Operational amplifier with feedback problem

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Discussion Overview

The discussion revolves around a problem related to operational amplifiers (op-amps) and feedback, specifically focusing on the modeling of the op-amp's output resistance and its implications for circuit analysis. Participants explore the correct representation of the op-amp's output resistance in relation to voltage sources and the Thévenin equivalent resistance.

Discussion Character

  • Homework-related
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant expresses uncertainty about their calculation of beta, questioning whether their model is incorrect.
  • Another participant asserts that the op-amp's output resistance (ro) should be shown in series with the controlled voltage source.
  • A question is raised about whether ro is always in series with a voltage source, to which another participant confirms this assertion.
  • Participants discuss the Thévenin equivalent resistance when a resistance is placed in parallel with an ideal voltage source, with one participant suggesting it would be a voltage source in series with the resistance.
  • There is a challenge to determine the Thévenin equivalent resistance, with a participant suggesting it could be zero based on circuit analysis rules.
  • A later reply confirms that a resistance in parallel with an ideal voltage source does not affect the effective source resistance, which remains zero.
  • One participant indicates they will attempt the question again after the discussion.

Areas of Agreement / Disagreement

Participants generally agree on the placement of the op-amp's output resistance in relation to the voltage source, but there is some uncertainty regarding the implications of parallel resistances and the resulting Thévenin equivalent resistance. The discussion remains unresolved regarding the initial problem of calculating beta.

Contextual Notes

There are limitations in the discussion regarding assumptions about ideal components and the specific conditions under which the rules apply. The discussion does not resolve the mathematical steps involved in determining beta.

TheRedDevil18
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Homework Statement


upload_2016-11-23_22-8-8.png


Homework Equations

The Attempt at a Solution


IMG_20161123_220433.jpg

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Using the equations above, I'm not getting the right answer for beta which is 0.4926 V/A.
Is my model wrong ?
 
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The op-amp's ro should be shown in series with the controlled voltage source.
 
NascentOxygen said:
The op-amp's ro should be shown in series with the controlled voltage source.
So with a voltage source, ro is always in series ?
 
TheRedDevil18 said:
So with a voltage source, ro is always in series ?
Yes.

If you were to place a resistance in parallel with an ideal voltage source, what would be the Thévenin equivalent resistance of that combination?
 
NascentOxygen said:
Yes.

If you were to place a resistance in parallel with an ideal voltage source, what would be the Thévenin equivalent resistance of that combination?

Would it be a voltage source in series with the resistance ?, I know that a current source with a parallel resistance is equal to a voltage source in series with that resistance
 
TheRedDevil18 said:
Would it be a voltage source in series with the resistance ?
Guessing is not allowed.

Can you determine it?
 
NascentOxygen said:
Guessing is not allowed.

Can you determine it?

Would it be zero ?, I determined it by using the rules. When you have a voltage source you short it and remove the load, so that means all the current will flow through the short and not through the parallel resistor
 
TheRedDevil18 said:
Would it be zero ?, I determined it by using the rules. When you have a voltage source you short it and remove the load, so that means all the current will flow through the short and not through the parallel resistor
Right. A resistance has no effect if you erroneously add it parallel to an ideal voltage source: the effective source resistance here would stay as zero.
 
NascentOxygen said:
Right. A resistance has no effect if you erroneously add it parallel to an ideal voltage source: the effective source resistance here would stay as zero.

Ok, thanks, I'll attempt the question again
 

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