Gm-C FILTER

PROBLEM TO BE SOLVED: To provide a Gm-C filter wherein its filter characteristic is made constant without depending on the amplitude of the differential input signal inputted to its Gm amplifier. SOLUTION: A V/I converting portion 21 so inputs from a Gm amplifier 1 the differential output voltage thereof as to convert it into a corresponding differential output current. Based on the converted differential output current, a Gm adding portion 22 generates an output voltage VB including the voltage represented by the sum of both the transconductances of MOS transistors M2, M3. The output voltage VB is so changed into a level-shifted voltage VC by a level shifter 24 as to apply it to the gate of a MOS transistor M21 of a current comparing portion 25. The unbalance portion between a current I21 generated from the MOS transistor M21 and a reference current generated from a transistor M6 so appears as the variation of the drain voltage of the MOS transistor M21 as to feedback it to the Gm amplifier 1.