A write driver includes two field-effect/bipolar transistors with
feedbacks between the drains/collectors and gates/bases, a write/read
switch connected between the drains/collectors and a positive power
supply, an impedance matching circuit connected to the drains/collectors
to match both differential and common mode output impedance of the write
driver to an input impedance of a flex circuit, a common mode output
voltage control circuit coupled to the transistors, and a differential
mode output voltage control circuit coupled to the transistors.