A preamplifer for use with a magneto-resistive playback head in which a d.c. current source provides current through a field effect transistor and the magneto-resistive element to ground. An operational transconductance amplifier included in a feedback loop provides a bias voltage for the field-effect transistor, whereby the voltage across the said magneto-resistive element is held constant while the resistance of said element changes. The resulting a.c. current flowing through the magneto-resistive element flows only through the feedback loop and an output resistor connected across the inputs of the operational transconductance amplifier. The amplification of the circuit is approximately the resistance of the output resistor divided by the resistance of the magneto-resistive element.
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