elektret mike amplification

Joe Hey · 2016-01-12T19:02:37+00:00

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. joe

Jan 12, 2016 88 Replies

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Joe Hey 10 years ago

Exactly, hence the struggle. :)

joe

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Joe Hey 10 years ago

About ten messages down this thread we just found out that the electret is *not* a fixed capacitor with a moving electret foil in between.

One side of the electret is metallised and forms one plate of the capacitor, the other side of the electret faces the opposite capacitor plate.

It can be that the electret foil moves due to the sound pressure, it can be that the electret is mounted on the backplate, while another metallized foil is moved by the sound. The latter gives more freedom in the choice of the moving membrane and more opportunity to optimise with regard to acoustic properties.

So it indeed works 'just' by (open voltage) U = Q.d/(eps0.A), with Q the charge not on the capacitor, but on the electret...

joe

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Ralph Barone 10 years ago

The fixed plates are at opposing voltages. It's a push-pull kinda thing.

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Phil Hobbs 10 years ago

We're in violent agreement about that. It wasn't a mystery to begin with, because (as you say) it couldn't work otherwise.

Depends on the load. Do google "displacement current" some time--it's a good concept to keep in the toolkit.

Cheers

Phil Hobbs

Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net

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Phil Allison 10 years ago

** Electrostatic speakers have two fixed, perforated plates that are driven with anti-phase voltages. The diaphragm is charged to a high voltage ( ie 5kV) with respect to both plates and the stored charge does not vary in operation.

The electric field strength between the fixed plates is proportional to the voltage difference and is quite linear. So the force on the diaphragm is always proportional to signal voltage at any usable displacement.

Although the diaphragm may be large, it experiences an even force applied all over its surface - similar to microphone diaphragms when exposed to sound waves arriving on axis.

.... Phil

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Joe Hey 10 years ago

Yes, and more so in the electrostatic loudspeaker than in the electret microphone, as the surface to circumference ration of the former is much larger than of the latter.

I guess that in the electret, the bending of the diaphragm leads to more non-linear distortion than the bending of the foil in the electrostatic loudspeaker.

joe

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Phil Allison 10 years ago

** But it takes enormous SPLs to displace the diaphragm of a condenser mic more than a tiny amount - and it is not frequency sensitive.

Like most speakers, electrostatic speaker diaphragms move far more at low f requencies and can run out of available excursion. At any audio frequency a nd at levels over 100dB SPL, the needed drive voltage becomes excessive and arcing is induced. Amplifier power must be restricted or the speakers cont ain overvoltage protection, as in the Quad ESL63 etc.

At least one maker ( Dayton Wright) used Sulphur hexafluoride gas in their speakers to overcome this.

.... Phil

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