A complex filter circuit which receives a signal of an in-phase component and a signal of a quadrature component, and which has a circuit on the in-phase side and a circuit on the quadrature side being identically constructed, each circuit comprising:
- a first OTA (Operational Transconductance Amplifier) which receives differential signals differentially;
- a second OTA which receives differential outputs of said first OTA differentially;
- a first capacitor connected between commonly connected differential outputs of said first and second OTAs;
- a third OTA having differential inputs connected via second capacitors of positive and negative phases to respective ones of the commonly connected differential outputs of said first and second OTAs;
- and
- a third capacitor connected between differential inputs of said third OTA;
- the differential outputs and differential inputs of said third OTA being commonly connected to differential output terminals;
- wherein said complex filter circuit further comprises first to sixth OTA pairs as a coupler of the in-phase and quadrature sides, each OTA pair having two terminals at which differential inputs of one OTA thereof and differential outputs of the other OTA thereof are respectively connected, and another two terminals at which differential outputs of the one OTA and differential inputs of the other OTA are respectively connected;
- the two terminals of said first OTA pair are connected to both ends of the first capacitor on the in-phase side, and the two other terminals of said first OTA pair are connected to both ends of the first capacitor on the quadrature side;
- the two terminals of said second OTA pair are respectively connected to one end of one of said first and third capacitors on the in-phase side via a capacitor and to one end of the other of said first and third capacitors on the in-phase side directly, and the other two terminals of said second OTA pair are respectively connected to one end of one of said first and third capacitors on the quadrature side via a capacitor and to one end of the other of said first and third capacitors on the quadrature side directly;
- the two terminals of said third OTA pair are respectively connected to the other end of one of said first and third capacitors on the in-phase side via a capacitor and to the other end of the other of said first and third capacitors on the in-phase side directly, and the other two terminals of said second OTA pair are respectively connected to the other end of one of said first and third capacitors on the quadrature side via a capacitor and to the other end of the other of said first and third capacitors on the quadrature side directly;
- the two terminals of said fourth OTA pair are connected to both ends of the positive-phase second capacitor on the in-phase side and the other two terminals are connected to both ends of the positive-phase second capacitor on the quadrature side;
- the two terminals of said fifth OTA pair are connected to both ends of the negative-phase second capacitor on the in-phase side and the other two terminals are connected to both ends of the negative-phase second capacitor on the quadrature side;
- and
- the two terminals of said sixth OTA pair are connected to both ends of the third capacitor on the in-phase side and the other two terminals of said sixth OTA pair are connected to both ends of the third capacitor on the quadrature side.