RC Active Filters

Inductorless filters are attractive for numerous practical reasons. Passive RC filters, however, suffer from the defects of high in-band loss and poor economy of elements. These defects are overcome in active RC filters in which amplifying elements supply power to the filter in addition to that applied by the signal. A class of active filters is described in which one active component, a transistor negative-impedance converter, is employed. Simple unbalanced network configurations are obtained in which the number of capacitors in the RC circuits is equal to the total number of reactive elements in the corresponding LC filter. The ultimate limit in performance in this class of active filters is the drift in the converter. The drift in input impedance in converters employing Darlington´s compound transistors is only a few tenths of a per cent of the load impedance for a wide range of loads. Such stability is more than adequate for many practical filter applications. The theory of this type of active RC filters is discussed and experimental tests are reported on low-pass, high-pass and band-pass filters.