Connecting "microphone" to smartphone

[petergriffin56]

Hi everyone,

I'm working on a side project with the one of the goals being to leverage the average smartphone's audio frontend, specifically to record a small AC signal. Looking at your typical electret microphone circuit below, I'm keeping things pretty much the same, but instead of the vibrating element is an antenna, a JFET is also still used after it.



Is my assumption that the biasing resistor and DC blocking capacitor are both inside the phone correct ? The MIC connection on headphones is biased, so the resistor must be on the phone side right ? And adding a series cap would remove the bias, so it would only make sense that this part of the circuit is all inside the phone.

But android specifications and other sources state that the output impedance of a microphone should be around 1Kohm to a few Kohm at most. From what I understand the output impedance of a common source JFET circuit is dictated by the biasing resistor, so doesn't the phone hardware itself dictate output impedance ? Why do they specifiy it then ?

I suspect this last question is somewhat related to the previous one, can I adjust the gain of the JFET by adding an additional series resistor ? Is this resistor what they mean by the output impedance of the microphone ?



How else would I be able to add gain to my JFET ? I'm not familiar with transistors, from what I understand gain is -Gm (transconductance) * R. But most JFETs I look at have Gm values in the milli-siemens or micro-siemens range, makes it difficult to add gain when the output impedance is so limited.

Can anybody help a n00b understand some of this a little better ?