Q factor, Q stability and gain in active filters

This letter sets out a simple theory relating the maximum Q factor of a second-order active-filter section to the stability of that Q factor and the available gain in the active elements used. The theory is shown to apply to both the `negative-feedback¿ or `Rauch¿-type filters and `positive-feedback¿ or `Sallen¿Key¿-type filters and by implication to negative-impedance convertors (n.i.c.s) and the whole family of circuits based on minimum-phase feedback networks.