Electret Condenser Microphone - Basic Question

[gonsays]

Hello guys,

I'm trying to understand the operation of a electret condenser microphone, as you can see the circuit on this figure:


How does the implanted charge (electret material) polarizes the transistor?

Why the JFET acts as a buffer?

I've search a lot, but I'm stuck on these question to understand further about the operation of a ECM.

Thank you very much in advance

 

[betwixt]

During manufacture the plastic diaphragm is heated while between two high voltage electrodes, it aligns the atoms which are then 'locked' as the plastic cools. It works like a battery but has almost zero power capability, even the slightest load would drop the voltage to zero. The JFET is a transistor with an extremely high gate resistance but relatively low resistance between source and drain pins. The electrostatic charge from the diaphragm is conducted to a pick-up disc connected to the gate where it controls the source to drain resistance and therefore lets you convert the current through the JFET into a usable voltage. As sound pressure waves bend the diaphragm, it's position relative to the disc changes and so does the gate voltage and therefore JFET current.

So in a nutshell, the actual electret element can only provide a voltage into a load of maybe 1,000,000,000 Ohms or more, with the JFET it may be 1,000 Ohms or less.

Brian.