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Log opamp, with negative Linear side?

Discussion in 'Electronic Design' started by Warren, Jan 12, 2011.

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  1. Warren

    Warren Guest

    This is a homebrew pet project that
    I'm hoping there is a simple answer to.

    I want to create a logarithmic opamp stage
    that operates _linear_ during the
    alternate (negative) half of the input wave.

    Here is a textbook log amp with simple diode
    in the feedback loop (display in fixed font):

    | |
    | |\ |
    o-----R1----+---(-)\ |
    | \ |
    | >-----+---------o out
    | /
    | |/

    The above suggests to me that the alternate
    half of the wave will be linear BUT with open
    loop gain. But I want to establish/lower the
    gain to be about 3 or so.

    The obvious thing would seem to be to add a
    reverse biased diode in series with a
    resistor in the feedback path (the resistor
    to establish the gain). But this also suggests
    to me that I will still have a _log_ function but
    in the reverse. I'm trying to keep that linear.

    I'm trying to avoid the need for precision

  2. Warren

    Warren Guest

    Warren expounded in
    Cancel that.. I'm going to design in a different
    direction now.

  3. Download for free LTSpice and you can spice model these circuits.

  4. Warren

    Warren Guest

    Joe G (Home) expounded in
    Been doing that. :)
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