An active filter is some combination of integrated-circuit operational amplifiers, resistors, and capacitors that does things that normally could be done only with expensive inductor-capacitor passive filter combinations. Active filters are versatile, low-cost items that are easy to design and easy to tune. They have gain and have a number of other benefits. Active filters are well suited for most subaudio, audio, and ultrasonic filtering or equalizing applications. Important areas of use for active filters include communications, electronic music, brainwave research, quadrature art, speech and hearing studies, telephony, psychedelic lighting, medical electronics, seismology, instrumentation, and many other areas.
This book is about active filters. It is user-oriented. It tells you everything you need to know to build active filters, and does so with an absolute minimum of math or obscure theory. — Active-Filter Cookbook
