Abstract: A bidirectional filter circuit having an input and an output and substantly constant input and output impedance characteristics over the desired frequency range. A low-pass filter network is connected between filter input and output and has identical low-pass filter half sections cascaded back-to-back with a symmetrical all-pass network interposed therebetween, the all-pass network providing isolation between the low-pass filter half sections and the desired terminating impedance therefor. First and second identical compensating networks are respectively connected to the filter input and output and have reciprocal characteristic functions with respect to the low-pass filter half sections to compensate for impedance changes in the low-pass filter half sections over the desired frequency range without affecting amplitude characteristics thereof.