A disk drive is disclosed employing an offset-nulling amplifier for detecting
a
back EMF voltage of a voice coil motor (VCM) in a velocity mode servo control system.
The offset-nulling amplifier comprises a differential amplifier, an input stage
for swapping the polarity of the input voltage at a periodic interval, and an output
stage comprising a single-ended amplifier having an input switched at the periodic
interval between the positive and negative outputs of the differential amplifier.
