LOW NOISE op amp

Inside the electret mike there is a FET amplifying or buffering the extremly high impedance diaphragm signal. The output impedance is usually in the 2k2 to 6k8 region. You could take a OPA134, which will work nicely with only

4mA of supply current and from 5V on. It will not increase the noise and can amplify a lot with very little distortion. It can also drive low impedance loads. Very important for high gain amplifiers is that you know about layout and associated issues, so you do not produce a generator instead of an amplifier. But the best choice would probably be the LT1677 Rail-to-Rail, low supply current, high gain and CMRR if you pay the price for it($4.50 at Digikey). maybe they can send you samples? On the first page of the datasheet is an electret preamp (tho I do not think it is the best design possible with it).

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ciao Ban
Bordighera, Italy

Inside the electret mike there is a FET amplifying or buffering the extremly high impedance diaphragm signal. The output impedance is usually in the 2k2 to 6k8 region. You could take a OPA134, which will work nicely with only

4mA of supply current and from 5V on. It will not increase the noise and can amplify a lot with very little distortion. It can also drive low impedance loads. Very important for high gain amplifiers is that you know about layout and associated issues, so you do not produce a generator instead of an amplifier. But the best choice would probably be the LT1677 Rail-to-Rail, low supply current, high gain and CMRR if you pay the price for it($4.50 at Digikey). maybe they can send you samples? On the first page of the datasheet is an electret preamp (tho I do not think it is the best design possible with it).

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ciao Ban
Bordighera, Italy

will run on an 8V supply and is very low noise. cb

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You are very detail about explaining it. Thank you very much.

I hope you don't mind, how do I measure the "output impedance" of the electret microphone.

Also how to choose the suitable resistor which connected to +, the positive voltage suply.

About the LT1677, why it is not the best for the design.

Again Thank you very much Padmow

The O/P impedance of the mike is mainly governed by the resistor to supply, in the datasheet

?navId=H0,C1,C1154,C1009,C1026,P1844,D2102 this is resistor R1 with 10k. You can eliminate R2 all together, it only attenuates the signal and adds noise. You can then reduce the feedback resistor R3 until the gain is right. I would also add a 270R resistor from pin6 to output, to isolate the cable capacitance. This preamp will work on 2AA batteries(better would be a 9V battery, but require some additional circuitry) and consume very little current (3mA). Nice easy circuit.

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ciao Ban
Bordighera, Italy

Thanks again.

I did try to use LM 386 before but it wasn't that successfull.

the microphone was couple with capacitor to LM386 , while LM386 drive loudspeaker directly. The voice is weak, even I increase the LM386 gain , its still weak.

Any idea about this.

Thanks Padmow