One circuit generates a constant positive potential, another circuit generates
a constant negative potential. The positive potential and the negative potential
are applied to a magneto-resistive head that reads data from and writes data to
a magnetic disk. The positive potential is generated using a positive voltage source,
an operational amplifier, and an NMOS transistor. The negative potential is generated
using a negative voltage source, an operational amplifier, and a PMOS transistor.
