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

[jim hardy]

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

 

[LvW]

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

 

[meBigGuy]

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.

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 ...

 

[Averagesupernova]

Well I DID say it was subtle.