Gain-tuned active filters with constant percent bandwidth

It is shown that the amplifier gains in a two-amplifier active filter section can be used to tune the center frequency of its bandpass characteristic in such a manner that the percent bandwidth remains essentially constant over the tuning range. Low-pass and high-pass tunable sections with constant pole- are also derived. Altogether, fourteen basic bandpass sections and six circuits each for the low-pass and the high-pass sections have been derived. Formulas for their parameters are given. A design example and experimental results are given.