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I was wondering if you guys could help me or lead in the right direction. I need to find the voltage across a resistor with an op amp. I was originally going to try solving for nodal voltage. But we can't do that. And usually we can do the Rf/Rs*Vin but it is throwing me off cause there is no Rf so it would be 0?

What is the voltage before and after the op amp?

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    \$\begingroup\$ Here's a hint: Vout = V-, since it's the same node. \$\endgroup\$ – klamb Apr 4 '17 at 21:41
  • \$\begingroup\$ So can I solve for nodal voltage at point between 8k and 4k resistor and that voltage is equal to the voltage leaving the op amp? So that voltage at V1 would be equal to Va? \$\endgroup\$ – noreturn Apr 4 '17 at 21:45
  • \$\begingroup\$ Rf/Rs*Vin is for an inverting opamp configuration. You have a non-inverting circuit there. \$\endgroup\$ – brhans Apr 4 '17 at 21:52
  • \$\begingroup\$ I added my work, does that look correct? \$\endgroup\$ – noreturn Apr 4 '17 at 21:57
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Since you are using an op amp as a non-inverting amplifier, your gain equation is wrong. Rf/Rin applies for an inverting amp, and you have left off the - sign in your relation. The general configuration for a non-inverting amplifier is

schematic

simulate this circuit – Schematic created using CircuitLab

So for your configuration (called a voltage follower) the gain is 1 plus (zero /infinty) or 1.

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  • \$\begingroup\$ I added a picture of my work. Does that look correct? \$\endgroup\$ – noreturn Apr 4 '17 at 21:52
  • \$\begingroup\$ Yup. Well, assuming a perfect op amp, of course. A real op amp will have a (typically as much as a few millivolts) small error due to the fact that the voltage between the + and - inputs is not actually zero. It's called input offset error, combined with the fact that the gain of the op amp is not actually infinite. But some op amps can have input errors in the microvolt range. \$\endgroup\$ – WhatRoughBeast Apr 4 '17 at 21:59

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