In the past I've tried to (and actually did it) design and build some pre-amplifiers for use with a elektret microphones and found out that inside the electret there's one ingeneous little JFET which 'makes it all work'.
Lately I was wondering however in how far this JFET is the optimal solution and whether those mikes could be improved by using a slightly more complicated circuit inside.
The case is that some elektrets
have an internal capacitance of about 10 pF and some JFETs, used inside, have a Cgs of 5 pF (or centimetre ;).
Now the way these mikes function seems to be that a pre-polarised dielectric membrane is moving to and from its backplate, attracting more and less charge to it.
This charge (the amount of it) I think contains the real 'information' that one wants to amplify. But the JFET is merely amplifying the voltage over the mike's + JFET's capacitances.
With a mike internal capacitance of 10 pF and a Cgs of 5 pF, this would mean that 2/3 of the moved charge is sticking in the elektret itself, and only 1/3 is moved to the JFET's gate, generating a voltage about 67% lower compared to if all charge would have gone into Cgs.
Wouldn't it be better to incorporate the JFET in an amplifier with a negative overall feedback loop? Then the input voltage of the JFET, and also the voltage over the elektret would be practically (for engineering purposes) zero and all the charge would be absorbed by the feedback loop. If for instance the component in the feedback loop would be an external capacitor of 5 pF and the input stage of the amplifier would be the same JFET, then wouldn't we have a true charge amplifier and a 3x times better S/R?
Actually I think the answer is yes. Do these mikes actually exist?
My guess is that you'd only need to put 1 small IC plus a 5 pF capacitor (or whatever value one might specifically prefer) inside the elektret to make this work.