What does a coupling capacitor do in a circuit?

• A. It blocks direct current and passes alternating current.
• B. It blocks alternating current and passes direct current.
• C. It increases the resonant frequency of the circuit.
• D. It decreases the resonant frequency of the circuit.

Answer: (A)

A coupling capacitor serves to pass the AC part of a signal between stages while blocking the DC part. A capacitor blocks direct current because, after it charges, no steady current can flow through it. When the signal changes, the capacitor continuously charges and discharges, creating a path for the alternating current to move from one stage to the next. The impedance of a capacitor decreases as frequency increases, so at the frequencies of interest it presents only a small opposition to the signal, letting the AC information pass through. This setup keeps the DC bias of each stage separate, preventing one stage’s DC level from shifting the next. In practice, the capacitor and the next stage’s input resistance form a high-pass filter, setting the lowest frequency that can be transmitted with minimal attenuation.