Op Amp Circuit Help: Solving KCL Eq with Source Transform

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Discussion Overview

The discussion revolves around solving a circuit problem involving an operational amplifier (op-amp) using Kirchhoff's Current Law (KCL) and source transformations. Participants explore different approaches to analyze the circuit and express their challenges in deriving the correct equations.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant describes performing a source transformation and applying KCL at a specific node, but struggles with the resulting equations, questioning whether the source transformation is complicating their analysis.
  • Another participant suggests that leaving the source un-transformed might simplify the problem and recommends writing KCL equations for both inputs of the op-amp.
  • Several participants discuss the behavior of currents at the op-amp inputs, with one asserting that for an ideal op-amp, the currents should equal zero, while another notes that practical models may differ.

Areas of Agreement / Disagreement

Participants express differing views on the effectiveness of source transformations and the behavior of currents in op-amp circuits. There is no consensus on the best approach to solve the problem, and the discussion remains unresolved.

Contextual Notes

Participants reference specific assumptions about ideal versus non-ideal op-amp behavior, which may affect their analyses. The discussion includes unresolved mathematical steps and varying interpretations of circuit behavior.

Who May Find This Useful

This discussion may be useful for students and practitioners working on circuit analysis involving operational amplifiers, particularly those interested in KCL applications and source transformations.

wclawson
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It was much easier scanning the pictures and problem than trying to recreate:

circuit1.jpg


circuit2.jpg


I began by doing a source transform, making Vs = is*Rs, and placing Rs in series instead of parallel. I then did KCL at Vn.

I tried to break the KCL eq down to il/is, but just ended up with some un-godly mess that ended up approximating to 0 for part b. Is the source transform throwing me off? Or is there just a much easier way to solve this than what I tried.

Thanks a lot.
 
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wclawson said:
It was much easier scanning the pictures and problem than trying to recreate:

circuit1.jpg


circuit2.jpg


I began by doing a source transform, making Vs = is*Rs, and placing Rs in series instead of parallel. I then did KCL at Vn.

I tried to break the KCL eq down to il/is, but just ended up with some un-godly mess that ended up approximating to 0 for part b. Is the source transform throwing me off? Or is there just a much easier way to solve this than what I tried.

Thanks a lot.

Since they want you to calculatet Il/Is, I'd probably leave the source un-transformed. Write the 2 KCL equations (one for each input on the opamp), and see how it shakes out. And yes, the approximation when the gain is infinite helps simplify it a lot.
 
I happen to be working on this same problem right now (University of rochester, ECE 111?)

Wouldn't the currents of each input always equal zero?
 
danhamilton said:
I happen to be working on this same problem right now (University of rochester, ECE 111?)

Wouldn't the currents of each input always equal zero?

For an ideal opamp, yes. But Figure 4-4 shows some of the more practical (non-ideal) model bits...
 
danhamilton said:
I happen to be working on this same problem right now (University of rochester, ECE 111?)

Wouldn't the currents of each input always equal zero?

Haha yes, ECE 111 at U of R.

Thank for the help, I've got it now.
 

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