Circuits That Change
Filters & Frequency Response
14 min · 2 worked examplesFree
Building a circuit that keeps the frequencies you want and rejects the rest, and reading its behaviour off a graph.
After this lesson you can
- Work out the corner frequency of an RC filter
- Read a rolloff in dB per decade
- Explain what a decoupling capacitor is doing
The RC low-pass filter
A resistor in series with a capacitor to ground makes a low-pass filter: low frequencies pass through, high ones are shunted to ground by the capacitor's falling reactance.
The cutoff frequency is where reactance equals resistance: fc = 1/(2πRC). At that point the output has fallen to 0.707 of the input, which is the −3 dB point from the decibels lesson, half the power.
Swap the two components and you have a high-pass filter instead, blocking DC and passing fast changes.
Note that RC appears again, exactly as in the time-constant lesson. A filter's cutoff and a circuit's step response are two views of the same physics: a slow time constant is a low cutoff, and a filter that smooths well necessarily responds slowly.
In the full lesson
- The RC low-pass filter
- Rolloff and order
- Decoupling as a filter
Free with an account. Read it, work the examples, answer the questions, then build it in the simulator.