Patent ID: 7078892

Claim:
A rotation detecting device comprising: a magnetic rotary member having a plurality of teeth on its outer periphery; a biasing magnet having a magnetic center axis for providing said rotary member with a magnetic field; a magnetic sensor unit disposed between said teeth and said biasing magnet, said magnetic sensor unit including at least four bridge circuits of magnetoresistive elements being arranged to respectively provide voltage signals V 1 , V 2 , V 3 and V 4 that change with a change in the direction of the magnetic field; and a differential amplifier circuit for providing a differential output signal Vd, said differential amplifier circuit including a first differential amplifier for providing a first differential signal that is a difference between the voltage signals V 1 and V 2 , a second differential amplifier for providing a second differential signal that is a difference between the voltage signals V 3 and V 4 , first means for providing a first amplified signal of the first differential signal amplified by a variable gain α, second means for providing a second amplified signal of the second differential signal amplified by a fixed gain and a third differential amplifier for providing the differential output signal Vd that is a difference between the first amplified signal and the second amplified signal, wherein the variable gain is adjusted to provide a minimum air-gap characteristic point that corresponds to a fixed rotation angle relative to one of the teeth of said magnetic rotary member.