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Single-supply op-amp help!

Discussion in 'Electronic Design' started by Hw, Aug 1, 2005.

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  1. Hw

    Hw Guest

    Hello.

    I am trying to make a single-supply op-amp circuit for an electret
    microphone so I can pick-up voice 2-3 feet away (i.e. like a
    speakerphone on a desk).

    Circuit is split into two stages, 40 dB gain first stage with some
    filtering and I'm trying to get 40 dB in second stage (no additional
    filtering). I am having problems getting the 2nd stage to work
    correctly.

    The first stage is built following this app. note:

    http://www.st.com/stonline/books/pdf/docs/8698.pdf

    2nd stage is a standard non-inverting op-amp configuration with gain =
    40 dB via 100k and 1k resistors.

    Questions:

    - Is 80 dB enough gain for a typical electret mic to pick-up voice 2-3
    feet away?

    - Should I AC couple between stages to avoid offset issues?

    - If no to the above, do I NEED to have the second stage vcc/2
    referenced, given my first stage biases the AC signal about vcc/2 ?

    In my SPICE simulations, if I don't ref. the second stage to vcc/2, I
    get no output, but I find that confusing since the first stage already
    biases the signal about vcc/2.

    - Does inverting/non-inverting make a difference for second stage? I
    don't think the 180 deg phase shift makes a difference since there is
    only one mic but perhaps I am neglecting something.

    - Anyway to keep the input impedance low for the second stage?

    The ST PDF claims the input impedance is equal to R4 (18k), but
    following this PDF PDF (no, I am not a Cornell student, but I couldn't
    find this PDF on TI's site) the impedance would be closer to 811 ohms.

    http://instruct1.cit.cornell.edu/courses/bionb440/datasheets/SingleSuppy
    ..pdf

    Why in ST's configuration is the input impedance not equiv to R8||R7?
    C4 seems like it forms a low-pass with R3...

    Thanks in advance.
    HW.
     
  2. A single stage of 40dB should be quite OK for an electret mic capsule.
    The majority of professional micamps rarely go above 66dB, and that is
    usually for use with very low output ribbon mics.

    Dont bother with a second stage.

    Re: the input Z, the non inv input is more or less = to 18K, but this
    is swamped by the low source Z of the microphone. Its only there to
    set it at 1/2Vcc
    but the inverting input is needs to see a low source z for noise
    considerations, which is r8//r7. This is not the input z of the opamp,
    but what it sees

    Both inputs are Hi Z, but its the networks around the beast that
    determin the operating impedances

    I would replace R7 with a 5K or 2K pot in series with 470R, so you can
    tweak the gain. that will give you up to 45dB gain approx.

    AC couple, yes, in this type of setup

    Most of this is covered in the data sheet, it just needs reading a bit
    more carefully


    martin
     
  3. Pooh Bear

    Pooh Bear Guest

    Excessively complicated. You can do the same probably better with any half
    decent modern op-amp.

    Sounds plenty to me.
    Yes. Indeed essential if using a single rail supply.

    Each stage needs to be biased to Vcc/2 individually.

    You don't understand circuit basics. This is common with ppl who use Spice
    to avoid understanding circuits.

    No - phase / signal polarity is irrelevant to your application.
    Low where ? Any op-amp output has quite low impedance.

    Indeed - see R4.
    The page cannot be found
    Uh ? R8 and R7 form the feedback network. Irrelevant to input impedance.
    Time to study circuit basics.

    I suggest you learn some op-amp basics.

    TI have an excellent app note - it's a book in effect something like 400
    pages - but I forget the name and its title ( may be Op-amps for Everyone )
    I may recall it later.

    National semiconductor have the rather old ( refers to too many old devices
    ) but still useful Audio Handbook ( now reprinted ) and the Linear
    Applications Handbook - may be available in hardcopy but should be available
    as a pdf.


    Graham
     
  4. Robin

    Robin Guest

    If the +ve input goes, say, a little high then the output (of the
    op-amp) will try to slam hard against the positive rail (because the
    op-amp's gains is very high).

    As soon as the output starts to move high, it is fed back to the -ve
    input (reduced by the potential divider of R7,8) and this "cancels" the
    above change that caused the output to move high.

    This "action" makes +ve look like a "high impedance" - its the same
    thing as power steering: a slight displacement allows a powerful fource
    to "null it out", moving the wheels the while.

    It is fun to work out; the input impedance of +ve... and then you will
    never have to depend on a "PDF" again!

    Here is an example of how to do it for the simpler inverting op amp
    arrangement:

    1) If -ve input changes by delta voltage "dV" (w.r.t. the +ve input).
    2) Then the current at -ve will change by "dI".
    3) And evidently dV / dI == input Z at -ve.
    ....
    a) Meanwhile the output moves -dV * A (A == open loop gain).
    b) And most of dI flows via the FB resistor (very little via -ve).
    c) And most of -dV * A volts is across the FB resistor (dv << -dv * A).
    d) So dI == -dV * A / FB resistor.
    ....
    4) So from 3) above input Z at -ve == FB resistor / A


    Cheers
    Robin
     
  5. Ban

    Ban Guest

    There are a few mistakes in the ST pdf, 1Pa corresponds to 94dB SPL not
    110dB. I also think the circuit can be improved.
    You do not need 80dB of gain either. At the normal conversation we have 60dB
    SPL at 1m. The speaker may talk louder (80dB) and speak closer to the mike
    12.5cm (+18dB), so we have to process a max. SPL of 98dB, which corresponds
    to 10mVeff or 28.25mVpp. We want to amplify this to the max. possible O/P
    voltage of 2.5Vpp, so the gain needs to be 88.5 or 38.9dB. Exactly what the
    amp is supposed to do.
    The mike has 58dB S/N referenced to 94dB, so the noise is 7.94uVrms, which
    is amplified to 715uVrms. This voltage will be -62dB below maximum, so it
    will not make sense to amplify more, since the details get lost in the noise
    anyway.
    You are wrong with the idea of the low input impedance. C4 is for the audio
    frequencies a dead short to gnd, so the input impedance of the amp is indeed
    18k. This is important for the calculation of C5. The output impedance of
    the mike(2.2k) is in series with it as written in the PDF. The output of the
    mike is loaded with the 2k2 in parallel with the 18k, so it is 1.96k plus
    100R for the EMI filter. This will reduce the output voltage by 10% or -1dB.

    As to the improvement of the circuit, I can respond later, too much to do
    now.
     
  6. Hw

    Hw Guest

    Thank you all for your responses, I appreciate all the helpful comments.

    My confusion initially stemmed from getting one single-stage amp drive
    another single-stage amp.

    The first stage, 39 dB gain, works, but 2-3 feet away the output
    amplitude drops and it is hard to hear the voice, hence my desire to put
    a second stage.

    I guess it makes sense to AC couple from stage to stage since any DC
    offset would pass through the gain stages otherwise and because my
    signal is AC, I can strip away (this potentially large) DC without
    "worries". I was hoping to avoid loading the Vcc/2 reference generator
    too much by using it in the second stage, but it is needed anyway.

    The confusion I had regarding the input impedance came from page 6 of
    the second PDF I was referencing (now in TinyURL form should load):

    http://tinyurl.com/dutqx

    In the non-inverting configuration shown, he doesn't put any bias
    resistors for the AC signal coming in which seems odd since the cap
    should strip away the Vcc/2 DC bias added (presumably?) in a previous
    gain stage. Regardless, I suppose for his circuit the input impedance
    is considered at the V+ stage since:

    * V- has the signal directly coming in after the AC cap
    * V+ has the feedback resistors which are referenced to Vcc/2, hence
    there is a bias current flowing.

    Ban of course you are right, C4's impedance for the audio frequencies is
    essentially a short (~36 to ~7.2 ohms from 2kHz to 10kHz), and the
    Zin_V+ for the ST op-amp circuit is 18k.

    Thanks again.
    HW.
     
  7. Pooh Bear

    Pooh Bear Guest

    The page cannot be found

    Sorry - no luck still. Are you perhaps logged into the Cornell site ?

    Graham
     
  8. Hw

    Hw Guest

    Graham,

    Please try this (and no, I am not a student at Cornell or anywhere
    else):

    http://tinyurl.com/ck9l4

    HW.
     
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