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Microphone Electret Circuit

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Larissa3m

New Member
Hi,

I am Working in a project for the Uni, is a circuit that measure frequency between 50Hz - 1KHz.
The circuit is based in a Microphone electret and then connected to a Impedance Matching using a JFET.
I read about the "Impedance Matching" but i still don't get very well how it works, the main idea is that help to reduce the lost of Impedance Signal from the Entrance to the exit of the circuit. (that is what i understood)

This is the Schematic of the Circuit,
What I really don't understand:

1. How i know which one is the impendance in the entrance
2. How calculate the value of the Resistance, what kind of data i need to do that.

I have more questions but atm this one i need help first.

Thanks in advance
Cheers,
Larissa.

 

cowboybob

Well-Known Member
Most Helpful Member
Welcome to ETO, Larissa3m!

Ordinarily, this matching issue is ignored for these types of circuits.

Nonetheless, ...

There is the impedance of the electret output AND the impedance of the input to the JFET which are determined prior to actually hooking them together. Ideally, the two would be matched (the same value) to achieve the lowest noise level and the best audio reproduction (we are ignoring, for the moment, the amp output and load impedance).

This often is achieved with a single resistor.

See: http://www.epanorama.net/circuits/microphone_powering.html
 
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Larissa3m

New Member
Hi, Thank you for the answer.

I am still lost, i don't understand very well.
the Impedance in the output in the electret is the same one, that is give in the datasheet?
so this one, is the one that goes to the input in the jfet? or there is another one that comes in the jfet?
 

alec_t

Well-Known Member
Most Helpful Member
The impedance of the polarised capacitor forming the mic's sound sensor is very high. Therefore, if you were to connect it directly to a lower impedance load (such as the input stage of a conventional audio amplifier) the sensor's output signal would be attenuated because of the voltage-divider constituted by the two impedances. To overcome this, an electret mic capsule includes a built-in JFET. The input impedance at the FET gate is extremely high, so the sensor signal is not attenuated by the load presented by the FET. The FET output at its source/drain has a low impedance, so the following audio amplifier input stage does not cause significant attenuation.
 

Larissa3m

New Member
Another thing that i am really struggling and i didnt found something about in internet is the gate to the drain.

I see that the Vgd = Vgs ?
 

audioguru

Well-Known Member
Most Helpful Member
Most electret mics have only 2 wires, the output wire which also connects to the resistor that powers the Jfet inside and a ground wire.
But some electret mic have 3 wires and can properly play very high sound levels. Their wires are the positive supply wire that connects to the drain of the Jfet inside, the output wire which connects to the source of the Jfet inside and a resistor to the ground wire and the ground wire.

A 2-wires electret mic has an output impedance of about 3.3k ohms. Its Jfet inside it can be powered through a 10k ohms resistor from 8V or 9V so that its current is about 0.4mA.

The 3.3k of the mic is effectively parallel with the 10k resistor which creates a total impedance of 2.5k ohms. It should be loaded with a preamp having an input impedance of 25k ohms or more so that its small output signal is not reduced.
 

audioguru

Well-Known Member
Most Helpful Member
We all got the deleted reply in an e-mail that said, "I thought it was best to match impedances to reduce noise?".
But matching impedances causes the signal level to be cut in half that DOUBLES the noise level when you amplify it back up.
 

Mosaic

Well-Known Member
Yeah, I remembered after. Low to high maximises voltage transfer, thereby improving S/n for AF frequencies.
 
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