RC Filters

Short Topical Videos

• RC Filters (by Karol Sanchez)
• Passive RC low pass filters (by Afrotechmods)
• Passive RC high pass filters (by Afrotechmods)

Reference Material

• Horowitz & Hill, The Art of Electronics, 2nd Ed., Ch. 1

RC Filters

RC filters use resistors ($R$) and capacitors ($C$) to make circuits that have frequency-dependent responses to input waveforms. They are passive circuits that operate on the same principle as the voltage divider, but make use of the imaginary, frequency-dependent impedances of capacitors. Recall that the impedances of resistors and capacitors are given by:

${\begin{aligned}Z_{R}&=R\\Z_{C}&={\frac {1}{j\omega C}}\\\end{aligned}}\,\!$

where $j\equiv {\sqrt {-1}}$. Regardless of whether you are constructing a low-pass or high-pass filter, RC filters have a characteristic frequency at which their frequency response evolves most rapidly. This timescale is given by the product $RC$. In a great triumph of SI units,

$1\ s=1\Omega \cdot 1F.\,\!$

The frequency response of filters is typically given by the cutoff frequency at which a signal is attenuated by 3dB. For RC filters (both low-pass and high-pass), this happens when the magnitude of the impedance of the resistor and the capacitor are equal:

${\begin{aligned}|Z_{R}|&=|Z_{C}|\\|R|&=\left|{\frac {1}{j\omega C}}\right|\\\omega _{-3dB}&={\frac {1}{RC}}\\\end{aligned}}\,\!$

Low-Pass Filter

A passive RC low-pass filter

High-Pass Filter

A passive RC hi-pass filter