I've been wondering if there is a simple means of extracting a DC signal from an AC+DC signal for the purpose of providing a small DC supply to a CMOS device.

Essentially the signal I'm considering is a DC microphone bias supplied to an electret microphone. The electret mic will add its own small signal AC, and this will pass through some circuit that requires the DC bias only, leaving the microphone AC undistorted to be passed to another part of the circuit uneffected. The rest of the audio chain is passive, and no external supply is available other than the 2.5V - 5.0V DC microphone bias superimposed with audio AC.

I'd like to add a small CMOS comparator (or similar) that requires a DC supply, and the microphone bias is the only DC source available for the CMOS device supply. Usually removing the DC from a signal is straight forward with a DC block capacitor, but any thoughts how to do this DC extraction simply without effecting the audio signal ??

Cheers, Mike

  • \$\begingroup\$ Forgot to mention, prefereably no inductors in this case ... \$\endgroup\$ – citizen Aug 19 '19 at 8:42
  • \$\begingroup\$ Bridge diodes + cap & 3.3V FET LDO and 10k to 20k series to open drain electret for extracting amplified audio \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Aug 19 '19 at 9:29
  • \$\begingroup\$ Bridge rectifier + cap and FET LDO seems good for obtaining a DC supply good enough for a CMOS device etc. However I'm worried the small signal audio AC will be significantly attenuated without some sort of buffering... (without a circuit, in mind I can see a low-impedance AC path somewhere in there for the AC signal ... This superimposed AC needs to continue unaffected without distortion, as the DC is simply tapped off to generate the CMOS supply. This is, unless you think otherwise ... \$\endgroup\$ – citizen Aug 19 '19 at 10:11
  • \$\begingroup\$ do as I said and but add 100 Ohms in series to bride to 200 to your load. Any ripple on LDO should be low peak current on audio depends on driver \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Aug 19 '19 at 10:29
  • \$\begingroup\$ Especially if you rule out inductors, you cannot really do this without attenuating the audio signal, however, if the consumption of your circuit is small, you may be able to feed it through a resistor (and then a filter capacitor) such that the attenuation may be reasonable - electret microphone capsules with internal circuitry work that way, after all. You've not stated the power consumption of your circuit; if you find something that has an open drain output (which in fact many do), or make sure that whatever the comparator is driving has very high impedance, this may be minimized. \$\endgroup\$ – Chris Stratton Aug 19 '19 at 15:33

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