Circuit Diagram

How do I calculate Vo? I used the nodal voltage method combined with KCL, but it is not working out for me. I have:

At non-inverting port: $$ (v-.4V)/4kohm=0 $$ At inverting port: $$ (v-0)/2kohm+(v-Vo)/35kohm=0 $$

I solve these equations and end with \$v=.4V\$ and \$Vo=.75V\$.

I make the assumptions that the voltage at the input node equals the voltage at the inverting node and that the entire bottom wire is ground. The bottom wire is a super node connected to the negative side of Vo, so the entire bottom wire should have a voltage of 0.

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    \$\begingroup\$ Why is this downvoted? Albert has shown work and gave an idea of how he should solve for what he's looking for. \$\endgroup\$ – KingDuken Apr 1 '18 at 16:22
  • \$\begingroup\$ You must be stumbling somewhere while solving, these exact same equations give me Vo = 7.4, which means the output is limited by the upper voltage rail (downvoter was not me, btw). \$\endgroup\$ – Vicente Cunha Apr 1 '18 at 16:27
  • \$\begingroup\$ The bias equalising resistor on the non-inverting input seems to be causing you some confusion. For your gain calculations you can assume that the bias current is zero and that the voltage is the same on both sides of the 4k resistor. So V+ = 0.4 V. Try working from there and update your question. \$\endgroup\$ – Transistor Apr 1 '18 at 16:42
  • \$\begingroup\$ You guys were right. I got 7.4 after doing the math again. I must've typed something wrong while doing my math. And then, since the OpAmp is limited to +-7V, you round down to 7V. Thank you all. \$\endgroup\$ – Albert Garcia Apr 1 '18 at 18:55

Second equation: v/2 = (Vo-v)/35 ; Vo -v = 35*v/2 = 7; Vo = 7.4

Vo cannot be higher then V+ then Vo = 7 for ideal conditions.

| improve this answer | |
  • \$\begingroup\$ Thank you. I got 7.4 on a previous attempt, but I forgot that I needed to drop it down to 7V since the OpAmp limits the voltage output. I appreciate the help. \$\endgroup\$ – Albert Garcia Apr 1 '18 at 18:57

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