I've done front ends for surface voltage tools used in semiconductor manufa cturing. Turns out that within a few thou of the silicon surface, there's a persistent E field due to variations in the local work function, and that' s a very sensitive indication of surface contamination. You can see stuff d own to way under a monolayer thick.
The reason the field persists is that there aren't any surface quantum stat es available for the neutralizing charges to sit in, so although air ions s hield it out eventually, you can still measure it close to the surface. If the field is big, some charges will collect in the conduction band, but the re's still a volt or so worth of slop due to the band gap.
Dunno exactly about plastic, but the situation is probably similar. Metalli zed surfaces don't show the effect, of course.
I think the main question here is whether elektret mikes 'are PVDF' or not. Would you agree to the following?
"Modern electret microphones use PTFE plastic, either in film or solute form, to form the electret."
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This page shows the internal build-up of the microphone:
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It is easily seen that the polarised diaphragm makes no electrical contact with any part of the microphone. Are you suggesting that through piezo-electric action a charge separation takes place which leads to a charge displacement in the backplate? That's an interesting option and I think in that case you just invented a new type of piezo-electric condensator microphone by using PVDF instead of PTFE.
Well, in that case my almost 40 years old Philips N 8410 'electret Hi-Fi stereo microphone system' with their 1.5 V penlight batteries inside should have ceased working by now? Fortunately they haven't. Just like these here below :)
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I've never heard of piezo mikes with (PVDF) diaphragms. This doesn't mean that I think they can't exist. And it's also not the type that I proposed this amplifier for.
Maybe that's exactly why I practice this 'odd sense of humour'. To not get myself involved with (counter productive) negative emotions as soon as some surprise pops up.
I understand the plastic is melted and mixed with polar molecules, the let cool down in a strong electric field, and then metallised. Would that be able to keep the contamination out?
With electrets, this happens; it just takes a very long time. West and Sessler used a metallized Teflon foil for the first ones at Bell Labs ( yes, I cheated and looked - couldn't remember the details ).
The charge on the back plate doesn't actually move much. The other plate is just a dielectric, possibly also sputtered or otherwise metallized for reasons I'm not sure of.
I'm probably forgetting some detail or another. The JFET has really high input Z...
I'm not sure an external field is required... don't capacitors in general have 0 field?
At any rate, I'd still class them as condenser mics because there's two plates, one's moving and this causes the signal.
Those are "front electrets." Back electrets were more common because you could use anything that was a dielectric for the diaphragm, which opened up more design choices.
I recall back in the '80s repairing a Sennheiser MKH816T shotgun mic. Even after I traced the circuit, it still took a while to work out that the capacitance of the mic capsule controlled the frequency of an 8MHz oscillator, and this was followed by an FM discriminator. I have to say that coming across a Foster Seeley circuit (which looks a bit like a full wave rectifier with a diode on each leg of a centre- tapped inductor) when you're expecting to find an audio amplifier, is rather weird.
I can't find a schematic online, so I'm trusting my memory from almost 3 decades ago.
** This schem should be close enough to illustrate the point.
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The mic capsule does not directly modulate the 8Mhz xtal oscillator, but rather the tuning point of the FM discriminator instead, with the same outcome.
Impedances are kept low and hence so is the noise level with no need for any DC bias on the capsule.
They depolarize internally, which is slow at room temperature. The air ion thing is pretty fast by comparison, but the charge has to have somewhere to go, i.e. an empty quantum state at the right energy. Metals always have th ose, but dielectrics and semiconductors often don't.
My proposal involves using the very same JFET that's already in the elektret microphone so I simply don't understand why that would increase the noise. Can you explain how this works?
With respect to the RF oscillator I can see (and Phil Allison already showed it) how one could use that in a 'real' condenser mike, but not how this should be done with an elektret.
BTW I found a comparable approach in this paper:
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and they do it much simpler than I had anticipated: the JFET _is_ the whole pre-amplifier and the feedback basically is a capacitor from drain to gate. I'm sure however that if you'd increase the loop gain by adding 2 more stages the voltage swing on the gate can be further reduced, thereby increasing the sensitivity a little more. Stability can then still be assured by placement of local poles and/or zeros. A technique Nordholt explains in his book.
They get a noise of 20 nV.Hz^(-1/2) @ 1 kHz. I get the feeling that if one would replace the PVDF/TrFE foil with a teflon, mylar of PTFE electret foil, one would get (almost) exactly what I was after (except for the loop gain).
That's not cheating, that is studying or refreshing.
I feel that as a little over-abridged. In a non-polarised condenser microphone like that Sennheiser MKH 105 they use the capacitor's change in capacity to modulate a frequency. In both the externally polarised condenser microphone as well as the electret microphone they use the varying voltage, but what exactly (physically/fundamentally) causes that voltage to vary, I wonder?
My take is that the electret's polarisation causes an electric field between the plates electret diaphragm and the opposite capacitor's plate. This field causes this opposite plate attract or push away electrons, hence it gets polarised also. When the diaphragm moves, the distance between it and the opposite plate changes and with that the electric field (E) changes, which in turn causes the number of excess electrons (or 'holes') on the opposite plate to change. That leads to a movement of charge. Either exclusively through the capacitor wires if it is shorted, or through polarisation of the opposite plate, including Cgs, leading to a change in voltage. Of course a combination is also possible if the electret is not really shorted, as in a nfb amplifier with low loop gain.
My hunch was that if you were able to move all the charge through the amplifier that you would get a roughly 3x better S/N-ratio and more sensitivity.
One could easily reason why that would be the case, at least with the components that I presented in my OP (electret 10 pF, Cgs 5 pF) because using a nfb amplifier with 5 pF feedback capacitor would immediately give a 3x higher voltage and the same noise (unless someone explains me why that would not be viable) if you use the same JFET as input stage.
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