Jinx wrote:
> Electrets are pretty easy to work with. Attached is a typical
> front end from a "bug" transmitter. Very sensitive and can
> pick up speech across a room, no problem.

Hmm.  The circuit you show is hardly complete since it doesn't show what
happens to the relatively high impedence signal produced by it.  It has a
voltage gain of about 10 up to about 3.4KHz, above which the gain goes
higher.  That is a strange choice for voice, so this circuit is probably
intended for some other application.  It does have over 10 times the input
impedence of the OP's circuit, which seems a lot more right to me.

It still bugs me though, just like the OP's circuit, that the DC bias
resistor to the mic comes straight from the supply.  Unless the mic has a
much lower impedence than I think, I would want to see the top end of that
resistor go to a filter from the supply voltage.  Otherwise supply noise
will couple directly into the input of the amp.

> As I mentioned,
> I've never needed more than one stage for amplification, either
> opamp or transistor

So can you tell us what the output impedence of a properly biased electret
is, and what a typical output voltage level is for normal voice input?


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