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How would I calculate the input impedance in the diagrams above?

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This is what I have tried in my exam, however I got this wrong, and my teacher had told me not to take into account the 2 10k resistors.

  • \$\begingroup\$ Where are you stuck? \$\endgroup\$ – Spehro Pefhany Dec 3 '20 at 7:29
  • \$\begingroup\$ This is homework-please tag it appropriately. What is your initial working/thinking on this? \$\endgroup\$ – mhaselup Dec 3 '20 at 7:29
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    \$\begingroup\$ You should make a distinction between the input impedance of the opamp (usually this is a very high impedance, see the datasheet of the opamp), and the input impedance of the circuit (which depends on the circuit AND the opamp). How the input impedance of the circuits you show are determined is described in many textbooks as these are very common circuits, the non-inverting and inverting amplifier configurations. So consult your textbook and see what you find. \$\endgroup\$ – Bimpelrekkie Dec 3 '20 at 8:46
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    \$\begingroup\$ I’m voting to close this question because this looks like a homework question, which is OK to ask about but you have to show what you have tried. You show nothing. The circuits you ask about are very common and with some searching you will find out how to determine their input impedances. \$\endgroup\$ – Bimpelrekkie Dec 3 '20 at 8:50
  • \$\begingroup\$ Your teacher is right. The 10k resistors don't have anything to do with input impedance. \$\endgroup\$ – Justme Dec 3 '20 at 18:07

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