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I have a problem with understanding a text's paragraph about the use of FET as a preamplifier. It claims that it reduces the coupling noise for an electret microphone before it is amplified in the next stage. Here is that section:

The electret microphone has a very high output impedance because in practical microphones the capacitance of the diaphragm is very low. This requires a very high impedance load for the microphone. To prevent noise pickup by the high-input impedance amplifiers, an FET preamplifier is provided within the microphone and provides a low output impedance.

What is the text trying to explain here? The FET in the capsule for preamplification already has a very very high input impedance. So it can already pickup noise. What is the point of saying "to prevent noise pickup by the high-input impedance amplifiers" here? It seems to me if the noise pickup issue is due to the "high-input impedance"; then by adding a preamplifer FET they are again coupling the mic capacitor to a high-input impedance isn't it?

I don't understand how they mitigate the noise by adding another high input impedance device(FET) if the noise is related to high input impedance.

Edit:

enter image description here

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  • \$\begingroup\$ It seems to be talking about electret + external fet vs. electret with bultin fet \$\endgroup\$
    – PlasmaHH
    Oct 8, 2018 at 11:35
  • \$\begingroup\$ But if the noise coupling is increased due to the high input impedance, there must be inserted a low input impedance device in the chain. But everything FET opAmp ect. has high input impedance. I dont get it. \$\endgroup\$
    – cm64
    Oct 8, 2018 at 11:37
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    \$\begingroup\$ "FET preamplifier is provided within the microphone and provides a low output impedance" there you have your low impedance. \$\endgroup\$
    – PlasmaHH
    Oct 8, 2018 at 11:42
  • \$\begingroup\$ @PlasmaHH Please see my drawing in Edit. This clarifies my question. If there is no FET and only opamp like in 2nd(below) one the total impedance seen by the noise is HIGH HIGH. But if there is FET and opamp there will be HIGH HIGH and LOW HIGH for the noise to couple. How come the 1st(above) one is better in this case?? \$\endgroup\$
    – cm64
    Oct 8, 2018 at 12:05

2 Answers 2

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The (electrical) noise is picked up by the wires between the microphone capsule and the amplifier.

When the amplifier is attached directly to the capsule, you have almost no wire length between the capsule and the amplifier. It pretty much doesn't matter that it is a high impedance circuit.

Once you are past that stage, the low impedance output of the FET amplifier helps to minimize the noise picked up on the long wires from the FET amplifier to whatever is actually going to use the signal.

So, if you have a capsule without a built in FET amplifier, and you use long wires to connect it to your own amplifier, then you will pickup lots of noise because of the high impedance circuit.

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  • \$\begingroup\$ Please see my drawing in Edit. This clarifies my question. If there is no FET and only opamp like in 2nd(below) one the total impedance seen by the noise is HIGH HIGH. But if there is FET and opamp there will be HIGH HIGH and LOW HIGH for the noise to couple. How come the 1st(above) one is better in this case?? \$\endgroup\$
    – cm64
    Oct 8, 2018 at 12:05
  • \$\begingroup\$ Because in your first example, the section marked "A" is a fraction of a millimeter in a typical microphone with FET amplifier. \$\endgroup\$
    – JRE
    Oct 8, 2018 at 12:08
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I guess, between your microphone and your amp you will have some cable, or perhaps long traces.

Having an amplifier very close to the signal source allows to minimise catching noise, as the leads, cable, trace acts as antenna.

Highest is the amp impedance input, highest is the noise you gonna get, so by adding a low impedance driver near the source help reduce the noise, as the longer trace / cables will be driven at low impedance.

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  • \$\begingroup\$ Please see my drawing in Edit. This clarifies my question. If there is no FET and only opamp like in 2nd(below) one the total impedance seen by the noise is HIGH HIGH. But if there is FET and opamp there will be HIGH HIGH and LOW HIGH for the noise to couple. How come the 1st(above) one is better in this case?? \$\endgroup\$
    – cm64
    Oct 8, 2018 at 12:05
  • \$\begingroup\$ I think it's explain in both answers, the goal is to go from high to low impedance as close as possible from the source. Note that you need to have an opamp with low impedance or add a resistor near the opamp. The datasheet should give you a currant or an impedance it can drive so you can calculate what value you need to add (by keeping some margin). \$\endgroup\$
    – Damien
    Oct 8, 2018 at 12:08

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