# 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?

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):

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

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:

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)

• give us some figures and links! Mar 9, 2011 at 7:15
• 95 cents qty 1 at digikey search.digikey.com/scripts/DkSearch/… Mar 9, 2011 at 13:04
• 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. Mar 9, 2011 at 22:43
• Can the LM386 handle a 40kHz input signal? Mar 10, 2011 at 1:21
• @wallacoloo: Bandwidth (BW) VS = 6V, Pins 1 and 8 Open 300 kHz biltek.tubitak.gov.tr/gelisim/elektronik/dosyalar/6/LM386.pdf
– nak
Mar 10, 2011 at 17:58