to as "condensers" for a reason that goes back to the days of the Leyden Jar where electric charges were thought to accumulate on plates through a condensation process.
The property of capacitance that opposes a change in voltage is exploited for the purpose of conducting signals with a higher frequency component while preventing signals of lower frequency components from passing. A common application of a capacitor in an RF (radio frequency) circuit is where there is a DC bias voltage that needs to be blocked from being present in a circuit while allowing the RF signal to pass. DC power supplies use large capacitance values in parallel with the output terminals to smooth out low frequency ripples due to rectification and/or switching waveforms.
When used in series (left drawing) or parallel (right drawing) with its circuit compliment, an inductor, the inductorcapacitor combination forms a circuit that resonates at a particular frequency that depends on the values of each component. In the series circuit, the impedance to current flow at the resonant frequency is zero with ideal components. In the parallel circuit (right), impedance to current flow is infinite with ideal components.
Realworld capacitors made of physical components exhibit more than just a pure capacitance when present in an AC circuit. A common circuit simulator model is shown to the left. It includes the actual ideal capacitor with a parallel resistive component ('Leakage') that responds to alternating current. The equivalent DC resistive component ('ESR') is in series with the ideal capacitor and an equivalent series inductive component ('ESL') is present due to metal leads (if present) and characteristics of the plate surfaces. This inductance, in combination with the capacitance, creates a resonant frequency at which point the capacitor looks like a pure resistance.
As the operational frequency is increased past resonance (aka selfresonant frequency, or SRF), the circuit behaves as an inductance rather than a capacitance. Hence, careful consideration of the SRF is required when selecting capacitors. SPICEtype simulators use this or an even more sophisticated model to facilitate more accurate calculations over a wide range of frequencies.
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The Noble Art of DeCoupling
Equations for combining capacitors in series and parallel are given below. Additional equations are given for capacitors of various configurations. As these figures and formulas indicate, capacitance is a measure of the ability of two surfaces to store an electric charge. Separated and isolated by a dielectric (insulator), a net positive charge is accumulated on one surface and a net negative charge is stored on the other surface. In an ideal capacitor, charge would be stored indefinitely; however, real world capacitors gradually lose their charge due to leakage currents through the nonideal dielectric.

