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Voltage controlled resistor (LM13700) not working properly

Discussion in 'LEDs and Optoelectronics' started by george2525, Sep 23, 2016.

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  1. george2525

    george2525

    165
    0
    Jan 30, 2015
    VCRproblems.PNG

    Hello. I have taken the above from the LM13700 datasheet (datasheet version below) which is a voltage controlled resistor, controlled by the labelled "Cv". I have attached this as the feedback resistor Rf of a standard non-inverting amp just to test whether or not the supplied formula actually works. Eventually Cv will be supplied by an R2R ladder but anyway...

    Datasheet states Rx = 2R/gmRA where R are the two 100k resistors shown and RA is labelled.

    I have spent hours trying to configure this so that the formula version of Rx matches the one determined by the non inverting op amp gain (1+Rx/R2).

    Using the Vo = 3.215, Rx should be 12.14k ohms

    but using Rx=2R/gmRA I get Rx = 11.35k ohms

    so - they dont match and the VCR isnt working properly

    I may have done something stupid but I cant spot it so any fresh eyes woud be appreciated. I will attack the datasheet version below. PS - if anyone has ever actually derived the formula shown I would be very interested to know how it was done.

    Thanks.

    VCRdatasheet.PNG
     
    Last edited: Sep 23, 2016
  2. george2525

    george2525

    165
    0
    Jan 30, 2015
    I should add for anyone not familiar with the chip that gm = 19.2*Iabc

    where 2*Iabc is labelled on the ammeter
     
    Last edited: Sep 23, 2016
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