Abstract: A magnetic detector, such as is used in a frequency generator, for detecting the rotational rate of a motor includes magnetic resistance elements and an operational amplifier. The magnetic resistance elements have a predetermined resistance ratio and are connected serially between a power source and ground. The common node between the two magnetic resistance elements serves as an output terminal for providing an output voltage. The output voltage from the output terminal being supplied to one of the two input terminals of the amplifier. The voltage from a resistance voltage divider disposed between the power source and ground is supplied to the other input terminal of the amplifier. A change of the resistance ratio between the two magnetic resistance elements is effected by a resistor disposed in series with the elements or in parallel with one of the elements.