The invention relates to a filter circuit comprising input and output terminals and a differential amplifier, said amplifier having a (-) inverting input being connected to the output via a series circuit including a first capacitor and a first resistor, the output of said amplifier being coupled via a series circuit of a second resistor and a second capacitor to a terminal carrying a reference voltage, the (+) non-inverting input to the amplifier having a DC connection to said reference voltage terminal, one said input to said filter circuit being connected to said reference terminal, the other input to said filter circuit being in circuit with said (-) inverting input to said amplifier, the junction between the second resistor and the second capacitor being connected to said output terminal of the filter circuit.
Such a filter circuit is described in the periodical "IEEE Transactions on Consumer Electronics", Volume CE-26, August 1981, pages 303 and 304. Amplitude modulation is mentioned there as a field of application of the filter circuit, the differential amplifier and the first capacitor and resistor operating as an active filter and the second capacitor and resistor operating as a capacitor-resistor filter. The filter circuit may be incorporated in a phase-locked loop circuit.