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In electronics, capacitive coupling is the transfer of energy within an electrical network by means of the capacitance between circuit nodes. This coupling can have an intentional or accidental effect. Capacitive coupling is typically achieved by placing a capacitor in series with the signal to be coupled. ==Use in analog circuits== In analog circuits, a coupling capacitor is used to connect two circuits such that only the AC signal from the first circuit can pass through to the next while DC is blocked. This technique helps to isolate the DC bias settings of the two coupled circuits. Capacitive coupling is also known as ''AC coupling'' and the capacitor used for the purpose is also known as a ''DC-blocking capacitor''. A coupling capacitor's ability to prevent a DC load from interfering with an AC source is particularly useful in Class A amplifier circuits by preventing a 0 volt input being passed to a transistor with additional resistor biasing; creating continuous amplification. Capacitive coupling has the disadvantage of degrading the low frequency performance of a system containing capacitively coupled units. Each coupling capacitor along with the input electrical impedance of the next stage forms a high-pass filter and the sequence of filters results in a cumulative filter with a −3dB frequency that may be higher than those of each individual filter. So for adequate low frequency response, the capacitors used must have high capacitance ratings. They should be high enough that the reactance of each is at most a tenth of the input impedance of each stage, at the lowest frequency of interest. See Impedance bridging. Coupling capacitors can also introduce nonlinear distortion at low frequencies. This is not an issue at high frequencies because the voltage across the capacitor stays very close to zero; both sides move up and down together. However, if a signal is allowed to pass through the coupling that is low relative to the RC cutoff frequency, voltages can develop across the capacitor, which for some capacitor types results in changes of capacitance, leading to distortion. This is avoided by choosing capacitor types that have low ''voltage coefficient'', and by using large values that put the cutoff frequency far lower than the frequencies of the signal. These disadvantages of capacitively coupling DC-biased transistor amplifier circuits are largely minimized in directly coupled designs. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Capacitive coupling」の詳細全文を読む スポンサード リンク
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