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I am designing a preamp intended to interface with a microphone in common headphones (e.g. apple earpods, or sony headphones with a microphone) which will then connect to my ADC's inputs.

I've developed a circuit based on a reference design, however the reference does not take into account the need to bias the microphone. From what I gather, I need to provide ~1.65V to the mic.

My circuit is using an AC voltage source @ 4mVrms to simulate output from the microphone (not sure if that's a fair model) and I'm not including any mic resistance.

My questions:

  • How would I bias this mic? providing power to net 8 does not show a bias (in simulation).
  • How do I ensure the gain of this pre-amp is correct? Finding datasheets for the mic in apple earpods is next to impossible so I don't have any real specs to go from.
  • Are there any reference designs that perform the same task?

enter image description here

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  • \$\begingroup\$ A TI design for an electret microphone preamplifier. \$\endgroup\$ – JRE Jun 25 at 6:20
  • \$\begingroup\$ That includes biasing for the microphone. \$\endgroup\$ – JRE Jun 25 at 6:21
  • \$\begingroup\$ @JRE I saw that reference as well, however I can't be certain the gain is correct for these types of microphones. Do you know of any other with this specific application? \$\endgroup\$ – bcoo Jun 25 at 6:21
  • \$\begingroup\$ Then look at how TI provides the bias, and apply that to your circuit. \$\endgroup\$ – JRE Jun 25 at 6:23
  • \$\begingroup\$ It shows R1 = (Vcc - Vmic)/Is. I don't know Is (no datasheet for these mics) which makes getting that resistor value difficult. \$\endgroup\$ – bcoo Jun 25 at 6:27
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The simplest solution to the microphone bias problem is to use a a resistor to provide current to the electret microphone.

It usually looks like this:

schematic

simulate this circuit – Schematic created using CircuitLab

The correct value for R1 would normally depend somewhat on the microphone.

Since you are going to be plugging just any old microphone in a cellphone headset into it, you can't really do any kind of calculations to make it work better.

The Android headset specifications show a 2.2k ohm resistor to provide bias to the headset microphone.

That's when using a 2.2V source. You could reduce the voltage to your microphone to 2.2V, or just do what everyone one else does: just ignore the voltage and use 2.2k regardless.


If you were doing something with low noise and microphones purpose bought with datasheets, you would probably want to go with something like this Texas Instruments verified design note.

That walks you through properly designing a low noise transimpedance microphone preamplifier. It does require that you have the specifications of the microphone and the desired output level for a specific input level.

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  • \$\begingroup\$ Thanks again for your help @JRE. Moving forward with this approach. \$\endgroup\$ – bcoo Jun 25 at 18:28

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