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why is my op amp doing this?

Discussion in 'General Electronics Discussion' started by AFex54, Aug 8, 2015.

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  1. AFex54

    AFex54

    144
    2
    Apr 10, 2015
    Im using an lm324 and a 9V supply with this circuit.
    the output will sit around 8V and will not hold a sample it justs climb back to this voltage after opening the switch,
    I tried this with a tl074 and it behaves correctly , what is the issue with using an lm324?
    [​IMG]
     
  2. Laplace

    Laplace

    1,252
    184
    Apr 4, 2010
    The issue is input bias current charging the capacitor and thereby changing the capacitor voltage. According to the spec sheets the typical input bias current for the LM324 is 40 nA whereas for the TL074 it is 65 pA. Both op amps have the same problem, but it is 615 times worse for the LM324.
     
    BobK and AFex54 like this.
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