the present invention achieves technical advantages as a preamplifier write driver having a varying common - mode output voltage . this varying common - mode output voltage also adjusts the derivative of the common - mode voltage , which is proportional to the amount of current coupled onto the mr head through parasitic capacitance . currents of a first circuit and a second circuit are matched to overcome process variations and modeling errors . a pair of transresistance amplifiers are driven by control lines to achieve these matched currents .