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Can i ask a very basic question please about Op Amps, specifically non inverting op amps with a negative feedback. I know two conditions are present a priori to any voltage input, and that is input current is 0 for both inputs. If input current is zero, how does voltage get applied in the input?

And is negative feedback a constant loop that is constantly fed back until the voltage difference is 0, but isn't this an a prior condition?

apoliteperson
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  • Take a look at this https://electronics.stackexchange.com/questions/123199/why-do-op-amps-keep-amplifying?rq=1 – mhaselup Oct 25 '20 at 08:42
  • Thank you. I tried looking at the link but couldn't get it. How is voltage applied at the input when current is zero at both inputs? – apoliteperson Oct 25 '20 at 09:27
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    I don't understand the statement you are making. Please can you draw of diagram of the circuit you are envisaging and then use that to explain your thinking at t=0 and then at some later point in time. – mhaselup Oct 25 '20 at 09:44
  • The current through the opamp inputs is not zero. It will be close to zero (~nA or uA range). This is due to the high input impedance at the input stage. This is similar to the way MOSFETs operate where the gate current is close to zero but not zero even on applying a Gate Voltage. – Srinivasan M P Oct 25 '20 at 11:11
  • A voltage may be present with no current. Consider an open circuit for instance. That is not the case here but you are making a statement that does not make sense. – Peter Smith Oct 25 '20 at 13:56
  • Thank you for all the responses. Just to clarify my question. I meant there are two conditions for an op amp with a negative feedback: zero current for both inputs. If this is the case, how does the voltage get dropped on the resistor for the negative feedback if there is no current bringing the voltage to the resistor? – apoliteperson Oct 26 '20 at 04:50

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