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.
Best Answer
The LM108 is an old part, and the 30 pf capacitor is required for stability, especially at low gains, and as @Brian Drummond states this has a gain of 1. A more modern op amp will probably not require external compensation.
In answer to your question, the circuit's response is almost independent of the piezo characteristics in this circuit. This is because when the piezo voltage increases, the output tracks it and the voltage is pulled up to equal the piezo voltage by C1. Since the voltage is the same on both sides of R2, there is no current discharging the piezo device. The frequency response of this circuit is therefore determined not by the piezo characteristic, but by the RC time constant of C1 and R1. You may even want to make this time constant a little faster - right now it is 110 seconds (less than 0.01 Hz), way below any bass requirement. Also, be careful with layout because the impedance is so high that the circuit will pick up any hum.