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.