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LOW NOISE op amp

Discussion in 'Electronic Basics' started by padmow, Jun 6, 2005.

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  1. padmow

    padmow Guest

    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
     
  2. Chris Berry

    Chris Berry Guest

  3. Ban

    Ban Guest

    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).
     
  4. Ban

    Ban Guest

    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).
     
  5. padmow

    padmow Guest

    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
     
  6. Ban

    Ban Guest

    The O/P impedance of the mike is mainly governed by the resistor to supply,
    in the datasheet
    http://www.linear.com/pc/downloadDo...076?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.
     
  7. padmow

    padmow Guest

    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
     
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