A question on ideal op-amp (finding a current).

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Discussion Overview

The discussion revolves around a homework problem involving an ideal operational amplifier (op-amp) where participants are trying to find the output current (io) along with other unknowns in a circuit. The conversation includes attempts at solving the problem, equations used, and reflections on potential errors in the calculations.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Technical explanation

Main Points Raised

  • One participant expresses skepticism about their calculation of the output current (io) and shares their equation: ((va-vo)/R6) - (vo/R7) = io.
  • Another participant suggests that two additional equations are needed to solve for the three unknowns: io, va, and vo.
  • A participant questions whether it is possible for io to equal zero, given their calculations led to that result.
  • Another participant argues that io must be greater than zero for a positive voltage input, stating that only when Vin = 0 can io be zero.
  • One participant reflects on their calculations, noting that both Va and Vo are negative due to the inverting nature of the op-amp, and suspects a possible error in their substitution process.
  • A later reply reveals that the participant identified an error in their approach, specifically in writing the nodal equation at the output node without accounting for io, which led to an incorrect value for Vo.

Areas of Agreement / Disagreement

Participants express differing views on the possibility of io being zero, with some asserting it cannot be zero under certain conditions while others reflect on their calculations leading to that result. The discussion remains unresolved regarding the exact values and relationships between the variables.

Contextual Notes

Participants mention missing values for resistors and the DC voltage source, which may affect their calculations. There is also uncertainty regarding the correct application of nodal analysis in the context of the op-amp circuit.

CDTOE
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Homework Statement



I had an exam question on ideal op-amp asking to find a number of unknowns. I found them all except that I'm skeptic of my attempt in finding the output current which is symbolized on the circuit (io), so I have come to ask to throw my skepticism away. See the circuit below in the link including my attempt of solution:

http://tinypic.com/view.php?pic=2evrzap&s=6

Homework Equations

The Attempt at a Solution



As shown on the image, my attempt was: ((va-vo)/R6)) - (vo/R7)= i0
 
Last edited:
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The equation you wrote in your handwriting is correct, but you need two more equations. You have 3 unknowns: io, va and vo.

Hint: two of the resistors can be ignored completely.
 
I found these three unknowns, then I substituted in the 'io' equation and it resulted to zero amps. I don't remember the values that were given in the exam for the resistors and the DC voltage source, but is it correct that 'io' may equal zero in this question specifically?
 
CDTOE said:
I found these three unknowns, then I substituted in the 'io' equation and it resulted to zero amps. I don't remember the values that were given in the exam for the resistors and the DC voltage source, but is it correct that 'io' may equal zero in this question specifically?

No. io has to flow into the op amp (i0 > 0) for a positive voltage input, and vice-versa for a negative input. Only for Vin = 0 is io = 0.

Assume a positive input: then Va and therefore Vb must be < 0 (why?). But Vb can only be < 0 if the op amp draws current into itself (io > 0).

The value of the dc source does not alter the above statement (unless of course it was zero).
 
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That's what actually resulted for 'io' when I substituted in the equation, which is zero amps. I've become worried that I may have some error made in finding one of the voltages, probably the output voltage 'vo'. However, yes, both Va and Vo are negative voltages because it's an inverting op-amp, and that's what actually resulted for me in the exam. I probably forgot a minus sign somewhere when I substituted the resulted values into the equation.
 
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I got the exam paper back and I found the error that I made. When I found the output voltage (Vo), I wrote the nodal equation at the output node while I don't have the value of the output current (Io), so it was missing in the equation, and for this reason, I got the value of Vo wrong. I should have done the nodal equation at node Va to get the value of Vo.
 

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