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Dropping voltage at output of opamp

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by Minh Thanh, Oct 5, 2017.

  1. Minh Thanh

    Minh Thanh New Member

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    I have problem at output of the first opamp . About 3 -4 minutes , the opamp's output voltage decreases by 0.1V . Although I reduced gain of opamp to 2, but the opamp 's output was still reducing 0.1V about 3 -4 minutes . Could you me about this ? Thank You !
     

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    Last edited: Oct 5, 2017
  2. dknguyen

    dknguyen Well-Known Member

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    Decreases by 0.1V or does it decrease to 0.1V? If it decreases by 0.1V, are you sure it's not just the op-amp warming up?
     
  3. Minh Thanh

    Minh Thanh New Member

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    Decreasing by 0.1 every 3- 4 minutes . Opamp is still normally warm. Dual power supply +5.25 / -5.20
     
  4. dave

    Dave New Member

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  5. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    So what is the voltage at pins 1,2,3? (at start up) and (after 4 minutes) ?
    Is C1 an Ele. Cap. ?
    upload_2017-10-5_20-30-26.png
     
  6. Dick Cappels

    Dick Cappels Member

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    What, if anything, is connected to the input?
     
  7. crutschow

    crutschow Well-Known Member Most Helpful Member

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    and to the output?
     
  8. ci139

    ci139 Active Member

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  9. dknguyen

    dknguyen Well-Known Member

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    It almost sounds like there's no DC path to ground at all terminals of your op-amp to allow bias currents to flow so the capacitances are just charging up slowly and killing the signal...except I don't see that in your circuit anywhere.

    But Ron does have a point about your capacitor being backwards in the circuit if you are using a polarized capacitor (electrolytic or tantalum).
     
  10. throbscottle

    throbscottle Well-Known Member

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    If you are testing it with no signal source connected, try connecting a resistor between pin 3 and ground and see if the problem goes away.
     
  11. ci139

    ci139 Active Member

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    ((((damnit - silly me)))) buffer your reference at IC1.b (R7:R6)
    ? also the "BNC-Core" INPUT before inverting level shifting

    or it never works precise - floating or not
     
    Last edited: Oct 7, 2017
  12. Minh Thanh

    Minh Thanh New Member

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    Recently, my circuit is stability . However, I have question is I could use C1 as Ele Cap or I should use C1 as non polarized cap ?
     
  13. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Use a film capacitor for C1. An electrolytic capacitor will have current leakage and will not discharge completely when it should discharge.
     
  14. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    You can find out if the voltage is positive all the time, negative all the time, or goes + and -. Measure across the cap. not to ground. Some types of caps can only handle DC and some can work with AC.
    AND
    What AudioGuru said.
     

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