I'm building a guitar tuner that can be used with a (piezo) microphone clip for acoustic, and from the pickup for electric. In my experiments they have comparable signal levels.
However, the piezo has a very small capacitance and large resistance. So the input circuit needs to have a high input impedance.
So far I've been using a good old non-inverting opamp with a 100k biasing resistor to ground. This seems to work okay, but then I found this collection of opamp circuits that includes a "Amplifier for Piezoelectric Transducer" that I don't really understand. What is going on here? How do I go about calculating the gain and impedance of this circuit? (and what is the 30pF cap doing?)
I've also found this other question about This guitar piezo opamp buffer lacks any sort of bass, why? which has the common problem of having too low input impedance. Their suggestion seems to be to just increase the biasing resistor of the classic circuit, but in my simulation this only goes so far before the input current of the opamp starts to create a noticeable offset.