How to measure phase margin of an amplifier?
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Bob Cordell
diyAudio Member

Join Date: Sep 2006
Quote:
 Originally Posted by traderbam When I talk about delay I mean time delay. I use the term phase shift and sometimes phase lead or phase lag to describe what I assume you mean when you use delay. That is, the apparent phase delay of a simple sinusoid. I notice a number of novice's get easily confused between the concepts of inertial phase shift and time delay and I like to point this out now and again for clarification.
I meant time delay, as in so many nanoseconds. Phase lag that increases linearly with frequency is time delay. Well below a pole, the phase shift vs. frequency of that pole is somewhat linear, so it looks like time delay.

Cheers,
Bob

syn08
Account disabled at member's request

Join Date: Aug 2005
Location: Toronto
Quote:
 Originally Posted by traderbam I don't think you need tons of explanation so here are a few grams. CL performance is the goal. Seems self-evident to me.
I don't disagree with what you are saying (ok, at least high level), so let me reformulate:

Do you think there's any parameter in a closed loop system that cannot be evaluated by the loop gain analysis?

To me it's a "nay". That's all I'm saying, a loop gain analysis will always provide a correct and complete prediction of the closed loop behaviour.

diyAudio Member

Join Date: Jan 2002
Location: Earth
Quote:
 Originally Posted by syn08 I don't disagree with what you are saying (ok, at least high level), so let me reformulate: Do you think there's any parameter in a closed loop system that cannot be evaluated by the loop gain analysis? To me it's a "nay". That's all I'm saying, a loop gain analysis will always provide a correct and complete prediction of the closed loop behaviour.
In theory, if you know everything about the OL circuit and you know exactly how you will change it when you make it CL then you should be able to fully predict the CL behaviour.
In practice, it is not possible and so I am recommending relying only on the CL performance.
__________________
“Simplicity is the ultimate sophistication.” Leonardo da Vinci

diyAudio Member

Join Date: Jan 2002
Location: Earth
Quote:
 Originally Posted by Bob Cordell I meant time delay, as in so many nanoseconds. Phase lag that increases linearly with frequency is time delay. Well below a pole, the phase shift vs. frequency of that pole is somewhat linear, so it looks like time delay.
You seem to be muddling up common English and engineering terminology. And this is my point. The sentence "Phase lag that increases linearly with frequency is time delay." is an engineering no-no. So I recommend not using the word delay in the NFB context when describing phase shift. Because folk really do get confused.
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“Simplicity is the ultimate sophistication.” Leonardo da Vinci

wahab
diyAudio Member

Join Date: Nov 2009
Location: algeria/france
Quote:
 Originally Posted by traderbam In theory, if you know everything about the OL circuit and you know exactly how you will change it when you make it CL then you should be able to fully predict the CL behaviour. In practice, it is not possible and so I am recommending relying only on the CL performance.
using the open loop transfer function caracteristics,a nyquist plot
allow to define the closed loop stability behaviours of a negative feeddback
circuit..
thus, applying negative feedback doesn t change the intrinsical transfer
function of a system....

Bob Cordell
diyAudio Member

Join Date: Sep 2006
Quote:
 Originally Posted by traderbam You seem to be muddling up common English and engineering terminology. And this is my point. The sentence "Phase lag that increases linearly with frequency is time delay." is an engineering no-no. So I recommend not using the word delay in the NFB context when describing phase shift. Because folk really do get confused.
OK, I am having trouble understanding your objection. I think you understand what I am trying to say. Let me know how you would state it so I can better understand.

I do like to use terms that are commonly understood, and maybe I am taking too much liberty here. On the other hand, your objection may be based on a philosophical one of yours, in that you philosophically disagree with using the concept of delay in looking at how feedback works.

Let me state one more time, that I am referring to simple delay, as if it were physical delay. If I inserted 50 feet of perfect wire into the loop, it might introduce on the order of 50 ns of delay (what some call flat delay). This is the kind of delay I am talking about. That 50 ns of delay will add 18 degrees of phase lag to the loop at a frequency of 1 MHz. In other words, it will detract from phase margin by 18 degrees if the gain crossover frequency is at 1 MHz. Do you agree with this assertion?

Moreover, I am saying that three or four added poles at some frequency(s) well above 1 MHz will largely approximate the delay described above.

Cheers,
Bob

 20th January 2010, 03:24 PM #27 traderbam   diyAudio Member     Join Date: Jan 2002 Location: Earth Bob, The handy thing about using maths and science is that one don't have to faff about with philosophical interpretations. One can, instead, say this is X and is not Y and prove it. Provided the rules and terminologies are clear. A pole is not a time delay (Laplace: e^(-ts)). The roll-off and associated phase shift due to a pole is not caused by a time delay. You may quantify the phase shift of a sine wave that a pole causes and convert this to time displacement. But this time displacement is nothing to do with a time delay in the system. Is this clear? Brian "A time delay will cause a linearly increasing phase lag with frequency" is how I would restate what I think you are expressing. __________________ “Simplicity is the ultimate sophistication.” Leonardo da Vinci Last edited by traderbam; 20th January 2010 at 03:27 PM.
Bob Cordell
diyAudio Member

Join Date: Sep 2006
Quote:
 Originally Posted by traderbam Bob, The handy thing about using maths and science is that one don't have to faff about with philosophical interpretations. One can, instead, say this is X and is not Y and prove it. Provided the rules and terminologies are clear. A pole is not a time delay (Laplace: e^(-ts)). The roll-off and associated phase shift due to a pole is not caused by a time delay. You may quantify the phase shift of a sine wave that a pole causes and convert this to time displacement. But this time displacement is nothing to do with a time delay in the system. Is this clear? Brian "A time delay will cause a linearly increasing phase lag with frequency" is how I would restate what I think you are expressing.

I have no problem with your re-statement that "a time delay will cause a linearly increasing phase lag with frequency".

However, I never suggested that a single pole was the same as a pure time delay. This is where you seem to be missing the boat.

What I said, or meant to say, is that a group of multiple poles can cause a nearly linearly increasing phase lag with frequency (over a limited range of frequencies depending on the tolerance you allow for the approximation) that emulates a time delay with reasonable accuracy. To the feedback loop, this gives the appearance of a constant time delay, well in the vicinity of the gain crossover frequency. If you simulate three poles at 5 MHz, 10 MHz, and 20 MHz, and look at the phase vs. frequency +/- one octave about 1 MHz you will see what I mean.

Once again, I NEVER suggested that a single pole was the same as a time delay.

Cheers,
Bob

diyAudio Member

Join Date: Jan 2002
Location: Earth
Quote:
 Originally Posted by wahab since there s some expiremented people by there, i ve got a question.. what is, by your experiences, the accuracy of simulators in checking these parameters, phase margin and gain margin? is real world largely different from the virtual experiments?... thanks for the insights,
80% accurate.

This is a great question and probably best answered by Andyc. There is a great thread about simulators somewhere around here.
A real circuit contains many, many more "components" than you tend to see in a typical LT Spice model and the "typical" device model parameters will not be identical to the parts you actually use, so it's a garbage-in, garbage-out thing.
So my opinion is that simulation is hugely useful but it is no substitute for measuring the real circuit.
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syn08
Account disabled at member's request

Join Date: Aug 2005
Location: Toronto
Quote:
 Originally Posted by traderbam In practice, it is not possible and so I am recommending relying only on the CL performance.
This is where we disagree. Care to explain why would theory be here different to practice? Again, I'm talking about measurements, not simulations.

Last edited by syn08; 20th January 2010 at 04:17 PM.

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