Electronic – Amplifying sound from a microphone


I have a small piezoelectric ultrasonic microphone (receiving signals of around 40 kHz). But the voltages it generates are too small to be read by my micro-controller's ADC. So I want to amplify the voltage created by the microphone.

I'd like a solution with a low part count, if possible. I've been trying to use an op-amp + some resistors, but I can't figure it out.
So, how can I amplify the voltage from the microphone?

Best Answer

Piezos have a high output impedance (capacitive, really), so you need an even higher input impedance on your mic pre or you'll get a "tinny" sound from the low end being rolled off.

A piezo with 1000 pF capacitance would have a reactance of 3 MΩ at 50 Hz, for instance, so if you wanted to pick up 50 Hz, you'd need an amp with several MΩ input impedance.

Signal Conditioning Piezoelectric Sensors from TI shows a circuit with 10 MΩ input impedance, for instance. Interfacing Piezo Film to Electronics suggests 22 MΩ and shows the most basic circuit you could use: one of these big resistors in parallel with a unity-gain buffer op-amp. If that level is too low, add two resistors and make it into a non-inverting voltage amplifier (figure 9 b):

a charge amplifier and non-inverting voltage amplifier for piezo applications

The major advantage of a charge amplifier, therefore, can be found when a long cable is used between a piezo film sensor and electronics. In addition, it also minimizes charge leakage through the stray capacitance around the sensor. Otherwise, simple voltage amplifiers are sufficient for most applications.