Patent ID: 7471141

Claim:
A filter circuit having an order of three or more and a cutoff frequency f c , said filter circuit comprising at least one amplifying unit for amplifying an in-band signal, a frequency response at an output of said filter circuit having an attenuation characteristic obtainable with said order of said filter circuit, based upon an attenuation measured at a frequency 2 f c and an expected attenuation of about 6 dB/octave per order, comprising a plurality of stages, each comprising at least one of a 1st-order filter and a 2nd-order filter, which are cascade-connected to each other, a summation of orders of said 1st-order and 2nd-order filters being (2×m+1), where m is an integer equal to or greater than 1; wherein said amplifying unit comprises an amplifier arranged between an output of a first-stage filter and an input of a succeeding, second-stage filter, in at least one pair among a plurality of pairs of filters of adjacent stages.