# LOW NOISE op amp

P

Jan 1, 1970
0
Hi,

I wonder if there is any suggestion about which op amp I can use which
has Low Noise, and only need 9V from battery.

This is for amplifiying signal from electret microphone. The opamp
would then drive class AB transistor amplifier.

Thank you

B

#### Ban

Jan 1, 1970
0
Hi,

I wonder if there is any suggestion about which op amp I can use which
has Low Noise, and only need 9V from battery.

This is for amplifiying signal from electret microphone. The opamp
would then drive class AB transistor amplifier.

Thank you

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). B #### Ban Jan 1, 1970 0 padmow said: Hi, I wonder if there is any suggestion about which op amp I can use which has Low Noise, and only need 9V from battery. This is for amplifiying signal from electret microphone. The opamp would then drive class AB transistor amplifier. Thank you Padmow 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).

P

Jan 1, 1970
0
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

B

#### Ban

Jan 1, 1970
0
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

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

P

Jan 1, 1970
0
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.

Thanks

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