As frequency increases, the phase of the impedance of a parallel RC circuit approaches which value?

As frequency increases, the phase of the impedance of a parallel RC circuit approaches which value?

• A. 0 degrees
• B. +90 degrees
• C. -90 degrees
• D. 180 degrees

Answer: (C)

As frequency climbs, the capacitor's reactance gets smaller, so the capacitor branch dominates the parallel combination. The total impedance is Z = 1 / (1/R + jωC). When ω is large, the term jωC overwhelms 1/R, giving 1/Z ≈ jωC and thus Z ≈ 1/(jωC) = -j/(ωC). That means the impedance is mainly imaginary and negative, which corresponds to a phase of -90 degrees. Therefore, the phase approaches -90 degrees as frequency increases.

As frequency climbs, the capacitor's reactance gets smaller, so the capacitor branch dominates the parallel combination. The total impedance is Z = 1 / (1/R + jωC). When ω is large, the term jωC overwhelms 1/R, giving 1/Z ≈ jωC and thus Z ≈ 1/(jωC) = -j/(ωC). That means the impedance is mainly imaginary and negative, which corresponds to a phase of -90 degrees. Therefore, the phase approaches -90 degrees as frequency increases.