Troubles with mic preamp for headphones with integrated microphone

Question

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?

Answer 1

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:

^{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.

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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.