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Dropping voltage at output of opamp

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Minh Thanh

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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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dknguyen

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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?
 

ronsimpson

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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
 

dknguyen

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Most Helpful Member
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).
 

throbscottle

Well-Known Member
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.
 

audioguru

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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.
 

ronsimpson

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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 ?
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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