sound capture

Posted on Feb 4, 2014

After some new experiments on sound analysis, I used a different design for the microphone amplifying section, that I am happy to share with you (From the statistics of Arduinoos, I know that sound capture is a topic of major interest for many of you). The whole circuit is supplied with a clean +5 to +12V, filtered by C5. The microphone (electret t

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ype) is biased through R7, R8, using a filtering capacitor C2 for removing some power supply noise. The active pin of the micro phone is connected to the first stage of an inverting amplifier (IC2B) via a decoupling capacitor which stops the DC component of the signal and all the AC component to go through. The gain of this amplifier is set by R3 and R4, with G=R3/R4. The non inverting input of the amplifier is set to avirtualground at half the value of VCC (typcally 5/2=2. 5V), so that the signal at rest is about half VCC and swings around this value. Halving VCC is performed by the divider bridge R1/R2, stabilized by C3 and C4. Lacking to install thesecapacitors may lead to an unstable output. The output of this stage is available at pin 7 of IC2B. Someapplicationsmay require an even higher gain, which is achieved by a second amplifying stage. This second stage uses exactly the same design than for the first stage; its gain is set by R5 and R6 [erratic spot on R6 and GND: do not connect]. It shares the same virtual ground as for the first stage. The total gain of the amplifier is the product of the gains from the two stages. BTW, why would we use two stages It is very important not to violate the bandwidth limit of the op amp at the highest frequency seenby the circuit. Practical circuits can include gains of 100, but higher gains could causethe circuit to oscillate unless special care is taken during PC board layout. It...

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