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Exponential to log fit (linear)

  • 06-10-2008 10:31AM
    #1
    Guscat
    Registered Users, Registered Users 2 Posts: 71 ✭✭


    Hello all,

    I have a problem in relation to logarithmic amplifiers. I am trying to input exponential voltages (eventually pulse peaks) of 10, 20, 40, 80, 160, 320, 640, 1280, 5120 mv etc.. and get a linear response (or those values logged).
    I have a simple circuit with resistor, op-amp and tran NPN. I also have an AD8307 log amplifier. However the output is not as expected. Can anyone let me know of a simple circuit that might sort this.

    Thanks.


Comments

  • #2
    Delphi91
    Registered Users, Registered Users 2 Posts: 1,501 ✭✭✭Delphi91


    http://upload.wikimedia.org/wikipedia/en/5/51/Logamp.JPG

    Any good??


  • #3
    Guscat
    Registered Users, Registered Users 2 Posts: 71 ✭✭Guscat


    Hello Delphi,

    Thanks for the reply. Yes i have tried that method and can only get as far as 1 volt when the NPN reaches its 0.7 V. It then becomes linear. I would like something that ranges exponentially 0.01V, 0.02V, 0.04V, 0.08V, 0.16V, 0.32V, 0.64V, 1.28V, 2.56V, 5.12V, 10.24V to have a log response ie.
    0 :1
    0.30103 :2
    0.60206 :4
    0.90309 :8
    1.20412 :16
    1.50515 :32
    1.80618 :64
    2.10721 :128
    2.40824 :256
    2.70927 :512
    3.0103 :1024

    With this i will have a linear output from an exponential input and therefore be able to gain the output further and have a greater amount of detail in the lower ranges.


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