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how to tell if opamp is unity gain stable

Started by panfilero February 12, 2013
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 loo=
p phase margin?  Should I be saying open loop phase margin or does just say=
ing phase margin imply that you are talking about the open loop phase margi=
n of the opamp?

much appreciate your comments!
On Tue, 12 Feb 2013 08:42:46 -0800 (PST), panfilero
<panfilero@gmail.com> 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!
You need to check the phase margin with the OpAmp loaded with any expected capacitance. I like 45&#2013266096; 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.
On Tuesday, February 12, 2013 11:42:46 AM UTC-5, panfilero wrote:
> hello, >=20 >=20 >=20 > 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 dun= no... about 30 degrees... is this it? Don't most opamps have decent open l= oop phase margin? Should I be saying open loop phase margin or does just s= aying phase margin imply that you are talking about the open loop phase mar= gin of the opamp?
>=20 >=20 >=20 > 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 l= oop???
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. 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. ...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.
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. -- 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
On Tue, 12 Feb 2013 08:42:46 -0800, 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!
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. -- 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
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. 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
On Tue, 12 Feb 2013 13:57:46 -0600, Tim Wescott <tim@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.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?".
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 -- | 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.
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 -- | 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.
On Tue, 12 Feb 2013 08:42:46 -0800 (PST), panfilero
<panfilero@gmail.com> 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!
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