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Almost zero current is drawn by an opamp from its inputs (in theory), but still there must be tiny current causing the opamp to feel the voltage. What is the name of that sort of current in electrical engineering terminology?

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    \$\begingroup\$ It is not the current that causes the voltage - it is the other way round. Theoretically an opamp could have truly zero input current in the steady state. \$\endgroup\$ – Kevin White Mar 27 '17 at 15:43
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    \$\begingroup\$ Please, stop propagating this fallacy of voltage causing current (or vice-versa). electronics.stackexchange.com/questions/201533/… \$\endgroup\$ – Sredni Vashtar Mar 27 '17 at 17:07
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It's called "input bias current", if you're referring to the current that flows into the input terminals. See this tutorial from Analog Devices, which discusses the current in great detail. It points out that the input bias current \$I_B\$ can vary from a tiny 60 fA to many μA depending on the device. (60 fA is one electron every 3 microseconds, which is impressively low.)

You could also be referring to the input offset current \$I_{OS}\$, which is the difference between the two input bias currents.

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  • \$\begingroup\$ Obviously in that tutorial Ib+ and Ib- is flowing. But current must loop. Where do they flow to? To the ground of the amplifier? But there is no common ground connection between the source and the amplifier. Shouldn't Ib+ and Ib- flow from the source back to the source again creating a loop??? \$\endgroup\$ – user1245 Mar 27 '17 at 15:23
  • \$\begingroup\$ @user134429 Yes, the currents flow to ground. The source signals and the op amp will normally share a ground. To simplify, each op amp input is connected to the base of an input transistor in a "differential pair" inside the op amp. These base currents are the input currents. A while back I took apart a 741 and explained what's inside, which might help you understand what goes on internally: righto.com/2015/10/inside-ubiquitous-741-op-amp-circuits.html \$\endgroup\$ – Ken Shirriff Mar 27 '17 at 17:17
  • \$\begingroup\$ These bias currents are non-idealities, and they don't cause "the op amp to feel the voltage". They are reality, in that they are a manifestation of the functioning of input transistors, but they are not what makes the op amp work. \$\endgroup\$ – Scott Seidman Mar 27 '17 at 21:48

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