Comment on `Sallen and Key filter networks with amplifier gain larger than or equal to unity'

Refers to an article by G. S. Moschytz (see abstr. B6114 of 1968) that gives consideration to the realization of a second-order transfer function of specified form through utilization of RC elements plus a noninverting operational amplifier. Moschytz claims in the abstract that he has presented a suitable realization, a modified Sallen and Key network, that contains a noninverting amplifier having a gain that can always be made greater than or equal to unity. Such a gain then permits one to incorporate a noninverting operational amplifier into the design. This correspondence shows that there exists a subclass of second-order transfer functions that, when realized by Moschytz's realization procedure, results in a gain constant that is less than unity for all values of the available parameters. It is then not possible to incorporate a noninverting operational amplifier without altering the network structure. A simple modification of Moschytz's work is presented that assures the existence of a realization that includes a noninverting operational amplifier.