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Logarithmic amplifier

Other Parts Discussed in Thread: OPA132



I need to use a logarithmic amplifier in my design. My signal is differential form -1 V to + 1V, but I don't find an Application Note where it explains how I can do it. ¿Do you know any AN for this?



  • Manuel;

    If your input signal is -1V to +1V, you may not be able to use a log amplifier, if your input passes through zero, the log of zero is undefined.

    Regards, Neil P. Albaugh    ex-Burr-Brown

  • Hi Neil,

    I know it, but I work with bipolar signals. Do you know any audio Codec or other IC that integrates a mu-law or similar? Perhaps I can work with it.


    Best regards.



  • Manuel;

    A "true" log amp cannot go through zero, which is required for bipolar input signals, but there are degenerate forms of log amps which go smoothly through zero but in that region the response is only "quasi-logarithmic". I forget what the shape of that region is called; it is something like tanh or ?

    There is a very interesting website that has a very simple circuit for a degenerate bipolar log amp at:  That circuit may do what you want-- if you use a more modern FET op amp such as a BB/TI  OPA132. As an old Philco techrep used to say in lectures at our German-American Radio Club in Frankfurt years ago, "Now this circuit will not work without Philco tubes!"

    I'm beginning to sound like Bob Pease.......

    Regards, Neil P. Albaugh   ex-Burr-Brown