What Amplifier for Piezoelectric Ultrasonic Transducer?
(Replying based on memory, please verify), you'll need a high-voltage amp that can handle a large, capacitive load. Piezo actuators will look like big, slightly-lossy capacitors to the amplifier (the piezo material is, among other things, a high-k dielectric material.) Not all amplifiers can handle the peak voltage needed to drive (assumed, bare) piezo elements at high power (with any hope of linearity) when presented with a capacitive load that will draw peak current at almost no voltage -- leaving a linear output stage to dissipate a high voltage * a high current. If you can, buy an amp designed to drive a piezo with (as close as you can get to) same impedance, resonant frequency, and power level.