how to tell if opamp is unity gain stable

[panfilero]

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!

[Jim Thompson]

You need to check the phase margin with the OpAmp loaded with any
expected capacitance. I like 45° or more, but it's all personal
choice... how much overshoot can you tolerate?

(Some OpAmps are NOT unity-gain stable... check the data sheet.)

...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.

[bloggs.fred...@gmail.com]

"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???

[Jim Thompson]

Nope. Phase margin is defined at the point where the _open_loop_gain_
passes thru 0dB.

However, it is usually simulated with the loop closed to ensure proper
operating point conditions.

See...

http://www.analog-innovations.com/LoopGain.zip

for analysis and a simulation tool based on Dr. R.D. Middlebrook's
laboratory method.

[Tim Wescott]

On Tue, 12 Feb 2013 11:04:57 -0700, Jim Thompson wrote:

> On Tue, 12 Feb 2013 09:42:55 -0800 (PST),
> bloggs.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.

--
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

[Tim Wescott]

Strictly speaking, the amplifier circuit will be unity gain stable if the
open-loop gain has any phase margin at all. With 1 degree of phase
margin you will still have an amplifier that is stable in the strict
sense, but that is (a) going to show a lot of peaking and ringing, and
(b) is going to be a hair's breadth away from bursting into song.

Look at what Jim said about the operation of the op-amp in question in
the circuit for which it is designed: nearly all op-amps suffer from
additional phase shift if they're driving a capacitive load, or if they
have significant capacitance on the negative feedback. Thus you can have
an op-amp that is stable at some gain in a circuit with purely resistive
loads and sources, but which is unstable in _your_ circuit.

Jim quoted 45 degrees of margin as being OK. That implies 3dB of peaking
in the frequency domain (I can't remember what percentage of overshoot in
the time domain). You may need more phase margin if you care about the
amplifier being flat all the way out. An amplifier that has 30 degrees
of phase margin is going to be peaky, exhibit overshoot, and may have
problems with manufacturing variations, temperature variations, and large-
signal behavior (because phase shift often gets worse when the output is
really banging around).

Note that gain margin matters, too.

[Phil Hobbs]

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.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.

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

[Jim Thompson]

On Tue, 12 Feb 2013 13:57:46 -0600, Tim Wescott <t...@seemywebsite.com>

wrote:

>On Tue, 12 Feb 2013 11:04:57 -0700, Jim Thompson wrote:
>
>> On Tue, 12 Feb 2013 09:42:55 -0800 (PST),
>> bloggs.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?".

Because it's never called anything BUT "phase margin"... neither
"open" or "closed" are used as modifiers.

>
>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.

Phase margin is phase margin is phase margin is phase margin.....

You're making a context that has no meaning >:-}

[Jim Thompson]

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

[John Larkin]

On Tue, 12 Feb 2013 08:42:46 -0800 (PST), panfilero
<panf...@gmail.com> wrote:

Unity-gain unstable is rare enough that data sheets always point it
out. But sure, check the graphs for phase shift at the zero-gain
frequency, and consider loading too.




--

John Larkin Highland Technology, Inc

jlarkin at highlandtechnology dot com
http://www.highlandtechnology.com

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom laser drivers and controllers
Photonics and fiberoptic TTL data links
VME thermocouple, LVDT, synchro acquisition and simulation

[Phil Hobbs]

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!

[Jim Thompson]

On Tue, 12 Feb 2013 15:53:09 -0500, Phil Hobbs

Probably because it's oscillating ;-)

Middlebrook's method works quite well.

[Tim Wescott]

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

[Phil Hobbs]

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

[radam...@gmail.com]

Traditional op-amps with one dominant pole followed by parasitic poles at much higher frequencies can be adequately analyzed by gain/phase margin. Lately there has been increasing use of conditionally-stable op-amps (mostly on-chip) that achieve incredible gain-bandwidth products. These op-amps always have at least one frequency where there is more than 0 db gain around the loop with 0 degrees of phase shift at that frequency, and yet they do not oscillate when the loop is closed. Such op-amps have many low-frequency poles. The time-domain response is often pretty ugly, but usually you don't care.

Bob

[Tim Wescott]

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

[Phil Allison]

"panfilero"

does anyone know how to tell if an opamp is unity gain stable?

** FFS

1. Read the data sheet for that claim.

2. See if suggested apps show the device wired as a voltage follower.

3. Note that the "gain" being referred to is the "noise gain" of the device
in the topology - which cannot be less than unity.

4. Note also that George Orwell might have said:

" Some unity gain stable op-amps are more stable than others. "



.... Phil

[Phil Hobbs]

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?

[Tim Wescott]

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.

[Phil Hobbs]

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?

[brent]

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.

[josephkk]

On Tue, 12 Feb 2013 15:05:56 -0500, 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.

?-)