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Op Amp Stability

Op Amp Stability

I am not sure if it is a good suggestion to communicate via PM because other forum members should be able to participate, I think.
What kind of "compromise" are you speaking of (in case of right or wrong)?
Here are my simulation results for the phase margin (feedback via 10k-1k):
*Crossover frequency (unity loop gain) at Fco=71 kHz.
*Phase margin at Fc is PM=87 deg. (This PM also can be derived very easily from Your Fig. 3 at the 20 dB line).
I don´t know how you can arrive at other values.

Oh yes I see where I went wrong. I was doing so many different simulations late at night, I must have imported the wrong graph. I will amend this as soon as possible.
Adam
 
Ok Yeah I just didn't want message tennis taking up all this space. I found an article on noise gain you might be interested in.
http://electronicdesign.com/analog/whats-all-noise-gain-stuff-anyhow
Adam
Ohh yes - I know a lot of contributions from the late Bob Pease - however, I must admit that I do not like all of them.
And the present article about noise gain is such a contribution which - according to my experience - can cause a lot of confusions.
Every thing in the article is correct - but remember the given definition
"Noise gain is the reciprocal of the attenuation from the output of an op amp (or any feedback loop) to the input. In Figure 1, the attenuation is RIN/(RIN + RF). So the noise gain is (RF + RIN)/RIN."
That means: It is the reciprocal of the feedback factor beta. And it has NOTHING to do with actual noise properties of the circuit.
However, unfortunately Bob Pease continously uses the term "noise gain" - and that´s what some readers keep in mind: NOISE!
Why not use the term "invers feedback factor 1/beta"?
And it is so simple to DERIVE the rule which is used to check stability:
1.) The loop gain is LG=Aol*beta=Aol/(1/beta) >> LG(db)=Aol(db)-(1/beta,dB)
2.) Since LG=0 dB (LG=1) is the oscillation condition we have to check the phase at Aol(db) = (1/beta,dB); that means: Crossing point of both functions.

That is the explanation - based on the fundamental term "loop gain" - why we have to find the crossing point in the BODE diagram. No relation at all with noise.
 
That's interesting. I thought noise was any unwanted voltage at the input or output of a circuit. And this unwanted voltage that appears on the non inverting input get amplified by the gain +1. You don't mention the +1 in your formula. So everyone says noise gain is important for stability but you seem to disagree. This assumption comes from Analog devices an TI. So if noise appearing on the non inverting terminal from external coupled noise due to external capacitance and PCB trace resistance gets amplified then what do you call this. Your not thinking about the nV Sqrt Hz on some data sheets are you?. I know what I'll do I will contact Bob at linear tech and ask him his opinion on noise gain. I can't ask Jim anymore as he is no longer with us and Bob Pease died in a car crash coming back from a talk about Jim. Such a shame.
Adam
 
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That's interesting. I thought noise was any unwanted voltage at the input or output of a circuit. And this unwanted voltage that appears on the non inverting input get amplified by the gain +1. You don't mention the +1 in your formula. So everyone says noise gain is important for stability but you seem to disagree.

Hi Adam, please excuse me - but I think, your answer is a good example for the confusion which does exist regarding the term "noise gain".
To make it clear : I do NOT disagree. Of course, noise gain is important for stability because it is nothing else than the reciprocal of the feedback factor.

So if noise appearing on the non inverting terminal from external coupled noise due to external capacitance and PCB trace resistance gets amplified then what do you call this. Your not thinking about the nV Sqrt Hz on some data sheets are you?.
Do you see now the difference between NOISE GAIN (to be interpreted only using the feedback factor) and the actual noise value which has NO relevance as far as stability is concerned?
One can describe all the stability aspects WITHOUT using the term "noise gain" at all - but using instead the feedback factor only. That´s the point of confusion caused by invention of the term "noise gain".
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Hi.

I'm mostly addressing what I perceive as weaknesses in this article from the perspective of someone who really has little idea of the math or what this phase and magnitude stuff actually means. I am specifically not addressing the factual content. If I have worded something in a way which changes the meaning, that is inadvertent.

In closed loop gain, you have "Aol /1+Aβ" Did you mean "Aol / (1+Aβ)"?

In gain bandwidth, you have "where the gain is 1". I think it might be sensible to say "where the gain is 1 (i.e. the output is the same as the input)", and I'm not sure if you should also say "where the voltage gain is 1...".

In Noise gain, you say: "NG= R1/R2+1". I'm not sure if you mean "NG = (R1/R2)+1", or "NG = R1 / (R2+1)". It's a very easy mistake to make, and I would probably include parentheses even where the rules of precedence mean they are not strictly required.

It might be useful at least once to note how you get from a voltage gain of 1, or 800,000 to a gain in dB of 0 or 118). Perhaps you can place this in your definitions.

I might reword the following:

"Well that’s not far off. Unity gain at approx 800KHz and a gain of approx 118dB and a phase of -45˚.
Now if we look at the slope we see a nice gradual slope down to 0dB. This is the classic single pole response and is the same as the same as a single RC low pass filter.
Note: The solid line on the plot is the magnitude of the output and the dashed line is the phase."

as

"The convention in this document, unless otherwise noted, is that the solid red line on a plot is the magnitude of the output and the dashed red line is the phase difference between the input and the output -- we shall simply call this phase.

Well that’s not far off! Unity gain (0db on the magnitude line) is at approx 800KHz and a gain of approx 118dB. The black dotted lines highlight 0db and approximately 800kHz. The phase at this point (800kHz) is -45˚.

If we look at the magnitude line we see a nice gradual slope down to 0dB. This is the classic single pole response and is the same as the same as a single RC low pass filter."

The technical details I'll leave up to you, but I think it's important that the reader is totally aware of what you're talking about. As you go further, you can drop the parenthetical comments unless the meaning of the lines on the graphs change.

A little later you say:

"What would cause this to come up over the line and be within our unity gain bandwidth? Well in some cases all it needs it a bit of capacitance."

And this sounds like a dire and terrible thing!!! However I'm not sure exactly where to look on the graph, and you haven't told me what terrible thing is going to happen.

Much later you say:

"Why 0.707, I don’t really know where this came from but I think it was derived from Butterworth’s original calculations"

I think it's worth noting to the reader that 0.707 is 1/Sqrt(2). It's not like it's a number that was pulled out of thin air. So we know where it comes from, not necessarily why this particular value was chosen.

I also think you need to add some white space as I have done in one of my examples above.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Cool Steve. Thanks for that. Apart from the are you mostly happy with this?

I've been pretty busy this weekend. I haven't read it fully through. My main concern is that it's accessible and not confusing to people that don't have a lot of background.

by accident I have discoverd that you have updated the text and the pictures.

Yeah Adam, it might be good to add a post when you've made any significant changes.
 
Hi Adam,

As you will remember, I gave you already some comments to your contribution on "opamp stability" (see, for example, my reply#8).
Of course, you are free to implement my comments or not (for example regarding noise gain and loop gain definition).
As mentioned already, I have discovered (by accident) that you have revised some parts of your contribution on stability.
Thus, I went through the text/pictures. I am sorry, but I must admit that - for my opinion- some parts are still incomplete or misleading or even confusing.
However, I think, this is a quite normal situation because such non-idealities sometimes can be revealed only by somebody who has not written the text.
Because of the relatively large number I have summarized my comments in the attached pdf document - and I am ready, of course, for further discussion if you like.
Regards
LvW
 

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I like to add three general comments:
1.) I agree, of course, to Steve´s comments regarding the correct writing of mathematical formulas.
2.) For my opinion, a contribution dealing exclusively with stability aspects should mention at the beginning Nyquist´s stability criterion, which all the known methods to check stability are derived from.
3.) All the pictures and figures should be referenced in the text with the correct numbering - otherwise it is not always clear which graph belongs to which paragraph.
 
Hi Adam - by accident I have discoverd that you have updated the text and the pictures. I will go through it and give some comments (if any) later, OK?
Thanks Lvw
I always welcome a keen eye. Let me know if I have missed any pictures or plots that need updating.
Thanks
Adam
 
I've been pretty busy this weekend. I haven't read it fully through. My main concern is that it's accessible and not confusing to people that don't have a lot of background.



Yeah Adam, it might be good to add a post when you've made any significant changes.

Yeah sorry I thought you got an alert when I changed the post.
 
Hi Adam,

As you will remember, I gave you already some comments to your contribution on "opamp stability" (see, for example, my reply#8).
Of course, you are free to implement my comments or not (for example regarding noise gain and loop gain definition).
As mentioned already, I have discovered (by accident) that you have revised some parts of your contribution on stability.
Thus, I went through the text/pictures. I am sorry, but I must admit that - for my opinion- some parts are still incomplete or misleading or even confusing.
However, I think, this is a quite normal situation because such non-idealities sometimes can be revealed only by somebody who has not written the text.
Because of the relatively large number I have summarized my comments in the attached pdf document - and I am ready, of course, for further discussion if you like.
Regards
LvW

Thanks Lvw. Some good points made I will make some corrections over the next week.
Adam
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Adam - is that ironic?

I would assume he's taking your feedback seriously.

It might help if you suggested some rewording for parts that you think are not as clear or accurate as you would like. I'm sure that would ease Adam's task of incorporating your requests for edits.
 
I would assume he's taking your feedback seriously.
It might help if you suggested some rewording for parts that you think are not as clear or accurate as you would like. I'm sure that would ease Adam's task of incorporating your requests for edits.
Hi Steve - thank you. Regarding your suggestion, I think I have provided - in some cases - already a "rewording" in my pdf-attachement (starting with "Better:").
 
Adam - is that ironic?

No Lvw, hence the smiley face, It was a bit of humour. I am taking your comments seriously and you will see I have added some of your wording already. I haven't quite finished yet I am still working on some of your questions.
Adam
 
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