A filter arrangement with active elements.

摘要

An active filter arrangement particularly for the reception and transmission of signal sequences,which comprises three feed back operational amplifiers (1,2,3) connected in a closed loop chain. The feedback impedance of one of the amplifiers (1) comprises an ohmical and a capacitive member connected in parallel, while another (2) is fed back by a capacitive impedance. At least one of the capacitive branches includes an RC member. The quality factor can be changed by the adjustment of the ohmical branch while the frequency can be changed by adjusting the coupling between the operational amplifiers within wide ranges. One of the operational amplifiers comprises a feedback circuit (7) controlled by a pulse width modulator (8) by means of which an oscillator mode of operation can be set and the frequency of oscillation will be that of the filter. The oscillator amplitude can be controlled by means of the pulse width modulator (8). The adjustment of the frequency and/or the quality factor can be carried out by means of an attenuator comprising a series chain of resistors (R0 to Rn) coupled to an operational amplifier (17) which arrangement is capable of realizing virtual ground potential with discretely changable location in the chain. The active filter has a preferable field of application in a circuit arrangement for generating and receiving signal sequences in radio transmitter-receiver systems.