PURPOSE: To optionally decide the center point of notch frequency depending on 4 components comprising resistors and capacitors by providing a BPF, a circuit subtracting an output of the BPF from an input signal and a circuit supplying/interrupting the input signal to/from the subtraction circuit and constituting the BPF by an operational amplifier of differentiation type and integration type with double feedback.

CONSTITUTION: An arithmetic circuit 2 subtracts an output of the BPF 1 from the input Vin, an amplifier 11 gives the input Vin of a prescribed level to the arithmetic circuit 2 and the amplifier 11 is controlled by an electronic switch SW. The BPF consists of an operational amplifier Q applied with a double negative feedback made up of a differentiation type feedback circuit 3 comprising a capacitor C1 and a resistor R1 and an integration type feedback circuit 4 comprising a resistor R3 and a capacitor C2, and the resistor R2 decides the input coefficient of the BPF together with the resistor R1. The arithmetic circuit 2 consists of an adder circuit comprising the inverse operational amplifier OP, the coefficient of the arithmetic operation is decided by resistors R4, R5 and R6 to obtain the filter output. The center point of the notch frequency f7 of the active filter is varied easily with the selection of the components C1, C2, R1 and R3, and the band stop characteristic is realized by turning on the SW of the amplifier 11 and the band pass characteristic is attained by turning off the SW.