The Sallen-Key filter structure is a revered analog filter design topology. In Sallen-Key lowpass and highpass filters, the cutoff frequency and resonance (Q) controls are decoupled though the cutoff and resonant frequencies are not. In this paper we demonstrate novel variations on the Sallen-Key architecture and we decouple the resonant and cutoff frequencies. This produces multiple hybrid filter designs including resonant quasi-first order lowpass and highpass filters, resonant quasi-first order low and high shelving filters, decoupled resonant second order filters and doubly resonant quasi-second order lowpass and highpass filters. In the doubly-resonant filters all three frequencies may be decoupled and independently adjustable; they also self-oscillate at both resonant frequencies.
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