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I am breadboarding a circuit using a TL082CP Op-Amp, and I am reading consistent values with the multimeter. The Op-Amp stage is powered with a single 9V battery, like this: enter image description here The Op-Amp stage looks like this:

enter image description here

However, a thing is making me scratch my head: If there is no input signal, I would expect to read around 4,5V at the non-inverting input and at the output of the Op-Amp, but actually if I measure with the multimeter the voltage between the + pin and the GND, I read around 2,5V, while the output is consistently at 4,5V.

Why is that?

I measured the voltage across the R4 resistor and it was 0V, so there must be no current on that; I tried to use a 10K resistor instead of a 470K, and then on the + pin I read around 4,5, so it must be that some sort of current is flowing at the input of the op-amp, and a bigger resistor just causes a voltage drop with that current. Am I wrong?

Is that current coming from the power stage or is it coming from the Op-Amp?

I also tried to disconnect the Op-Amp to see if the C5 capacitor was leaking, but it seemed to work fine. I am kind of inexperienced with Op-Amps, and maybe I am doing a very basic error in measuring or breadboarding, but what confuses me is that the output of the circuit seems to be correct. I would greatly appreciate your help.

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    \$\begingroup\$ Is your multimeter only 1 Megohm input impedance? I would expect to see at least 4V after R4 with a 10Megohm DMM. \$\endgroup\$
    – user16324
    Feb 19, 2020 at 16:33
  • \$\begingroup\$ Sounds like an answer to me @brian. \$\endgroup\$
    – Andy aka
    Feb 19, 2020 at 16:53
  • \$\begingroup\$ @Andyaka except I've never heard of a 1M meter. Maybe the OP needs to link to the datasheet for whatever he has. \$\endgroup\$
    – user16324
    Feb 19, 2020 at 17:22
  • \$\begingroup\$ @BrianDrummond Thank you for your answer. I don't know about the imput impedance, but it is a very cheap multimeter for sure. The thing that doesn't make any sense is that there is no voltage drop on R4, and the output of the op-amp seems correct. How does the multimeter's impedance affect that? \$\endgroup\$
    – Dag
    Feb 19, 2020 at 21:34
  • \$\begingroup\$ "there is no voltage drop on R4" ... when you aren't measuring IN+ io GND... What would 1Meg from IN+ to GND do to the voltages in that area? \$\endgroup\$
    – user16324
    Feb 19, 2020 at 21:40

1 Answer 1

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Is your multimeter only 1 Megohm input impedance? I would expect to see at least 4V after R4 with a 10Megohm DMM.

As it is, you have a potential divider between 4.5V and VIN+, formed by R4 and the DMM itself.

When you move the DMM to make any other measurement, the potential divider goes away.

I think this is enough to explain all your observations.

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  • \$\begingroup\$ Thank you very much; very helpful. \$\endgroup\$
    – Dag
    Feb 19, 2020 at 22:21

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