Using a low voltage opamp for feedback with high voltage mosfet?
Your MOSFET or power suppy would blow up in a flash because as soon as MOSFET input goes high it will connect the 100V directly to ground.

Have a look at the way Australian Silicon Chip Magazine drives its piezos in its "Piezo Transducer boat anti-foul system", design published September 2010.

www.siliconchip.com.au