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A circuit may contain both a DC level and an AC signal riding on top of it. When that circuit is connected to another stage, the two stages often operate at different DC voltages. Those DC voltages must remain separate; otherwise, one stage will shift the bias point of the other and cause one or both circuits to operate incorrectly.
When measuring AC signals, the same problem appears. The DC level in the circuit can produce false readings in AC test equipment, and the meter may draw enough DC current to pull the circuit's DC level away from where it needs to be. This unwanted change in the operating point is called DC loading.
A capacitor solves both problems. Because a capacitor blocks steady DC but passes changing AC, it can be placed between stages—or between a circuit and test equipment—to prevent DC levels from interacting while still allowing the AC signal to flow. A capacitor used to prevent cross-coupling of DC between circuits or to prevent test equipment from DC-loading circuits is called a coupling capacitor.
To select a coupling capacitor, choose a value large enough that its reactance at the lowest signal frequency is much smaller than the input impedance of the circuit it feeds (we will learn how to calculate reactance in AC Circuits). A common rule is to make the capacitor's reactance no more than one‑tenth of the input impedance. This ensures that the capacitor and input impedance do not form a high‑pass filter that attenuates or distorts the desired AC signal.
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