Justifying the peak closed loop gain of an op-amp circuit

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Discussion Overview

The discussion revolves around understanding the peak closed loop gain of an operational amplifier (op-amp) circuit as represented in a Bode plot. Participants explore theoretical approaches to determine where peak gain should occur and whether such information is available in the op-amp's datasheet. The conversation includes considerations of resonance in the gain at specific frequencies.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant seeks to understand if a resonance in the gain at a measured frequency can be theoretically justified and whether this information is available in the datasheet.
  • Another participant describes the typical behavior of Bode plots for op-amps, noting that they usually show flat gain until the first pole and then a roll-off, suggesting that resonances are uncommon.
  • A link to an external resource is provided to illustrate the concept of open loop and closed loop gains of an op-amp.
  • A later reply reflects on the initial observation, suggesting that what was perceived as resonance may actually be a transition point in the gain curve rather than a true resonance.

Areas of Agreement / Disagreement

Participants express differing views on the presence of resonance in op-amp gain plots, with some suggesting it is unlikely while others are exploring the possibility. The discussion remains unresolved regarding the specific characteristics of the gain at the measured frequency.

Contextual Notes

The discussion does not clarify the definitions of open loop gain or feedback factor as they relate to the specific circuit in question, nor does it resolve the mathematical steps needed to analyze the Bode plot fully.

ergospherical
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I'm just looking at a Bode plot of log(gain) vs log(frequency) for a an amplifier circuit. I'm trying to figure out whether I'm expecting to see a resonance in the gain at the frequency where I measured one. How could I, in principle, work out theoretically where the peak gain should occur - is this sort of information hidden somewhere in the data sheet? It's not clear to me from the schematics what the open loop gain ##A## or the feedback factor ##b## are (the internal components are not displayed). Thanks.
 
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ergospherical said:
I'm just looking at a Bode plot of log(gain) vs log(frequency) for a an amplifier circuit. I'm trying to figure out whether I'm expecting to see a resonance in the gain at the frequency where I measured one. How could I, in principle, work out theoretically where the peak gain should occur - is this sort of information hidden somewhere in the data sheet? Thanks.
The Bode plot of open loop gain for most opamps in their datasheets will show flat gain until the first pole ("dominant pole", set by an explicit capacitance), and then a straight rolloff down to the 2nd pole where it decreases even faster.

Can you post an example Bode plot that you're asking about? Most opamps would not have resonances or peaking in their open loop gain plots, in my experience.
 
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Like so:

https://toshiba.semicon-storage.com...-loop-and-closed-loop-gains-of-an-op-amp.html

1638997015488.png
 
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I think you're right - at a second glance it doesn't look like a resonance so much as a little bump where the flat gain turns into a straight roll-off. I'll try and have a look for the manufacturer's version of that graph... thanks :)
 
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