Electret Condenser Microphone - Basic Question

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

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.