I have a Raspberry Pi outputting a 3.3v DC square wave that I'd like to be able to amplify to just under 20v, the maximum the piezo can handle. Just to clarify, the piezo does not have an internal oscillator and the Pi is outputting at anything from 50Hz to 3kHz to make audible tones. From looking elsewhere online, there seem to be many different approaches to amplification but most require an AC wave to work.
I'd like to be able to simply and cheaply amplify the wave generated by the Pi to make recognisable musical tones with the piezo as loud as possible. The wave doesn't have to be perfect, but I'd rather there was as little interference or noise as possible! I have 3.3v, 5v and 7.2v available on the project and I could add a pair of 9v batteries if 18v is needed. The piezo has a 7kohm impedance, if I remember according to the data sheets, if that helps.
A circuit diagram would be good but a bit of an explanation to go with it would be preferred!
Edit: The data sheet is at http://www.farnell.com/cad/356850.pdf
Another edit: I'll be testing Spehro's answer as soon as I can, assuming that school has all the extra components needed.