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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?

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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.

source

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Try looking up the LM386 amplifier. It's a low part count solution!
Here's a circuit diagram of the "minimum parts" required to use this IC:
Pic of min part schematic for LM386


I am sure you could be able to figure out which pieces you can removed depending on your project (the 10k pot for example) Kenny, below, has a link to the digikey page. I am sure I've seen these at Radioshack too (if you're in the US of A)

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    \$\begingroup\$ give us some figures and links! \$\endgroup\$ – Kortuk Mar 9 '11 at 7:15
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    \$\begingroup\$ 95 cents qty 1 at digikey search.digikey.com/scripts/DkSearch/… \$\endgroup\$ – kenny Mar 9 '11 at 13:04
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    \$\begingroup\$ The input impedance is only 50k (or 10k if you leave that pot in). A piezo needs to see a lot more than that, or all you'll hear is treble. \$\endgroup\$ – endolith Mar 9 '11 at 22:43
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    \$\begingroup\$ Can the LM386 handle a 40kHz input signal? \$\endgroup\$ – Ponkadoodle Mar 10 '11 at 1:21
  • \$\begingroup\$ @wallacoloo: Bandwidth (BW) VS = 6V, Pins 1 and 8 Open 300 kHz biltek.tubitak.gov.tr/gelisim/elektronik/dosyalar/6/LM386.pdf \$\endgroup\$ – nak Mar 10 '11 at 17:58

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