When talking about filters in the context of electronics, it means a
circuit that will block signals of a certain frequency and allow others
to pass; some examples are signal filters to block signals of a certain
frequency to be amplified, and source filters that remove an alternating
component from a DC power line.
Most filters rely on the ability of some components, capacitors and
inductors, to change their ability to conduct current at certain
frequencies to attenuate a signal to the point where it's no longer a
problem.
One thing with this "passive" approach is that you can't use multiple
stages of filtering because the signal gets smaller smaller with every
stage, so the use of "active" filters, those that incorporate an
amplifying element, became a necessity.
In this series we have already used two of the most fundamental active
filters, even though we didn't see them as such at the time, now you are
going to revisit them with new eyes and new information on how exactly
their filtering properties emerge.
