High-pass filter ($H(s)=s\tau /(1+s\tau )$ for a first-order RC example)

A high-pass filter attenuates low frequencies relative to a specified higher-frequency pass-band. A simple first-order RC high-pass has the displayed transfer function, with a zero at zero frequency and a pole at $s=-1/\tau$. Combining a high-pass with a low-pass makes a band-pass, but its attainable selectivity depends on pole structure rather than gain alone.