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Input signal appearing on the +Vpin of an opamp

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Yamin:
A weird question I know, but some years back at college I remember while we were working on some op amp circuitry we detected our sine wave input appear on the +V pin of the op amp. Can't remember what we made of it back then.
This can't be normal right? I think I am having a similar issue on a project I am working on right now. Unfortunately I haven't got a scope to test it out at the moment.
Anyone had a similar experience? Is it a common failure mode for op amps?
Thanks

Zero999:
What configuration are you talking about?

Are you talking about the positive power supply rail? It sounds like the isn't enough supply decoupling, or the impedance of the power supply is too high.

Yamin:

--- Quote from: Zero999 on September 13, 2020, 09:28:04 pm ---What configuration are you talking about?

Are you talking about the positive power supply rail? It sounds like the isn't enough supply decoupling, or the impedance of the power supply is too high.

--- End quote ---
Yep on the positive supply. I am working on a guitar practice amp at the moment and this is on the preamp stage. It's in an inverting amplifier configuration.
Could I please ask you to explain what you meant by "the impedance of the power supply is too high".
Thanks a million

niconiconi:

--- Quote from: Yamin on September 13, 2020, 09:41:38 pm ---
--- Quote from: Zero999 on September 13, 2020, 09:28:04 pm ---What configuration are you talking about?

Are you talking about the positive power supply rail? It sounds like the isn't enough supply decoupling, or the impedance of the power supply is too high.

--- End quote ---
Could I please ask you to explain what you meant by "the impedance of the power supply is too high".

--- End quote ---

What Zero999 said is that, when the opamp tries to amplify a signal, say, a sine wave, its current consumption increases. If the power supply impedance is too high (high resistance connection, no bypass/decoupling, or if the circuit is somehow broken and draws more current than expected), the supply voltage at +V drops according to Ohm's Law. And after the sine wave peaks, voltage starts to decrease, and the current consumption of the opamp decreases again, voltage drop at +V decreases. The final result is that you see the output signal of the opamp has "leaked" into the +V rail. Another related problem is common-impedance coupling - if many chips are connected to the same power supply without any bypass/decoupling capacitance, the behavior of one chip can cause the power rail voltage to drop, and this drop affects other chips as well.

Not sure if it's related to your problem, just to explain "the impedance of the power supply is too high".

Zero999:
See the simulation of an inverting amplifier, with a power supply with a resistance of 100R. The red trace shows the output signal superimposed on the supply voltage.

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