Instrument makers have found an astonishingly wide variety of ways of creating the negative resistance which is needed to ensure that instead of dying away due to damping, the note increases in amplitude over time (until the effect saturates and the anti-damping just compensates for the natural damping of the oscillation). What is also surprising is how wide a variety of seemingly completely different schemes can produce the same effect. This is another example of "trunks" (as in my joke about steamships and elephants) where seemingly completely different phenomena are seen as being fundamentally the same when looked at is a certain way by physicists.