Q value correction of filter circuit

A filter circuit using an operational amplifier for keeping a cut-off frequency constant by making a resistance value or a capacitance value variable, comprising:

• a unity gain angular frequency stabilizing circuit provided with a circuit obtained by copying an input differential stage of the operational amplifier, for connecting a voltage source to an input of the copy of the input differential stage,
• provided with a current generating source for generating a current under the control of keeping the cut-off frequency constant, for connecting an output of the current generating source to an output of the copy of the input differential stage, for adjusting the output of the copy of the input differential stage,
• feeding back/inputting the adjusted output to a gate of a transistor for generating a bias current of the copy of the input differential stage;
• and
• a circuit for inputting an output of the unity gain angular frequency stabilizing circuit to a gate of the transistor for generating the bias current of the input differential stage of each operational amplifier constituting the filter circuit.