Patent ID: 6998802

Claim:
A motor driving device comprising: a motor; and a motor rotation rate detecting circuit, wherein said motor rotation rate detecting circuit includes a differentiating circuit that produces a rotation rate detection signal by differentiating a plurality of alternating signals having different phases that are output according to a rotation position of said motor, wherein a frequency response of differentiating operation of said differentiating circuit is switchable according to a voltage of said rotation rate detection signal, wherein said motor rotation rate detecting circuit further includes a selecting circuit, wherein said selecting circuit switches the frequency response of the differentiating operation of said differentiating circuit according to the voltage of said rotation rate detection signal, wherein said differentiating circuit includes a capacitor, at least two resistors, at least one MOSFET, and an operational amplifier, wherein said selecting circuit switches the frequency response of the differentiating operation of said differentiating circuit by turning on and off said at least one MOSFET according to the voltage of said rotation rate detection signal, and wherein said differentiating circuit has a constant alternating-current gain irrespective of the frequency response of the differentiating operation thereof.