Patent ID: 7315199

Claim:
A filter circuit utilizing an operational amplifier for keeping a cut-off frequency constant by varying a resistance value or a capacitance value, comprising: a unity gain angular frequency stabilizing circuit including: a circuit obtained by replicating an input differential stage of the operational amplifier having an input connected to a voltage source and having an output; a current generating source for generating a current based on an operation keeping the cut-off frequency constant and having an output connected to the output of the circuit obtained by replicating an input differential stage of the operational amplifier, for adjusting the output of the circuit obtained by replicating an input differential stage of the operational amplifier; a transistor having a gate connected to the adjusted output of the current generating source for generating a bias current of the circuit obtained by replicating an input differential stage of the operational amplifier; and a circuit for inputting an output of the unity gain angular frequency stabilizing circuit to the gate of the transistor for generating the bias current of the input differential stage of each operational amplifier constituting the filter circuit.