RC Filters: Low-Pass & High-Pass

Why pairing a resistor with a capacitor lets a circuit pass some frequencies through while blocking others.

You already know that a capacitor resists sudden changes in voltage (Lesson 7) — it takes time to charge and discharge through a resistor. That charging delay is the entire basis of an RC filter: a resistor and capacitor wired together to treat fast-changing (high-frequency) and slow-changing (low-frequency) signals differently.

In a low-pass filter, the resistor sits in the signal path and the capacitor connects from after the resistor down to ground. Slow (low-frequency) signals change gradually enough that the capacitor has time to charge and discharge along with them, so they pass through mostly unaffected. Fast (high-frequency) signals change too quickly for the capacitor to keep up — it effectively short-circuits them to ground before they reach the output. The result: low frequencies pass, high frequencies are attenuated.

Swap the resistor and capacitor's positions and you get a high-pass filter, with the opposite behavior: high frequencies pass through, low frequencies (including a steady DC voltage) get blocked.

The frequency where a filter transitions from "passing" to "blocking" is called the cutoff frequency, and it depends only on the resistor and capacitor values you choose: larger resistor or capacitor values give a lower cutoff frequency. RC filters show up everywhere — smoothing out a noisy sensor reading, blocking a DC offset before an audio amplifier, or cleaning up a switching regulator's output ripple.