Question regarding ideal OP-amps

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Discussion Overview

The discussion revolves around the behavior of ideal operational amplifiers (OP-amps), particularly focusing on the relationship between input and output voltages and the implications of feedback mechanisms. Participants explore theoretical aspects, practical implications, and configurations of OP-amps.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant questions how an increase in output voltage can occur if the + and - inputs of an ideal OP-amp are always equal, referencing the equation Vo = Gain(Vpos-Vneg).
  • Another participant clarifies that the + and - inputs are not exactly equal, suggesting that a difference is necessary for output voltage change and poses a calculation regarding the input voltage difference needed for a specific output change given a high gain.
  • A third participant discusses the role of feedback resistors in maintaining balance between the inputs, noting that the "always equal" condition is subject to limitations such as slew-rate, which affects performance at higher frequencies.
  • Another participant recommends analyzing a non-ideal OP-amp configuration to better understand the effects of imperfections such as offset currents and finite resistance between inputs.

Areas of Agreement / Disagreement

Participants express differing views on the nature of the input voltages in ideal OP-amps and the implications of feedback mechanisms. There is no consensus on the interpretation of the ideal conditions versus practical scenarios.

Contextual Notes

Limitations include assumptions about ideal behavior, the effects of feedback mechanisms, and the impact of non-ideal characteristics on OP-amp performance. The discussion does not resolve these complexities.

Who May Find This Useful

Readers interested in operational amplifier theory, feedback systems, and practical applications in electronics may find this discussion relevant.

Ry122
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Since both the + and - inputs of an ideal OP-amp are supposedly always equal to each other, how is it that there can be any increase in voltage on the output of the OP-amp when the equation which describes the relationship between the OP-amp input and output is Vo = Gain(Vpos-Vneg) ?
 
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The + and - inputs are not exactly equal to each other. As you have guessed, there must be a difference between the inputs in order to get a change in output voltage. How much of a difference in input voltages would you need to get a change in output voltage of 1 volt if the gain of the opamp is 100,000?

The output of an opamp is not Vo = Gain(Vpos - Vneg), it is Vo = Vpos + Gain(Vpos - Vneg).
 
Think of the feedback resistor (that sets the gain) as allowing the output to force the +/- inputs to be equal. With a high ratio resistor divider in the feedback, the output needs to swing further to counteract whatever is happening on the input. The "always equal" bit is subject to slew-rate limitations, meaning it can't keep up at higher frequencies. The magic of feedback is that (below the limit speed) everything is kept in balance with pretty much undetectable signal fluctuations.

In motor control systems you can see the feedback signal overshoot when the motor speed changes because of mechanical delay in the motor. Putting a capacitor to ground in the middle of the opamp feedback divider (from - to ground) slows the feedback response in the same way, so, with the right capacitance, you should be able to see an overshoot on the output when trying to track a pulse input.
 
You should probably analyze at least one op amp configuration where the op amp is not ideal (use offset currents, finite resistance between inputs, etc.) Try the negative feedback setup with Vout = G(V+ - V-) and all the imperfections. Everything will become clearer.
 

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