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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):

    Diode
    +--------->|---+
    | |
    | |\ |
    o-----R1----+---(-)\ |
    | \ |
    | >-----+---------o out
    | /
    ----(+)/
    | |/
    |
    _
    V

    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
    rectifier(s).

    Warren
     
  2. Warren

    Warren Guest

    Warren expounded in 81.169.183.62:
    Cancel that.. I'm going to design in a different
    direction now.

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

    Joe
     
  4. Warren

    Warren Guest

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