High-pass filter
A high-pass filter (HPF) is a circuit that lets through signals with frequencies above a chosen cutoff and reduces signals below that cutoff. The point where the output is about 70.7% of the input (−3 dB) is the cutoff frequency.
The simplest HPF uses a capacitor in series with a resistor, and the output is taken across the resistor. At DC (0 Hz), the capacitor blocks the signal, so nothing below the cutoff passes. The speed of the filter is set by the time constant RC, and the cutoff is fc = 1/(2πRC). For example, with a 10 Ω resistor and a 5 kHz cutoff, a 3.2 μF capacitor would do the job.
Active HPFs use an operational amplifier to provide gain and buffering, so the filter can amplify high frequencies or have a defined passband gain. Higher-order HPFs, made by cascading several first-order sections, produce a steeper cut (more attenuation of frequencies beyond the cutoff).
Common uses include blocking DC from circuits that don’t need it, AC coupling inputs to amplifiers, and guiding high-frequency signals to tweeters in loudspeakers. In audio mixers, HPFs are often available on each channel, sometimes fixed and sometimes adjustable.
The idea of high-pass filtering also shows up outside electronics. In optics, a high-pass (often called a short-pass) filter passes wavelengths shorter than a cutoff. In digital processing, HPFs can be implemented in software or with discrete-time designs that mimic the same pass/stop behavior.
In short, a high-pass filter separates high-frequency content from low-frequency content and is foundational in audio, imaging, and many other signal-processing tasks.
This page was last edited on 3 February 2026, at 14:48 (CET).