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Need help understanding electret mic amplifier

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distort10n

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electret mic

Hello All,

I am researching the theory of operation of electret microphones and have come across very little information. I understand how the FET works, but I wanted more information in regards to how the electret itself works. Good news is I found a good explanation on Usenet.

During my research I found the National LMV1012 and am having trouble understanding how it is operating. The op-amp driving the FET has no negative feedback and looks to be operating as a comparator. I am also confused by the voltage source from the source back to the non-inverting input.

Can someone help clarify this?

**broken link removed**

Thanks in advance.
 

electret microphone preamplifier

The simplified block diagram shows an inverting opamp driving an inverting mosfet. Therefore the feedback is negative.
 

electret mic amp

Where is the simplified block diagram showing negative feedback? I understand the FET is inverting but the input shown in the schematic on pg. 1 is the inverting input and there is no feedback to this terminal. The only feedback that I can see is from the source of the FET back to the non-inverting input.
 

electret amplifier

The details of the opamp circuit are not shown.
The Mosfet is inverting (The drain is feedback to the opamp, not the source. The source has the arrow) and its output feeds the no-inverting input of the opamp making negative feedback.
We don't know what is the voltage gain of the opamp.
 

www.national.com/ds/lm/lmv1012.pdf

Sorry, I don't see how the JFET is providing any negative feedback direct or overall since the drain (you're correct, I recognize a N channel but a new symbol for me anyway) is connected to the non-inverting input.
What am I missing here? I am under the impression that any voltage on the non-inverting input reinforces positive feedback.
I would inclined to say that something is indeed missing from the schematic.
 

two wire mic amplifier schematics

The Mosfet inverts the signal. So when its output feeds the non-inverting input of the opamp then the feedback is negative.
 

electrecet microphone recognize simple words

It's a current controlled voltage feedback, in other words, the amp is working as a current source. The circuit is different from a usual voltage controlled feedback, you're probably trying to see, but it's a common topology as well. The FET is P-channel however, otherwise the feedback polarity would be positive.

If you don't want to design an electret microphone yourself, it's not that meaningful to analyze the LMV1012 details. The circuit is simplified in so far, as the input bias circuit is omitted. Strictly spoken, the circuit can't work without an input bias.

It's a general property of two wire electret microphones of course, to provide a current source output, also if a simple JFET amplifier stage is used, as mostly done.
 

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