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Old 19th January 2010, 12:41 AM   #11
syn08 is offline syn08  Canada
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Originally Posted by tomchr View Post
Most amps are higher order than this. An amp can have PM > 45 deg but a not-so-well behaved transient response. This is usually resulting from high frequency poles or - even worse - from right half-plane zeros. In many cases, I've found that amps may have excellent PM but marginal GM. These amps tend to show more HF ringing on transients than those with excellent PM and excellent GM.
Measuring the phase/gain margin cannot substitute for a competent design, but only to check against component models and parameter variations.

An audio (and not only) amp that is higher order (usually relying on the asymptotic frequency characteristic given by the HF poles) is poorly designed. Also an audio amp in which the stability margins largely depend on the signal level (the most common reason for large signal oscillation bursts) is an incompetent design. Both of these circumstances can though be determined by closely examining the measured Bode diagrams.
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Old 19th January 2010, 12:56 AM   #12
tomchr is offline tomchr  United States
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Originally Posted by syn08 View Post
Measuring the phase/gain margin cannot substitute for a competent design, but only to check against component models and parameter variations.
Agree.

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Originally Posted by syn08 View Post
An audio (and not only) amp that is higher order (usually relying on the asymptotic frequency characteristic given by the HF poles) is poorly designed.
It's very possible to design an audio amp where the bandwidth and, hence, stability is determined dominantly by discrete RC's. If the bandwidth and/or stability of an audio amp is set by any other mechanism, I would agree that the design is poor. However, when designing op-amps -- especially designing for maximum bandwidth within a given power budget, it is very common that the second pole in the response is set by circuit parasitics. That said, these parasitics, while technically uncontrolled, are quite repeatable from device to device and lot to lot. It all depends on how close to the bleeding edge you want to be...

~Tom
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Old 19th January 2010, 01:05 AM   #13
syn08 is offline syn08  Canada
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However, when designing op-amps -- especially designing for maximum bandwidth within a given power budget, it is very common that the second pole in the response is set by circuit parasitics.
That is a completely different kettle of fish. Another example is using opamps with >1GHz bandwidth. Ever snipped a SOT unused pin to get rid of that pesky 0.5pF parasitic (aka "frequency decompensating" )?

Then again, this is supposed to be DIYAudio (although it lately looks like a cuckoo's nest).
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Old 19th January 2010, 07:43 PM   #14
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Originally Posted by tomchr View Post
If we're isolating the debate to stability only, then implementing a circuit with a reasonable phase margin and gain margin should be the goal. With PM > 0 and GM < 0, the amp is stable. Most people would prefer some margin, though...

However, if the debate is expanded to include amplifier closed-loop transient response, then PM does not tell the entire story. Most textbooks include calculations of the Q and overshoot as function of PM. Those equations are only valid for 2nd order systems. Most amps are higher order than this. An amp can have PM > 45 deg but a not-so-well behaved transient response. This is usually resulting from high frequency poles or - even worse - from right half-plane zeros. In many cases, I've found that amps may have excellent PM but marginal GM. These amps tend to show more HF ringing on transients than those with excellent PM and excellent GM.

In the op-amps and voltage regulator circuits I've designed, I've usually aimed for a PM of at least 60 degrees (worst case). I use AC analysis as it tends to be one of the fastest simulation types. But I also look at the transient response. This exercise is then repeated in the lab once I get the circuit built. Having access to a network analyzer does have its advantages...

~Tom
Hi Tom,

These are all very good points. Indeed, I have seen many amplifiers with adequate phase margin and poor gain margin. I was always taught to look for 6 dB gain margin.

In most conventional Miller-compensarted amps, gain margin can be reasonably verified to be at least 6 dB by changing the closed-loop gain to be 1/2 of what it normally is.

I agree that the second-order system view is not very accurate, since there are usually numerous parasitic poles that detract from the starting phase margin of 90 degrees. For a number of multiple poles far out, the concept of excess delay is better. In that case, excess delay translates to a given amount of phase margin at the gain crossover frequency. Under those conditions a plot of peaking vs. gain margin can be quite useful.

Think of maybe three poles out around 10 MHz for an amplifier with a 1 MHz gain crossover. Those poles begin to look more like a constant delay at 1 MHz. Moreover, they add phase delay while adding very little amplitude attenuation.

Cheers,
Bob


Cheers,
Bob
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Old 19th January 2010, 08:51 PM   #15
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Originally Posted by syn08
Wow Brian, this requires tons of explanations...
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.
CL performance is affected by many more things than phase margin. Such as linearity, saturation, subtraction error, delays and more. When you apply feedback and make a system's input a function of its own output you create a new system and IMO its characteristics should be considered afresh.
Input filter components can load the i/p node and affect its impedance which is important both for stability of the LTP and for noise susceptibility.

By "kicking" the system I mean to excite it with a fast signal like a pulse or a square wave. Like you would test a car suspension by driving it over a curb (if it wasn't your car of course!). Technically, this is injecting high frequency energy and seeing how the system reacts. As you know, it is usually desirable that the system does not get excited and is well damped. It is easy to feed the output of a sig gen through series RC to the -'ve input node of the LTP as a convenient injection point.

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My only extra comment is that measuring the phase using a scope is impossible to any useful degree of accuracy.
Yes the accuracy is poor (10 degrees or so) but one can estimate the phase margin quite well by observing the square wave or step response on the scope, assuming it is predominantly a second order system.
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Old 19th January 2010, 08:55 PM   #16
wahab is offline wahab  Algeria
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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,
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Old 19th January 2010, 09:00 PM   #17
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Originally Posted by Bob Cordell
For a number of multiple poles far out, the concept of excess delay is better. In that case, excess delay translates to a given amount of phase margin at the gain crossover frequency.
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.
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Old 19th January 2010, 09:26 PM   #18
tomchr is offline tomchr  United States
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Originally Posted by Bob Cordell View Post
I was always taught to look for 6 dB gain margin.
Bob - thanks for your insight. When designing op-amps I was taught to design for GM better than 10 dB. But then we also usually designed for 60 degrees PM... In my experience, the 6 dB figure you mention is adequate if you can accept a little buzz on the edges of the transient response.

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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.
Potato - potahto... Time delay and phase delay are two sides of the same thing. One is in the time domain, the other the frequency domain. It's been scary long since I've done any Laplace or Fourier transforms but you should be able to figure the equivalent phase delay from a given time delay. However, a constant time delay will imply a frequency dependent phase delay. This may be where the confusion between phase and time delay arises.

~Tom
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Old 19th January 2010, 11:15 PM   #19
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Originally Posted by tomchr
Potato - potahto... Time delay and phase delay are two sides of the same thing. One is in the time domain, the other the frequency domain. It's been scary long since I've done any Laplace or Fourier transforms but you should be able to figure the equivalent phase delay from a given time delay. However, a constant time delay will imply a frequency dependent phase delay. This may be where the confusion between phase and time delay arises.

~Tom
I am sorry sir but they are certainly not two sides of the same thing. They are entirely different effects and have different implications. This will become clear to you when you do refresh yourself on Laplace transforms.
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Old 19th January 2010, 11:32 PM   #20
tomchr is offline tomchr  United States
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Originally Posted by traderbam View Post
I am sorry sir but they are certainly not two sides of the same thing. They are entirely different effects and have different implications. This will become clear to you when you do refresh yourself on Laplace transforms.
Could you elaborate or are you really suggesting that I - I?! - do the hard work of reading up on Laplace transforms?! I really don't like hard work...

~Tom
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