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Electret microphone biasing question.

Discussion in 'Electronic Basics' started by MRW, Dec 8, 2006.

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  1. MRW

    MRW Guest

    Hi again!

    Image link:
    http://img50.imageshack.us/img50/1584/micpreampac2.jpg

    In regards to the circuit above, can someone explain to me the part of
    the circuit circle in red? I understand that for most electret mics I
    need to bias it and that the resistor connected to my bias source is
    normally my source resistance. In the red circle, why is the bias
    network setup that way?

    Also, for the circuits circle in blue, I just need to verify that I'm
    looking at it the right way. For the 10k and 1uF, the corner frequency
    is around 15Hz and for the other network is around 16kHz. So are the
    following the right assumptions:

    - if my input signal is 20Hz, then my closed-loop gain is around 11?
    - at 17kHz, the opamp would just be a voltage follower?
    - at 17kHz, the 10kohm op amp is there to match the input bias
    currents?

    Thanks!
     
  2. James Beck

    James Beck Guest

    Looks like a power filter to keep noise from the +5 from inducing noise
    in the audio.
     
  3. john jardine

    john jardine Guest

    Yes. At 20Hz, gain is 11. At 17kHz you still have lots of gain though and at
    this 'corner' frequency, the voltage gain will '3db down' or 71% of it's low
    frequency value.
    I.e 11* 0.7 = x7.7
    In this particular circuit, the bias currents are irrelevant and the 2
    resistors are there entirely for gain setting.
    john
     
  4. default

    default Guest

    red part

    One 2K resistor is to drop some voltage and is decoupled with a cap
    the other is to develop signal with. Think in terms of an open
    collector inside the microphone cartridge.

    The electret is really a capacitor microphone - in the old days
    condenser mikes had high voltage supplies to bias them - today they
    use electrets. Electrets are bits of plastic insulator that have been
    impressed with an electrostatic field as they solidify (you can make
    one yourself with some curing epoxy and a high voltage source). The
    electret holds a static charge for a very long time and eliminates the
    need for a HV bias supply.

    Inside the cartridge is the capacitor. Sound causes one of the plates
    to change its distance from the other plate. Changing distance
    changes bias voltage. The problem is that the impedance is near
    infinite - two bits of foil with an insulator. There's a junction FET
    inside the mike with its gate connected to the capacitor plate its
    source at ground and drain to the output pin - you supply the external
    drain load resistor - on a two wire electret mike - three wire ones
    contain the resistor inside them. Two wire is more common
     
  5. On the low frequency end, you have another pole from the .01 uF and the
    two 1 Mohm resistors in parallel, at 31.83 Hz, so at 20 Hz the gain
    will be less than the full 11.
     
  6. MRW

    MRW Guest

    Thanks for the info everyone! :)
     
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