A low-pass filter circuit

PROBLEM TO BE SOLVED: To provide a Sallen-Key type low-pass filter which can vary characteristics (ω0 and Q). SOLUTION: An input signal X is supplied to a buffer 5 through a resistor R1. An output signal from the butter 5 is supplied to a buffer 11 through a resistor R2. An output signal Y from the buffer 11 is supplied to an output terminal 15 and also feedbacked to an input terminal of the buffer through a capacitor C1. A capacitor C2 is connected between an input terminal side of the buffer 11 and earth. One end of a capacitor C3 is connected to the input of the buffer 11 and the other end of the capacitor C3 is connected to a selector 9a of a switch circuit 9. When the selector 9a of the circuit 9 selects either the terminal 9a for opening or a terminal for connecting the earth, the characteristics (ω0 and Q) of a Sallen-Key type secondary low-pass filter is changed.