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OTA with linearization diodes

Discussion in 'Electronic Design' started by R. G., Sep 21, 2003.

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  1. R. G.

    R. G. Guest

    A small mystery (for me...)
    from the following datasheet of the LM13700 at page 7, figure 3:

    http://www.national.com/ds.cgi/LM/LM13700.pdf

    Using the linearization diodes, input-output equation is:

    Io = 2 * Is * Iabc / Id

    and I assume:

    Is = Vth / Rth

    And if Ra << Rin then Rth = Ra
    (Rb = Ra in all conditions for the sake of simplicity..)

    Let's say Rin = 10K and Ra = 500 ohms as in figure 4.
    Now if I reduce the value of Ra, Vth will also be reduced in the same
    proportion thus Is remains *unchanged* if I read this correctly.

    If so, no matter how low Ra is reduced, I should still get the same output
    from the same Vth input signal.
    But Vth at the OTA input will be much lower, is it what sets a lower limit
    to Ra?


    Ron
     
  2. I think the thing that limits how low Ra can go is S/N ratio.
    At the end of the day, Rin/Ra is a pot-down and is it desirable
    to have at least 50 or 100mV pk across Ra.
     
  3. R. G.

    R. G. Guest


    So I was not so far from the truth after all!

    Thanks!
    Ron
     
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