Reply by josephkk ●February 16, 20132013-02-16
On Tue, 12 Feb 2013 15:05:56 -0500, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:


>On 02/12/2013 02:57 PM, Tim Wescott wrote:
>> On Tue, 12 Feb 2013 11:04:57 -0700, Jim Thompson wrote:
>>
>>> On Tue, 12 Feb 2013 09:42:55 -0800 (PST),
>>> bloggs.fredbloggs.fred@gmail.com wrote:
>>>
>>>> On Tuesday, February 12, 2013 11:42:46 AM UTC-5, panfilero wrote:
>>>>> hello,
>>>>>
>>>>>
>>>>>
>>>>> does anyone know how to tell if an opamp is unity gain stable?  My
>>>>> guess is to check out the open loop phase margin and make sure it's
>>>>> over... I dunno... about 30 degrees... is this it?  Don't most =

opamps

>>>>> have decent open loop phase margin?  Should I be saying open loop
>>>>> phase margin or does just saying phase margin imply that you are
>>>>> talking about the open loop phase margin of the opamp?
>>>>>
>>>>>
>>>>>
>>>>> much appreciate your comments!
>>>>
>>>> "open loop phase margin" is an oxymoron, the very term phase margin =

is

>>>> part and parcel with feedback, how you could possibly have feedback =

in a

>>>> open loop???
>>>
>>> Nope.  Phase margin is defined at the point where the =

_open_loop_gain_

>>> passes thru 0dB.
>>
>> Then why aren't you calling it "open loop phase margin?".
>>
>> You're disagreeing with something that Bloggs didn't say.  He didn't =

say

>> that the phase margin isn't where the open loop gain passes through =

0dB.

>> He did say that the whole concept of phase margin (or gain margin, for
>> that matter) only means something in the context of a closed loop --
>> which statement I agree with, wholeheartedly.
>>
>
>But the phase margin isn't equal to any phase you measure on the=20
>closed-loop system--especially if it's negative.  It's inherently an=20
>open-loop quantity, which is why nobody says "open loop phase=20
>margin"--it would be redundant.
>
>Cheers
>
>Phil Hobbs


The concept is useful in all cases.  That aspect related to the
definition.  It can be measured and used both open and closed loop.

?-)
Reply by brent ●February 13, 20132013-02-13
On Feb 12, 11:42=A0am, panfilero <panfil...@gmail.com> wrote:

> hello,
>
> does anyone know how to tell if an opamp is unity gain stable? =A0My gues=

s is to check out the open loop phase margin and make sure it's over... I d=
unno... about 30 degrees... is this it? =A0Don't most opamps have decent op=
en loop phase margin? =A0Should I be saying open loop phase margin or does =
just saying phase margin imply that you are talking about the open loop pha=
se margin of the opamp?

>
> much appreciate your comments!


This is a strange post.  Your question shows that you know enough
about this to not need an answer from anyone.  Secondly, just how much
does anyone really worry about unity gain stability in op amps
anymore?  It seems that every op made for the last 20 years is unity
gain stable.
Reply by Phil Hobbs ●February 13, 20132013-02-13
On 2/13/2013 1:28 PM, Tim Wescott wrote:

> On Wed, 13 Feb 2013 09:07:23 -0500, Phil Hobbs wrote:
>
>> On 2/12/2013 11:38 PM, Tim Wescott wrote:
>>> On Tue, 12 Feb 2013 21:43:23 -0500, Phil Hobbs wrote:
>>>
>>>> On 2/12/2013 8:49 PM, Tim Wescott wrote:
>>>>> On Tue, 12 Feb 2013 15:05:56 -0500, Phil Hobbs wrote:
>>>>>
>>>>>> On 02/12/2013 02:57 PM, Tim Wescott wrote:
>>>>>>> On Tue, 12 Feb 2013 11:04:57 -0700, Jim Thompson wrote:
>>>>>>>
>>>>>>>> On Tue, 12 Feb 2013 09:42:55 -0800 (PST),
>>>>>>>> bloggs.fredbloggs.fred@gmail.com wrote:
>>>>>>>>
>>>>>>>>> On Tuesday, February 12, 2013 11:42:46 AM UTC-5, panfilero wrote:
>>>>>>>>>> hello,
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> does anyone know how to tell if an opamp is unity gain stable?
>>>>>>>>>> My guess is to check out the open loop phase margin and make
>>>>>>>>>> sure it's over... I dunno... about 30 degrees... is this it?
>>>>>>>>>> Don't most opamps have decent open loop phase margin?  Should I
>>>>>>>>>> be saying open loop phase margin or does just saying phase
>>>>>>>>>> margin imply that you are talking about the open loop phase
>>>>>>>>>> margin of the opamp?
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> much appreciate your comments!
>>>>>>>>>
>>>>>>>>> "open loop phase margin" is an oxymoron, the very term phase
>>>>>>>>> margin is part and parcel with feedback, how you could possibly
>>>>>>>>> have feedback in a open loop???
>>>>>>>>
>>>>>>>> Nope.  Phase margin is defined at the point where the
>>>>>>>> _open_loop_gain_
>>>>>>>> passes thru 0dB.
>>>>>>>
>>>>>>> Then why aren't you calling it "open loop phase margin?".
>>>>>>>
>>>>>>> You're disagreeing with something that Bloggs didn't say.  He
>>>>>>> didn't say that the phase margin isn't where the open loop gain
>>>>>>> passes through 0dB. He did say that the whole concept of phase
>>>>>>> margin (or gain margin,
>>>>>>> for that matter) only means something in the context of a closed
>>>>>>> loop -- which statement I agree with, wholeheartedly.
>>>>>>>
>>>>>>>
>>>>>> But the phase margin isn't equal to any phase you measure on the
>>>>>> closed-loop system--especially if it's negative.  It's inherently an
>>>>>> open-loop quantity, which is why nobody says "open loop phase
>>>>>> margin"--it would be redundant.
>>>>>
>>>>> If you're not closing a loop around an element, it's phase shift does
>>>>> not affect stability.  So "phase margin" doesn't mean anything
>>>>> (unless you have some other critical phase shift to worry about).
>>>>>
>>>>> And while I freely admit that this _is_ picking nits, I measure the
>>>>> open-
>>>>> loop response of things in closed loop all the time.  Googling on
>>>>> "frequency response measurement", "control system analyzer" or
>>>>> "transfer function measurement" should get some relevant hits.
>>>>>
>>>>> Or read this (it probably needs updating):
>>>>> http://www.wescottdesign.com/ articles/FreqMeas/freq_meas.html
>>>>>
>>>>>
>>>> But you have to transform from the closed-loop to the open-loop
>>>> response to do that.
>>>
>>> Clearly you are not at all familiar with the method.
>>
>> I'm unfamiliar with a whole lot of things.  That's one of the main
>> reasons that I like SED--I learn stuff.
>>
>>> No, you do not.  You need to build a spare summing junction into the
>>> system at the input of the portion of the system you wish to measure,
>>> but once you do that you just inject a sine wave into the system at the
>>> summing junction, and measure the gain and phase shift from the summing
>>> junction output to the output of whatever part of the system you want
>>> to measure (which, if it is the entire open-loop response, is the
>>> system's input to the summing junction).
>>
>> So lay this out for me a bit further.  Say you have a system with a
>> phase margin of 2 degrees.  You construct a point in the closed-loop
>> system where you actually measure 2 degrees directly, without needing to
>> calculate anything?  How does that work exactly?
>>
>> And this method works if the phase margin is -2 degrees?
>
> It doesn't work on unstable systems.  On the two-degree phase margin
> system you'd have to do a lot of hand-coaching (and hold your breath
> hoping that you don't hit a nonlinearity that takes the phase margin
> away).
>
> But if you can keep the system stable, and keep the amplitudes high
> enough to adequately measure things away from the loop closure point,
> while keeping them low enough to not knock into the rails at the loop
> closure point with its 38dB of peaking (or whatever 2 degrees works out
> to), then it'll pretty much work.
>
> Of course, in the normal course of events you'd notice the long ringing,
> and you'd adjust the loop to something more stable before you took your
> measurements.
>
> You can, however, use it for an unstable _portion_ of the system, as long
> as the _overall_ loop is stable.  Basically, as long as you have a
> reasonable facsimile of a sine wave running around the loop you have set
> up the conditions to do a single-frequency measurement of gain and phase
> shift.  Do that over and over again for multiple frequencies (or use an
> instrument that does it for you) and you can get a Bode plot of the bit
> you're interested in.
>
> Search on "control systems analyzer", or read the article on my web site
> -- it should be clear how it all works.
>


Thanks.  The point we were arguing about is whether, in order to obtain 
that 2 degree number, you measure it directly, or have to compute it, 
i.e. transform it back to the open-loop situation mathematically.

Which is it?

Cheers

Phil Hobbs

-- 
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510 USA
+1 845 480 2058

hobbs at electrooptical dot net
http://electrooptical.net
Reply by Tim Wescott ●February 13, 20132013-02-13
On Wed, 13 Feb 2013 09:07:23 -0500, Phil Hobbs wrote:


> On 2/12/2013 11:38 PM, Tim Wescott wrote:
>> On Tue, 12 Feb 2013 21:43:23 -0500, Phil Hobbs wrote:
>>
>>> On 2/12/2013 8:49 PM, Tim Wescott wrote:
>>>> On Tue, 12 Feb 2013 15:05:56 -0500, Phil Hobbs wrote:
>>>>
>>>>> On 02/12/2013 02:57 PM, Tim Wescott wrote:
>>>>>> On Tue, 12 Feb 2013 11:04:57 -0700, Jim Thompson wrote:
>>>>>>
>>>>>>> On Tue, 12 Feb 2013 09:42:55 -0800 (PST),
>>>>>>> bloggs.fredbloggs.fred@gmail.com wrote:
>>>>>>>
>>>>>>>> On Tuesday, February 12, 2013 11:42:46 AM UTC-5, panfilero wrote:
>>>>>>>>> hello,
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> does anyone know how to tell if an opamp is unity gain stable? 
>>>>>>>>> My guess is to check out the open loop phase margin and make
>>>>>>>>> sure it's over... I dunno... about 30 degrees... is this it? 
>>>>>>>>> Don't most opamps have decent open loop phase margin?  Should I
>>>>>>>>> be saying open loop phase margin or does just saying phase
>>>>>>>>> margin imply that you are talking about the open loop phase
>>>>>>>>> margin of the opamp?
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> much appreciate your comments!
>>>>>>>>
>>>>>>>> "open loop phase margin" is an oxymoron, the very term phase
>>>>>>>> margin is part and parcel with feedback, how you could possibly
>>>>>>>> have feedback in a open loop???
>>>>>>>
>>>>>>> Nope.  Phase margin is defined at the point where the
>>>>>>> _open_loop_gain_
>>>>>>> passes thru 0dB.
>>>>>>
>>>>>> Then why aren't you calling it "open loop phase margin?".
>>>>>>
>>>>>> You're disagreeing with something that Bloggs didn't say.  He
>>>>>> didn't say that the phase margin isn't where the open loop gain
>>>>>> passes through 0dB. He did say that the whole concept of phase
>>>>>> margin (or gain margin,
>>>>>> for that matter) only means something in the context of a closed
>>>>>> loop -- which statement I agree with, wholeheartedly.
>>>>>>
>>>>>>
>>>>> But the phase margin isn't equal to any phase you measure on the
>>>>> closed-loop system--especially if it's negative.  It's inherently an
>>>>> open-loop quantity, which is why nobody says "open loop phase
>>>>> margin"--it would be redundant.
>>>>
>>>> If you're not closing a loop around an element, it's phase shift does
>>>> not affect stability.  So "phase margin" doesn't mean anything
>>>> (unless you have some other critical phase shift to worry about).
>>>>
>>>> And while I freely admit that this _is_ picking nits, I measure the
>>>> open-
>>>> loop response of things in closed loop all the time.  Googling on
>>>> "frequency response measurement", "control system analyzer" or
>>>> "transfer function measurement" should get some relevant hits.
>>>>
>>>> Or read this (it probably needs updating):
>>>> http://www.wescottdesign.com/ articles/FreqMeas/freq_meas.html
>>>>
>>>>
>>> But you have to transform from the closed-loop to the open-loop
>>> response to do that.
>>
>> Clearly you are not at all familiar with the method.
> 
> I'm unfamiliar with a whole lot of things.  That's one of the main
> reasons that I like SED--I learn stuff.
> 
>> No, you do not.  You need to build a spare summing junction into the
>> system at the input of the portion of the system you wish to measure,
>> but once you do that you just inject a sine wave into the system at the
>> summing junction, and measure the gain and phase shift from the summing
>> junction output to the output of whatever part of the system you want
>> to measure (which, if it is the entire open-loop response, is the
>> system's input to the summing junction).
> 
> So lay this out for me a bit further.  Say you have a system with a
> phase margin of 2 degrees.  You construct a point in the closed-loop
> system where you actually measure 2 degrees directly, without needing to
> calculate anything?  How does that work exactly?
> 
> And this method works if the phase margin is -2 degrees?


It doesn't work on unstable systems.  On the two-degree phase margin 
system you'd have to do a lot of hand-coaching (and hold your breath 
hoping that you don't hit a nonlinearity that takes the phase margin 
away).

But if you can keep the system stable, and keep the amplitudes high 
enough to adequately measure things away from the loop closure point, 
while keeping them low enough to not knock into the rails at the loop 
closure point with its 38dB of peaking (or whatever 2 degrees works out 
to), then it'll pretty much work.

Of course, in the normal course of events you'd notice the long ringing, 
and you'd adjust the loop to something more stable before you took your 
measurements.

You can, however, use it for an unstable _portion_ of the system, as long 
as the _overall_ loop is stable.  Basically, as long as you have a 
reasonable facsimile of a sine wave running around the loop you have set 
up the conditions to do a single-frequency measurement of gain and phase 
shift.  Do that over and over again for multiple frequencies (or use an 
instrument that does it for you) and you can get a Bode plot of the bit 
you're interested in.

Search on "control systems analyzer", or read the article on my web site 
-- it should be clear how it all works.

-- 
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
Reply by Phil Hobbs ●February 13, 20132013-02-13
On 2/12/2013 11:38 PM, Tim Wescott wrote:

> On Tue, 12 Feb 2013 21:43:23 -0500, Phil Hobbs wrote:
>
>> On 2/12/2013 8:49 PM, Tim Wescott wrote:
>>> On Tue, 12 Feb 2013 15:05:56 -0500, Phil Hobbs wrote:
>>>
>>>> On 02/12/2013 02:57 PM, Tim Wescott wrote:
>>>>> On Tue, 12 Feb 2013 11:04:57 -0700, Jim Thompson wrote:
>>>>>
>>>>>> On Tue, 12 Feb 2013 09:42:55 -0800 (PST),
>>>>>> bloggs.fredbloggs.fred@gmail.com wrote:
>>>>>>
>>>>>>> On Tuesday, February 12, 2013 11:42:46 AM UTC-5, panfilero wrote:
>>>>>>>> hello,
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> does anyone know how to tell if an opamp is unity gain stable?  My
>>>>>>>> guess is to check out the open loop phase margin and make sure
>>>>>>>> it's over... I dunno... about 30 degrees... is this it?  Don't
>>>>>>>> most opamps have decent open loop phase margin?  Should I be
>>>>>>>> saying open loop phase margin or does just saying phase margin
>>>>>>>> imply that you are talking about the open loop phase margin of the
>>>>>>>> opamp?
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> much appreciate your comments!
>>>>>>>
>>>>>>> "open loop phase margin" is an oxymoron, the very term phase margin
>>>>>>> is part and parcel with feedback, how you could possibly have
>>>>>>> feedback in a open loop???
>>>>>>
>>>>>> Nope.  Phase margin is defined at the point where the
>>>>>> _open_loop_gain_
>>>>>> passes thru 0dB.
>>>>>
>>>>> Then why aren't you calling it "open loop phase margin?".
>>>>>
>>>>> You're disagreeing with something that Bloggs didn't say.  He didn't
>>>>> say that the phase margin isn't where the open loop gain passes
>>>>> through 0dB. He did say that the whole concept of phase margin (or
>>>>> gain margin,
>>>>> for that matter) only means something in the context of a closed loop
>>>>> -- which statement I agree with, wholeheartedly.
>>>>>
>>>>>
>>>> But the phase margin isn't equal to any phase you measure on the
>>>> closed-loop system--especially if it's negative.  It's inherently an
>>>> open-loop quantity, which is why nobody says "open loop phase
>>>> margin"--it would be redundant.
>>>
>>> If you're not closing a loop around an element, it's phase shift does
>>> not affect stability.  So "phase margin" doesn't mean anything (unless
>>> you have some other critical phase shift to worry about).
>>>
>>> And while I freely admit that this _is_ picking nits, I measure the
>>> open-
>>> loop response of things in closed loop all the time.  Googling on
>>> "frequency response measurement", "control system analyzer" or
>>> "transfer function measurement" should get some relevant hits.
>>>
>>> Or read this (it probably needs updating):
>>> http://www.wescottdesign.com/ articles/FreqMeas/freq_meas.html
>>>
>>>
>> But you have to transform from the closed-loop to the open-loop response
>> to do that.
>
> Clearly you are not at all familiar with the method.


I'm unfamiliar with a whole lot of things.  That's one of the main 
reasons that I like SED--I learn stuff.


> No, you do not.  You need to build a spare summing junction into the
> system at the input of the portion of the system you wish to measure, but
> once you do that you just inject a sine wave into the system at the
> summing junction, and measure the gain and phase shift from the summing
> junction output to the output of whatever part of the system you want to
> measure (which, if it is the entire open-loop response, is the system's
> input to the summing junction).


So lay this out for me a bit further.  Say you have a system with a 
phase margin of 2 degrees.  You construct a point in the closed-loop 
system where you actually measure 2 degrees directly, without needing to 
calculate anything?  How does that work exactly?

And this method works if the phase margin is -2 degrees?

Cheers

Phil Hobbs


-- 
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510 USA
+1 845 480 2058

hobbs at electrooptical dot net
http://electrooptical.net
Reply by Tim Wescott ●February 13, 20132013-02-13
On Tue, 12 Feb 2013 21:43:23 -0500, Phil Hobbs wrote:


> On 2/12/2013 8:49 PM, Tim Wescott wrote:
>> On Tue, 12 Feb 2013 15:05:56 -0500, Phil Hobbs wrote:
>>
>>> On 02/12/2013 02:57 PM, Tim Wescott wrote:
>>>> On Tue, 12 Feb 2013 11:04:57 -0700, Jim Thompson wrote:
>>>>
>>>>> On Tue, 12 Feb 2013 09:42:55 -0800 (PST),
>>>>> bloggs.fredbloggs.fred@gmail.com wrote:
>>>>>
>>>>>> On Tuesday, February 12, 2013 11:42:46 AM UTC-5, panfilero wrote:
>>>>>>> hello,
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> does anyone know how to tell if an opamp is unity gain stable?  My
>>>>>>> guess is to check out the open loop phase margin and make sure
>>>>>>> it's over... I dunno... about 30 degrees... is this it?  Don't
>>>>>>> most opamps have decent open loop phase margin?  Should I be
>>>>>>> saying open loop phase margin or does just saying phase margin
>>>>>>> imply that you are talking about the open loop phase margin of the
>>>>>>> opamp?
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> much appreciate your comments!
>>>>>>
>>>>>> "open loop phase margin" is an oxymoron, the very term phase margin
>>>>>> is part and parcel with feedback, how you could possibly have
>>>>>> feedback in a open loop???
>>>>>
>>>>> Nope.  Phase margin is defined at the point where the
>>>>> _open_loop_gain_
>>>>> passes thru 0dB.
>>>>
>>>> Then why aren't you calling it "open loop phase margin?".
>>>>
>>>> You're disagreeing with something that Bloggs didn't say.  He didn't
>>>> say that the phase margin isn't where the open loop gain passes
>>>> through 0dB. He did say that the whole concept of phase margin (or
>>>> gain margin,
>>>> for that matter) only means something in the context of a closed loop
>>>> -- which statement I agree with, wholeheartedly.
>>>>
>>>>
>>> But the phase margin isn't equal to any phase you measure on the
>>> closed-loop system--especially if it's negative.  It's inherently an
>>> open-loop quantity, which is why nobody says "open loop phase
>>> margin"--it would be redundant.
>>
>> If you're not closing a loop around an element, it's phase shift does
>> not affect stability.  So "phase margin" doesn't mean anything (unless
>> you have some other critical phase shift to worry about).
>>
>> And while I freely admit that this _is_ picking nits, I measure the
>> open-
>> loop response of things in closed loop all the time.  Googling on
>> "frequency response measurement", "control system analyzer" or
>> "transfer function measurement" should get some relevant hits.
>>
>> Or read this (it probably needs updating):
>> http://www.wescottdesign.com/ articles/FreqMeas/freq_meas.html
>>
>>
> But you have to transform from the closed-loop to the open-loop response
> to do that.


Clearly you are not at all familiar with the method.

No, you do not.  You need to build a spare summing junction into the 
system at the input of the portion of the system you wish to measure, but 
once you do that you just inject a sine wave into the system at the 
summing junction, and measure the gain and phase shift from the summing 
junction output to the output of whatever part of the system you want to 
measure (which, if it is the entire open-loop response, is the system's 
input to the summing junction).

-- 
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
Reply by Phil Hobbs ●February 12, 20132013-02-12
On 2/12/2013 8:49 PM, Tim Wescott wrote:

> On Tue, 12 Feb 2013 15:05:56 -0500, Phil Hobbs wrote:
>
>> On 02/12/2013 02:57 PM, Tim Wescott wrote:
>>> On Tue, 12 Feb 2013 11:04:57 -0700, Jim Thompson wrote:
>>>
>>>> On Tue, 12 Feb 2013 09:42:55 -0800 (PST),
>>>> bloggs.fredbloggs.fred@gmail.com wrote:
>>>>
>>>>> On Tuesday, February 12, 2013 11:42:46 AM UTC-5, panfilero wrote:
>>>>>> hello,
>>>>>>
>>>>>>
>>>>>>
>>>>>> does anyone know how to tell if an opamp is unity gain stable?  My
>>>>>> guess is to check out the open loop phase margin and make sure it's
>>>>>> over... I dunno... about 30 degrees... is this it?  Don't most
>>>>>> opamps have decent open loop phase margin?  Should I be saying open
>>>>>> loop phase margin or does just saying phase margin imply that you
>>>>>> are talking about the open loop phase margin of the opamp?
>>>>>>
>>>>>>
>>>>>>
>>>>>> much appreciate your comments!
>>>>>
>>>>> "open loop phase margin" is an oxymoron, the very term phase margin
>>>>> is part and parcel with feedback, how you could possibly have
>>>>> feedback in a open loop???
>>>>
>>>> Nope.  Phase margin is defined at the point where the _open_loop_gain_
>>>> passes thru 0dB.
>>>
>>> Then why aren't you calling it "open loop phase margin?".
>>>
>>> You're disagreeing with something that Bloggs didn't say.  He didn't
>>> say that the phase margin isn't where the open loop gain passes through
>>> 0dB. He did say that the whole concept of phase margin (or gain margin,
>>> for that matter) only means something in the context of a closed loop
>>> -- which statement I agree with, wholeheartedly.
>>>
>>>
>> But the phase margin isn't equal to any phase you measure on the
>> closed-loop system--especially if it's negative.  It's inherently an
>> open-loop quantity, which is why nobody says "open loop phase
>> margin"--it would be redundant.
>
> If you're not closing a loop around an element, it's phase shift does not
> affect stability.  So "phase margin" doesn't mean anything (unless you
> have some other critical phase shift to worry about).
>
> And while I freely admit that this _is_ picking nits, I measure the open-
> loop response of things in closed loop all the time.  Googling on
> "frequency response measurement", "control system analyzer" or "transfer
> function measurement" should get some relevant hits.
>
> Or read this (it probably needs updating): http://www.wescottdesign.com/
> articles/FreqMeas/freq_meas.html
>


But you have to transform from the closed-loop to the open-loop response 
to do that.

Cheers

Phil Hobbs

-- 
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510 USA
+1 845 480 2058

hobbs at electrooptical dot net
http://electrooptical.net
Reply by Tim Wescott ●February 12, 20132013-02-12
On Tue, 12 Feb 2013 15:05:56 -0500, Phil Hobbs wrote:


> On 02/12/2013 02:57 PM, Tim Wescott wrote:
>> On Tue, 12 Feb 2013 11:04:57 -0700, Jim Thompson wrote:
>>
>>> On Tue, 12 Feb 2013 09:42:55 -0800 (PST),
>>> bloggs.fredbloggs.fred@gmail.com wrote:
>>>
>>>> On Tuesday, February 12, 2013 11:42:46 AM UTC-5, panfilero wrote:
>>>>> hello,
>>>>>
>>>>>
>>>>>
>>>>> does anyone know how to tell if an opamp is unity gain stable?  My
>>>>> guess is to check out the open loop phase margin and make sure it's
>>>>> over... I dunno... about 30 degrees... is this it?  Don't most
>>>>> opamps have decent open loop phase margin?  Should I be saying open
>>>>> loop phase margin or does just saying phase margin imply that you
>>>>> are talking about the open loop phase margin of the opamp?
>>>>>
>>>>>
>>>>>
>>>>> much appreciate your comments!
>>>>
>>>> "open loop phase margin" is an oxymoron, the very term phase margin
>>>> is part and parcel with feedback, how you could possibly have
>>>> feedback in a open loop???
>>>
>>> Nope.  Phase margin is defined at the point where the _open_loop_gain_
>>> passes thru 0dB.
>>
>> Then why aren't you calling it "open loop phase margin?".
>>
>> You're disagreeing with something that Bloggs didn't say.  He didn't
>> say that the phase margin isn't where the open loop gain passes through
>> 0dB. He did say that the whole concept of phase margin (or gain margin,
>> for that matter) only means something in the context of a closed loop
>> -- which statement I agree with, wholeheartedly.
>>
>>
> But the phase margin isn't equal to any phase you measure on the
> closed-loop system--especially if it's negative.  It's inherently an
> open-loop quantity, which is why nobody says "open loop phase
> margin"--it would be redundant.


If you're not closing a loop around an element, it's phase shift does not 
affect stability.  So "phase margin" doesn't mean anything (unless you 
have some other critical phase shift to worry about).

And while I freely admit that this _is_ picking nits, I measure the open-
loop response of things in closed loop all the time.  Googling on 
"frequency response measurement", "control system analyzer" or "transfer 
function measurement" should get some relevant hits.

Or read this (it probably needs updating): http://www.wescottdesign.com/
articles/FreqMeas/freq_meas.html

-- 
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?

Tim Wescott, Communications, Control, Circuits & Software
http://www.wescottdesign.com
Reply by Jim Thompson ●February 12, 20132013-02-12
On Tue, 12 Feb 2013 15:53:09 -0500, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:


>On 02/12/2013 03:19 PM, Jim Thompson wrote:
>> On Tue, 12 Feb 2013 15:05:56 -0500, Phil Hobbs
>> <pcdhSpamMeSenseless@electrooptical.net>  wrote:
>>
>>> On 02/12/2013 02:57 PM, Tim Wescott wrote:
>>>> On Tue, 12 Feb 2013 11:04:57 -0700, Jim Thompson wrote:
>>>>
>>>>> On Tue, 12 Feb 2013 09:42:55 -0800 (PST),
>>>>> bloggs.fredbloggs.fred@gmail.com wrote:
>>>>>
>>>>>> On Tuesday, February 12, 2013 11:42:46 AM UTC-5, panfilero wrote:
>>>>>>> hello,
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> does anyone know how to tell if an opamp is unity gain stable?  My
>>>>>>> guess is to check out the open loop phase margin and make sure it's
>>>>>>> over... I dunno... about 30 degrees... is this it?  Don't most opamps
>>>>>>> have decent open loop phase margin?  Should I be saying open loop
>>>>>>> phase margin or does just saying phase margin imply that you are
>>>>>>> talking about the open loop phase margin of the opamp?
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> much appreciate your comments!
>>>>>>
>>>>>> "open loop phase margin" is an oxymoron, the very term phase margin is
>>>>>> part and parcel with feedback, how you could possibly have feedback in a
>>>>>> open loop???
>>>>>
>>>>> Nope.  Phase margin is defined at the point where the _open_loop_gain_
>>>>> passes thru 0dB.
>>>>
>>>> Then why aren't you calling it "open loop phase margin?".
>>>>
>>>> You're disagreeing with something that Bloggs didn't say.  He didn't say
>>>> that the phase margin isn't where the open loop gain passes through 0dB.
>>>> He did say that the whole concept of phase margin (or gain margin, for
>>>> that matter) only means something in the context of a closed loop --
>>>> which statement I agree with, wholeheartedly.
>>>>
>>>
>>> But the phase margin isn't equal to any phase you measure on the
>>> closed-loop system--especially if it's negative.
>>
>> Sure it is... at unity feedback and no capacitive loading.
>>
>>> It's inherently an
>>> open-loop quantity, which is why nobody says "open loop phase
>>> margin"--it would be redundant.
>>>
>>> Cheers
>>>
>>> Phil Hobbs
>> 		
>>                                          ...Jim Thompson
>
>In that one case it's close, just off by a factor of A/(A+1) ish, _as 
>long as it's positive_.
>
>However, as I was pointing out, a negative phase margin can't be 
>measured anywhere on a closed-loop system!
>
>Cheers
>
>Phil Hobbs


Probably because it's oscillating ;-)

Middlebrook's method works quite well.
		
                                        ...Jim Thompson
-- 
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
I love to cook with wine.     Sometimes I even put it in the food.
Reply by Phil Hobbs ●February 12, 20132013-02-12
On 02/12/2013 03:19 PM, Jim Thompson wrote:

> On Tue, 12 Feb 2013 15:05:56 -0500, Phil Hobbs
> <pcdhSpamMeSenseless@electrooptical.net>  wrote:
>
>> On 02/12/2013 02:57 PM, Tim Wescott wrote:
>>> On Tue, 12 Feb 2013 11:04:57 -0700, Jim Thompson wrote:
>>>
>>>> On Tue, 12 Feb 2013 09:42:55 -0800 (PST),
>>>> bloggs.fredbloggs.fred@gmail.com wrote:
>>>>
>>>>> On Tuesday, February 12, 2013 11:42:46 AM UTC-5, panfilero wrote:
>>>>>> hello,
>>>>>>
>>>>>>
>>>>>>
>>>>>> does anyone know how to tell if an opamp is unity gain stable?  My
>>>>>> guess is to check out the open loop phase margin and make sure it's
>>>>>> over... I dunno... about 30 degrees... is this it?  Don't most opamps
>>>>>> have decent open loop phase margin?  Should I be saying open loop
>>>>>> phase margin or does just saying phase margin imply that you are
>>>>>> talking about the open loop phase margin of the opamp?
>>>>>>
>>>>>>
>>>>>>
>>>>>> much appreciate your comments!
>>>>>
>>>>> "open loop phase margin" is an oxymoron, the very term phase margin is
>>>>> part and parcel with feedback, how you could possibly have feedback in a
>>>>> open loop???
>>>>
>>>> Nope.  Phase margin is defined at the point where the _open_loop_gain_
>>>> passes thru 0dB.
>>>
>>> Then why aren't you calling it "open loop phase margin?".
>>>
>>> You're disagreeing with something that Bloggs didn't say.  He didn't say
>>> that the phase margin isn't where the open loop gain passes through 0dB.
>>> He did say that the whole concept of phase margin (or gain margin, for
>>> that matter) only means something in the context of a closed loop --
>>> which statement I agree with, wholeheartedly.
>>>
>>
>> But the phase margin isn't equal to any phase you measure on the
>> closed-loop system--especially if it's negative.
>
> Sure it is... at unity feedback and no capacitive loading.
>
>> It's inherently an
>> open-loop quantity, which is why nobody says "open loop phase
>> margin"--it would be redundant.
>>
>> Cheers
>>
>> Phil Hobbs
> 		
>                                          ...Jim Thompson


In that one case it's close, just off by a factor of A/(A+1) ish, _as 
long as it's positive_.

However, as I was pointing out, a negative phase margin can't be 
measured anywhere on a closed-loop system!

Cheers

Phil Hobbs

-- 
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net