Electret microphone question

[ashwick]

This is a general question with regard to very inexpensive electret mics
freely available . There is generally a FET inside the capsule itself for impedance conversion purposes.The question is: What is the quality of these FETs? Can we take them out and use them for for low noise audio applications(like front end of tape head amps etc?)
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[EC8010]

Originally posted by ashwick
This is a general question with regard to very inexpensive electret microphones. There is generally a FET inside the capsule itself for impedance conversion purposes. The question is: What is the quality of these FETs?

Because I need to make a microphone amplifier, I thought I'd try measuring the FET in a few capsules. The gate is effectively connected to the source internally, so Vgs = 0 and that's the only curve we can have. Here's a very cheap 3/8" capsule from Maplin. Note that the FET will be reasonably linear once Vds > 1V, but that Ids is only about 300uA. There's not going to be much gm in that FET, so we can expect plenty of noise...

[EC8010]

Here's a Panasonic WM60 1/4" capsule. The vertical scale is now 2mA (rather than 500uA) and with Ids four times greater at 1.3mA, we can expect twice the gm and commensurately lower noise. More interestingly, we need at least 5V across the capsule before we get proper performance out of the FET - no wonder the "recommended" circuit with only a volt or two performed so poorly! (The coggle at 2.52V is probably a measurement artefact.)