RC High-Pass Filter

Formula

Description

A first-order RC high-pass filter passes signals above the cutoff frequency and attenuates signals below it. The circuit is the complement of the low-pass filter: a series capacitor followed by a resistor to ground. At high frequencies, the capacitor presents low impedance and signals pass through to the output resistor. At low frequencies and DC, the capacitor blocks the signal. The cutoff frequency formula is identical to the low-pass filter. High-pass filters are commonly used for AC coupling (blocking DC bias), removing low-frequency drift from sensor signals, and audio crossover networks.

Variables

• fc — Cutoff frequency at -3dB point (Hz)
• R — Resistance (Ω)
• C — Capacitance (F)

Practical Notes

The DC blocking capacitor in audio circuits is effectively a high-pass filter. For audio coupling with a 10kΩ load, use fc well below 20 Hz (e.g., 1µF gives fc ≈ 16 Hz). The phase shift at fc is +45 degrees. Below fc, attenuation increases at 20 dB per decade. For steeper rolloff, use higher-order filters.