What Is a High-Pass Filter?
A high-pass filter passes signals with frequencies above a certain cutoff frequency while attenuating lower frequencies. In an RC high-pass filter, the capacitor blocks DC and low frequencies while passing high frequencies to the output. The cutoff frequency (-3 dB point) is where the output amplitude drops to 70.7% of the input.
High-pass filters are essential in audio engineering (removing rumble and hum), radio receivers (blocking DC bias), sensor signal conditioning, and communication systems. A first-order filter provides 20 dB/decade (6 dB/octave) roll-off below the cutoff frequency.
Cutoff Frequency Formula
Common Applications
| Application | Cutoff Freq | Purpose |
|---|---|---|
| Audio (rumble filter) | 20-80 Hz | Remove subsonic noise |
| Microphone preamp | 50-150 Hz | Block handling noise |
| DC blocking | <1 Hz | Remove DC offset |
| RF coupling | varies | Block lower bands |
Frequently Asked Questions
What is the difference between first and second order?
A first-order high-pass filter (single RC or RL stage) attenuates at 20 dB per decade below cutoff. Second-order filters (two stages or LC) attenuate at 40 dB per decade, providing sharper frequency selectivity. Higher orders give steeper rolloff but add complexity and potential phase issues.
How do I choose R and C values?
Start with the desired cutoff frequency and a practical resistor value (1k-100k for audio). Calculate C = 1/(2 pi R fc). For audio, 10k with 1.6nF gives about 10kHz cutoff. Use standard component values and verify with measurement, as component tolerances affect the actual cutoff frequency.