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Opamp circuit DC offset

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  • Opamp circuit DC offset

    Good day all
    I have built this circuit as part of a little audio mixer.
    Im getting a 200mV DC on the output pin. Opamp is not oscillating at all, actually perfectly clean DC.
    I have tried different bipolar and JFET opamps.
    This is a dual opamp and both sections have the same resistors.
    I also tried a resistor on pins3/5. No difference.
    Can anyone help? thanks!
    Attached Files

  • #2
    Instead of connecting pin 3 directly to ground, use a 1K resistor or a resistor equal to the input resistor going to pin 2.

    To be more precise the resistor should be equal to the paralleled resistance of the input and feedback resistors
    Last edited by dmeek; 05-31-2019, 02:05 PM.

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    • #3
      That circuit is an inverting amplifier.

      If you have ANY voltage across R1 (1K), then that voltage will be 'amplified' by the gain of the circuit . (47K/1K).

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      • #4
        Originally posted by diydidi View Post
        I have built this circuit as part of a little audio mixer.
        Im getting a 200mV DC on the output pin.
        What is the input connected to? If it's DC coupled to a low impedance source the op-amp will have a gain of 48 to its own input offset voltage which will turn a 4mV input offset voltage to 200mV at the output.
        Last edited by Dave H; 05-31-2019, 04:07 PM.

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        • #5
          ^^^^^ THAT

          AND add 10uF in series with input 1k resistor, may be a plain bipolar with the + terminal towards input jack.

          As is, it not only has input offset, which is corrected by dmeek´s suggestion, but you also amplify 50X any offset or leak which might be applied to input.

          The series cap means DC gain is 1X, so even uncompensated offset would become 200/50=4mV

          As a side note, try to upload images as image files (jpg / gif / png) so they can straight be opened inside the page.
          Juan Manuel Fahey

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