How to clasify if it's an inverting/non-inverting op amp

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Discussion Overview

The discussion revolves around how to classify an operational amplifier (op amp) as inverting or non-inverting based on its schematic. Participants explore the implications of feedback, the conditions under which an op amp operates, and the characteristics of specific configurations, including a circuit that may not function as a traditional amplifier.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants question the classification of an op amp as inverting or non-inverting based solely on which input the signal is applied to, suggesting that the circuit configuration ultimately determines its function.
  • One participant asserts that the shown circuit cannot function as an amplifier due to positive feedback, which leads to saturation at the supply rails.
  • Another participant agrees that the circuit does not work as an amplifier and suggests that the textbook's classification of the configuration as "inverting" is incorrect.
  • Some participants discuss the implications of feedback, noting that negative feedback is necessary for stabilizing the operating point of an op amp circuit.
  • There is a mention of a simulation of the circuit, with participants noting discrepancies between the input and output waveforms, indicating it does not behave as a linear amplifier.
  • One participant introduces the concept of a Schmitt comparator, suggesting that the configuration is common but not suitable for linear amplification.
  • Another participant elaborates on the operational principles of op amps, discussing how to determine if an op amp configuration inverts the signal based on the relationship between its inputs.

Areas of Agreement / Disagreement

Participants express disagreement regarding the classification of the op amp circuit in question. While some assert that it cannot function as an amplifier due to positive feedback, others explore the implications of input configurations and feedback types without reaching a consensus on the classification of the circuit.

Contextual Notes

The discussion highlights limitations in understanding the circuit's operation, particularly regarding the definitions of inverting and non-inverting configurations, and the necessity of negative feedback for proper amplification. There are unresolved mathematical steps and assumptions about the circuit's behavior that contribute to the ongoing debate.

  • #31
LvW said:
Hi Old Jim - just one short comment to your long and detailed explanation.
Since many decades the term "phase margin" is (a) defined and (b) extensively in use - in control theory as well as for all amplifier configurations with feedback.
I was not sure if it make sense to repeat the definition of such well-known parameters here in the forum.

Thanks LvW

sorry i got so long winded.
I estimated xconwing to be not yet an "Old Hand"
so took the risk of going back to the basic of basics ,
introducing those well known( to old hands) definitions.

It's important for beginners
to distinguish instability caused by running out of phase margin
from instability caused by noise, friction, loose connections, thermal runaway and the like.

For me the "light came on" when i grasped that bit about denominator going to zero because GH=1 with phase inversion; the math and my intuition suddenly agreed.
I wanted to offer that to xconwing

please take no offense at my remark about 'lack of introduction' for none was meant.
I just wanted to describe the concept clearly.

And i see i left the TI link out of post, will edit it in...
http://www.ti.com/lit/ml/sloa077/sloa077.pdf old jim
 
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  • #32
jim hardy said:
It's important for beginners to distinguish instability caused by running out of phase margin
from instability caused by noise, friction, loose connections, thermal runaway and the like.
old jim

Jim - thanks for clarification.
And yes - the above cited problem is an important one. That`s the reason we should strictly distinguish between "dynamic instability" and other forms of unwanted fluctuations ("static" instability - although not a very descriptive term).
LvW
 
  • #33
meBigGuy said:
No, he is not carrying it a bit far.
Negative feedback is not applied to "stabilize" an amplifier unless you are trying to compensate for existing positive feedback. That happens, but it isn't the common situation. Negative feedback is generally applied to linearize an amplifier, control its gain, control its frequency response, etc, etc. Your comments relate to its use for those reasons. If there is no positive feedback, the circuit will be stable.

Averagesupernova said:
Whoever said that negative feedback is seldom used to compensate positive feedback is quite wrong. This is very common although it may be subtle. .

When I apply negative feedback to control the gain, I naturally reduce the gain at high frequencies where positive feedback may be occurring. Did I apply feedback to control the gain? Or to reduce the effects of positive feedback? It may be used often to compensate for positive feedback, but it is used "oftener" for other reasons (that's a judgement call).

Maybe if the OP had just said that negative feedback CAN be applied to stabilize an amplifier ...
 
  • #34
Well I DID say it was subtle.
 

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