On Mon, 28 May 2018 15:26:58 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>On 05/28/18 14:56, Jim Thompson wrote: >> On Sun, 27 May 2018 15:09:33 -0400, Phil Hobbs >> <pcdhSpamMeSenseless@electrooptical.net> wrote: >> >>> Hi, all, >>> >>> I'm generally prejudiced against composite amplifiers (two op amps >>> inside one feedback loop) because they're generally squirrelly, with >>> poor settling performance and weird transient response. >>> >>> On the other hand, my aversion to them means that I don't have as much >>> experience with them as do composite-amp fans. So what do you folks say >>> about them? >>> >>> Orchids? Onions? Actual expertise? >>> >>> Cheers >>> >>> Phil Hobbs >> >> I don't have time (*) to join the discussion, but read this... >> >> <http://www.analog-innovations.com/SED/CompositeAmplifiers.pdf> > >I have that one, thanks. Lots of the circuits are summing-junction >snoopers, i.e. circuits wih a slow-but-accurate amp looking at the >time-averaged input error of a fast-but-cruder amp, and nulling it out. >That's a useful trick sometimes, and are examples of "putting a bandaid >on the fast circuit", which I was talking about upthread. > >There are other sorts of bandaids, e.g. the White cathode follower and >many sorts of local feedback. Often the key is to figure out a way that >the bandaid can be much slower than the main amplifier, as in the >snooper circuits. > >I often use op amps to force JFETs to run at exactly I_DSS, for >instance--the problem then is to keep the low-frequency noise from going >nuts. > >Cheers > >Phil HobbsI have a paper somewhere that discusses GBW-product and optimization of the two-OpAmp composite. Quite nicely done, as I recall... unfortunately I haven't re-found it :-( ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | STV, Queen Creek, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | Thinking outside the box... producing elegant solutions, by understanding what nature is hiding. "It is not in doing what you like, but in liking what you do that is the secret of happiness." -James Barrie
Composite amps
Started by ●May 27, 2018
Reply by ●May 29, 20182018-05-29
Reply by ●May 29, 20182018-05-29
On 05/29/2018 11:11 AM, George Herold wrote:> On Tuesday, May 29, 2018 at 10:26:04 AM UTC-4, bitrex wrote: >> On 05/29/2018 08:53 AM, George Herold wrote: >>> On Monday, May 28, 2018 at 7:07:51 PM UTC-4, bitrex wrote: >>>> On 05/28/2018 02:07 PM, George Herold wrote: >>>>> On Sunday, May 27, 2018 at 8:10:21 PM UTC-4, Phil Hobbs wrote: >>>>>> On 05/27/18 19:33, bitrex wrote: >>>>>>> On 05/27/2018 05:45 PM, Phil Hobbs wrote: >>>>>>>> On 05/27/18 16:28, John Larkin wrote: >>>>>>>>> On Sun, 27 May 2018 15:09:33 -0400, Phil Hobbs >>>>>>>>> <pcdhSpamMeSenseless@electrooptical.net> wrote: >>>>>>>>> >>>>>>>>>> Hi, all, >>>>>>>>>> >>>>>>>>>> I'm generally prejudiced against composite amplifiers (two op amps >>>>>>>>>> inside one feedback loop) because they're generally squirrelly, with >>>>>>>>>> poor settling performance and weird transient response. >>>>>>>>>> >>>>>>>>>> On the other hand, my aversion to them means that I don't have as much >>>>>>>>>> experience with them as do composite-amp fans. So what do you folks >>>>>>>>>> say >>>>>>>>>> about them? >>>>>>>>>> >>>>>>>>>> Orchids? Onions? Actual expertise? >>>>>>>>>> >>>>>>>>>> Cheers >>>>>>>>>> >>>>>>>>>> Phil Hobbs >>>>>>>>> >>>>>>>>> I've only done it a little, in very special cases, but if the intent >>>>>>>>> is to apply a slow DC offset correction, and the main amp and the DC >>>>>>>>> trim amp don't overlap in frequency response, it seems to work fine. >>>>>>>>> >>>>>>>>> If you want to make a general -6 dB/octave amp as a composite, the >>>>>>>>> risk is probably saturating one of the amps in large-signal/slewing >>>>>>>>> cases, or at leasy doing goofy things. A composite that clips clean >>>>>>>>> would be a challenge. >>>>>>>>> >>>>>>>>> A sorta similar case is where a fast signal needs to be DC coupled >>>>>>>>> across a big DC offset. A capacitor is the fast path and some slow >>>>>>>>> opamp thing does the DC part before the AC path decays. The gains have >>>>>>>>> to both be the same, about 1.00 usually, and the frequency responses >>>>>>>>> need to be matched, to get clean step response and no ISI. >>>>>>>> >>>>>>>> Not so easy! >>>>>>>> >>>>>>>>> >>>>>>>>> Tek called this "feed-beside", a brutally fast but ugly signal path, >>>>>>>>> and slow stuff in parallel to make it clean. >>>>>>>> >>>>>>>> Plus a lot a lot of hand work to get them to match. I've benefited >>>>>>>> greatly from their labours, but I have no interest in doing that myself! >>>>>>>> >>>>>>>> What I'm mostly talking about is using a nice quiet accurate amp such >>>>>>>> as an ADA4898 plus a faster but less accurate thing such as a THS3091 >>>>>>>> or LM6171. The output amp is run at some fixed gain like 10, and the >>>>>>>> input amp is run at high enough gain that the combination is stable at >>>>>>>> quiescent conditions. >>>>>>>> >>>>>>>> Other composite amps such as the one you mention or the common case of >>>>>>>> using a chopamp to control the offset voltage of some fast-but-ugly >>>>>>>> amplifier have a different set of problems. >>>>>>>> >>>>>>>> Cheers >>>>>>>> >>>>>>>> Phil Hobbs >>>>>>> >>>>>>> And this one from Burr-Brown: >>>>>>> >>>>>>> <http://www.ti.com/lit/an/sboa002/sboa002.pdf> >>>>>> >>>>>> Yeah, that's the idea. Problem is that it has horrible transient response. >>>>>> >>>>>> Cheers >>>>>> >>>>>> Phil Hobbs >>>>>> >>>>>> -- >>>>>> Dr Philip C D Hobbs >>>>>> Principal Consultant >>>>>> ElectroOptical Innovations LLC / Hobbs ElectroOptics >>>>>> Optics, Electro-optics, Photonics, Analog Electronics >>>>>> Briarcliff Manor NY 10510 >>>>>> >>>>>> http://electrooptical.net >>>>>> http://hobbs-eo.com >>>>> >>>>> Oh you've got some time delay front to back, >>>>> maybe feed back around the 1st opamp... >>>>> an RC in series? at some intermediate freq./time. >>>>> George H. >>>>> Inside cooling off, I'm grilling more meat later. >>>>> Hey a shoutout to any vet's on Memorial Day. >>>>> Thanks! >>>>> (I should give some buddies a call.) >>>>> >>>>> GH >>>>> >>>> >>>> I'm taking today to remember my Dad, who passed about two and a half >>>> weeks ago just shy of 92. S/Sgt 10th Mountain, served in Italy late 1944 >>>> to 1946. >>>> >>>> Thanks, Dad! >>>> >>>> And also sort through all his stuff, he was of the generation that never >>>> really liked to throw anything away that looked like it would come in >>>> handy someday. Sigh... >>> >>> I'm sorry for your loss. My dad was part of the 'greatest' generation too, >>> but he passed away in his 70's. Years ago. He served in the South Pacific. >>> >>> George H. >>> >> >> And thank you! Dad was kind of hit-or-miss as a father (like most >> fathers I suppose) but it was remarkable hearing the stories about the >> time he came from which was both so different and so much the same as th >> time I grew up in, I am happy to say he was a man I was proud to have >> had a chance to know. > > Sure, my dad died one night, massive heart attack, he was set to go > hunting the next day with buddies. It was totally unexpected from my > point of view, but we learned after the fact (from his girlfriend) > that he wasn't taking his high blood pressure meds. Some type of > advanced warning is mostly a good thing, let's you sorta prepare... > emotionally. > > George H. >No warning feels extremely unfair, but having a couple last conversations is extremely hard too. There's so much you want to say but gosh you don't want to be too emotional or look like you're coming unglued yourself! It feels "unfair" any way you slice it I suppose. We mostly talked about his grandkids, some old movies he wanted to see again, girls, and boats. Seems as reasonable an assortment of topics as any other :)
Reply by ●May 29, 20182018-05-29
On Tuesday, May 29, 2018 at 12:36:02 PM UTC-4, bitrex wrote:> On 05/29/2018 11:11 AM, George Herold wrote: > > On Tuesday, May 29, 2018 at 10:26:04 AM UTC-4, bitrex wrote: > >> On 05/29/2018 08:53 AM, George Herold wrote: > >>> On Monday, May 28, 2018 at 7:07:51 PM UTC-4, bitrex wrote: > >>>> On 05/28/2018 02:07 PM, George Herold wrote: > >>>>> On Sunday, May 27, 2018 at 8:10:21 PM UTC-4, Phil Hobbs wrote: > >>>>>> On 05/27/18 19:33, bitrex wrote: > >>>>>>> On 05/27/2018 05:45 PM, Phil Hobbs wrote: > >>>>>>>> On 05/27/18 16:28, John Larkin wrote: > >>>>>>>>> On Sun, 27 May 2018 15:09:33 -0400, Phil Hobbs > >>>>>>>>> <pcdhSpamMeSenseless@electrooptical.net> wrote: > >>>>>>>>> > >>>>>>>>>> Hi, all, > >>>>>>>>>> > >>>>>>>>>> I'm generally prejudiced against composite amplifiers (two op amps > >>>>>>>>>> inside one feedback loop) because they're generally squirrelly, with > >>>>>>>>>> poor settling performance and weird transient response. > >>>>>>>>>> > >>>>>>>>>> On the other hand, my aversion to them means that I don't have as much > >>>>>>>>>> experience with them as do composite-amp fans. So what do you folks > >>>>>>>>>> say > >>>>>>>>>> about them? > >>>>>>>>>> > >>>>>>>>>> Orchids? Onions? Actual expertise? > >>>>>>>>>> > >>>>>>>>>> Cheers > >>>>>>>>>> > >>>>>>>>>> Phil Hobbs > >>>>>>>>> > >>>>>>>>> I've only done it a little, in very special cases, but if the intent > >>>>>>>>> is to apply a slow DC offset correction, and the main amp and the DC > >>>>>>>>> trim amp don't overlap in frequency response, it seems to work fine. > >>>>>>>>> > >>>>>>>>> If you want to make a general -6 dB/octave amp as a composite, the > >>>>>>>>> risk is probably saturating one of the amps in large-signal/slewing > >>>>>>>>> cases, or at leasy doing goofy things. A composite that clips clean > >>>>>>>>> would be a challenge. > >>>>>>>>> > >>>>>>>>> A sorta similar case is where a fast signal needs to be DC coupled > >>>>>>>>> across a big DC offset. A capacitor is the fast path and some slow > >>>>>>>>> opamp thing does the DC part before the AC path decays. The gains have > >>>>>>>>> to both be the same, about 1.00 usually, and the frequency responses > >>>>>>>>> need to be matched, to get clean step response and no ISI. > >>>>>>>> > >>>>>>>> Not so easy! > >>>>>>>> > >>>>>>>>> > >>>>>>>>> Tek called this "feed-beside", a brutally fast but ugly signal path, > >>>>>>>>> and slow stuff in parallel to make it clean. > >>>>>>>> > >>>>>>>> Plus a lot a lot of hand work to get them to match. I've benefited > >>>>>>>> greatly from their labours, but I have no interest in doing that myself! > >>>>>>>> > >>>>>>>> What I'm mostly talking about is using a nice quiet accurate amp such > >>>>>>>> as an ADA4898 plus a faster but less accurate thing such as a THS3091 > >>>>>>>> or LM6171. The output amp is run at some fixed gain like 10, and the > >>>>>>>> input amp is run at high enough gain that the combination is stable at > >>>>>>>> quiescent conditions. > >>>>>>>> > >>>>>>>> Other composite amps such as the one you mention or the common case of > >>>>>>>> using a chopamp to control the offset voltage of some fast-but-ugly > >>>>>>>> amplifier have a different set of problems. > >>>>>>>> > >>>>>>>> Cheers > >>>>>>>> > >>>>>>>> Phil Hobbs > >>>>>>> > >>>>>>> And this one from Burr-Brown: > >>>>>>> > >>>>>>> <http://www.ti.com/lit/an/sboa002/sboa002.pdf> > >>>>>> > >>>>>> Yeah, that's the idea. Problem is that it has horrible transient response. > >>>>>> > >>>>>> Cheers > >>>>>> > >>>>>> Phil Hobbs > >>>>>> > >>>>>> -- > >>>>>> Dr Philip C D Hobbs > >>>>>> Principal Consultant > >>>>>> ElectroOptical Innovations LLC / Hobbs ElectroOptics > >>>>>> Optics, Electro-optics, Photonics, Analog Electronics > >>>>>> Briarcliff Manor NY 10510 > >>>>>> > >>>>>> http://electrooptical.net > >>>>>> http://hobbs-eo.com > >>>>> > >>>>> Oh you've got some time delay front to back, > >>>>> maybe feed back around the 1st opamp... > >>>>> an RC in series? at some intermediate freq./time. > >>>>> George H. > >>>>> Inside cooling off, I'm grilling more meat later. > >>>>> Hey a shoutout to any vet's on Memorial Day. > >>>>> Thanks! > >>>>> (I should give some buddies a call.) > >>>>> > >>>>> GH > >>>>> > >>>> > >>>> I'm taking today to remember my Dad, who passed about two and a half > >>>> weeks ago just shy of 92. S/Sgt 10th Mountain, served in Italy late 1944 > >>>> to 1946. > >>>> > >>>> Thanks, Dad! > >>>> > >>>> And also sort through all his stuff, he was of the generation that never > >>>> really liked to throw anything away that looked like it would come in > >>>> handy someday. Sigh... > >>> > >>> I'm sorry for your loss. My dad was part of the 'greatest' generation too, > >>> but he passed away in his 70's. Years ago. He served in the South Pacific. > >>> > >>> George H. > >>> > >> > >> And thank you! Dad was kind of hit-or-miss as a father (like most > >> fathers I suppose) but it was remarkable hearing the stories about the > >> time he came from which was both so different and so much the same as th > >> time I grew up in, I am happy to say he was a man I was proud to have > >> had a chance to know. > > > > Sure, my dad died one night, massive heart attack, he was set to go > > hunting the next day with buddies. It was totally unexpected from my > > point of view, but we learned after the fact (from his girlfriend) > > that he wasn't taking his high blood pressure meds. Some type of > > advanced warning is mostly a good thing, let's you sorta prepare... > > emotionally. > > > > George H. > > > > > No warning feels extremely unfair, but having a couple last > conversations is extremely hard too. There's so much you want to say but > gosh you don't want to be too emotional or look like you're coming > unglued yourself! It feels "unfair" any way you slice it I suppose. > > We mostly talked about his grandkids, some old movies he wanted to see > again, girls, and boats. Seems as reasonable an assortment of topics as > any other :)Yeah, life's not fair. :^) I keep telling my kids this, but I think you only learn it through hard knocks. Me mum passed away about 1 year ago now. Alzheimer's, which is a particularly icky way to go. When I heard, after shock, my next emotion was relief. George H.
Reply by ●May 29, 20182018-05-29
On Tue, 29 May 2018 08:11:42 -0700 (PDT), George Herold <gherold@teachspin.com> wrote:>On Tuesday, May 29, 2018 at 10:26:04 AM UTC-4, bitrex wrote: >> On 05/29/2018 08:53 AM, George Herold wrote: >> > On Monday, May 28, 2018 at 7:07:51 PM UTC-4, bitrex wrote: >> >> On 05/28/2018 02:07 PM, George Herold wrote: >> >>> On Sunday, May 27, 2018 at 8:10:21 PM UTC-4, Phil Hobbs wrote: >> >>>> On 05/27/18 19:33, bitrex wrote: >> >>>>> On 05/27/2018 05:45 PM, Phil Hobbs wrote: >> >>>>>> On 05/27/18 16:28, John Larkin wrote: >> >>>>>>> On Sun, 27 May 2018 15:09:33 -0400, Phil Hobbs >> >>>>>>> <pcdhSpamMeSenseless@electrooptical.net> wrote: >> >>>>>>> >> >>>>>>>> Hi, all, >> >>>>>>>> >> >>>>>>>> I'm generally prejudiced against composite amplifiers (two op amps >> >>>>>>>> inside one feedback loop) because they're generally squirrelly, with >> >>>>>>>> poor settling performance and weird transient response. >> >>>>>>>> >> >>>>>>>> On the other hand, my aversion to them means that I don't have as much >> >>>>>>>> experience with them as do composite-amp fans.� So what do you folks >> >>>>>>>> say >> >>>>>>>> about them? >> >>>>>>>> >> >>>>>>>> Orchids? Onions? Actual expertise? >> >>>>>>>> >> >>>>>>>> Cheers >> >>>>>>>> >> >>>>>>>> Phil Hobbs >> >>>>>>> >> >>>>>>> I've only done it a little, in very special cases, but if the intent >> >>>>>>> is to apply a slow DC offset correction, and the main amp and the DC >> >>>>>>> trim amp don't overlap in frequency response, it seems to work fine. >> >>>>>>> >> >>>>>>> If you want to make a general -6 dB/octave amp as a composite, the >> >>>>>>> risk is probably saturating one of the amps in large-signal/slewing >> >>>>>>> cases, or at leasy doing goofy things. A composite that clips clean >> >>>>>>> would be a challenge. >> >>>>>>> >> >>>>>>> A sorta similar case is where a fast signal needs to be DC coupled >> >>>>>>> across a big DC offset. A capacitor is the fast path and some slow >> >>>>>>> opamp thing does the DC part before the AC path decays. The gains have >> >>>>>>> to both be the same, about 1.00 usually, and the frequency responses >> >>>>>>> need to be matched, to get clean step response and no ISI. >> >>>>>> >> >>>>>> Not so easy! >> >>>>>> >> >>>>>>> >> >>>>>>> Tek called this "feed-beside", a brutally fast but ugly signal path, >> >>>>>>> and slow stuff in parallel to make it clean. >> >>>>>> >> >>>>>> Plus a lot a lot of hand work to get them to match.� I've benefited >> >>>>>> greatly from their labours, but I have no interest in doing that myself! >> >>>>>> >> >>>>>> What I'm mostly talking about is using a nice quiet accurate amp such >> >>>>>> as an ADA4898 plus a faster but less accurate thing such as a THS3091 >> >>>>>> or LM6171.� The output amp is run at some fixed gain like 10, and the >> >>>>>> input amp is run at high enough gain that the combination is stable at >> >>>>>> quiescent conditions. >> >>>>>> >> >>>>>> Other composite amps such as the one you mention or the common case of >> >>>>>> using a chopamp to control the offset voltage of some fast-but-ugly >> >>>>>> amplifier have a different set of problems. >> >>>>>> >> >>>>>> Cheers >> >>>>>> >> >>>>>> Phil Hobbs >> >>>>> >> >>>>> And this one from Burr-Brown: >> >>>>> >> >>>>> <http://www.ti.com/lit/an/sboa002/sboa002.pdf> >> >>>> >> >>>> Yeah, that's the idea. Problem is that it has horrible transient response. >> >>>> >> >>>> Cheers >> >>>> >> >>>> Phil Hobbs >> >>>> >> >>>> -- >> >>>> Dr Philip C D Hobbs >> >>>> Principal Consultant >> >>>> ElectroOptical Innovations LLC / Hobbs ElectroOptics >> >>>> Optics, Electro-optics, Photonics, Analog Electronics >> >>>> Briarcliff Manor NY 10510 >> >>>> >> >>>> http://electrooptical.net >> >>>> http://hobbs-eo.com >> >>> >> >>> Oh you've got some time delay front to back, >> >>> maybe feed back around the 1st opamp... >> >>> an RC in series? at some intermediate freq./time. >> >>> George H. >> >>> Inside cooling off, I'm grilling more meat later. >> >>> Hey a shoutout to any vet's on Memorial Day. >> >>> Thanks! >> >>> (I should give some buddies a call.) >> >>> >> >>> GH >> >>> >> >> >> >> I'm taking today to remember my Dad, who passed about two and a half >> >> weeks ago just shy of 92. S/Sgt 10th Mountain, served in Italy late 1944 >> >> to 1946. >> >> >> >> Thanks, Dad! >> >> >> >> And also sort through all his stuff, he was of the generation that never >> >> really liked to throw anything away that looked like it would come in >> >> handy someday. Sigh... >> > >> > I'm sorry for your loss. My dad was part of the 'greatest' generation too, >> > but he passed away in his 70's. Years ago. He served in the South Pacific. >> > >> > George H. >> > >> >> And thank you! Dad was kind of hit-or-miss as a father (like most >> fathers I suppose) but it was remarkable hearing the stories about the >> time he came from which was both so different and so much the same as th >> time I grew up in, I am happy to say he was a man I was proud to have >> had a chance to know. > >Sure, my dad died one night, massive heart attack, he was set to go >hunting the next day with buddies. It was totally unexpected from my >point of view, but we learned after the fact (from his girlfriend) >that he wasn't taking his high blood pressure meds. Some type of >advanced warning is mostly a good thing, let's you sorta prepare... >emotionally.My dad died suddenly at age 52, shoveling snow. I was 12 at the time.
Reply by ●May 29, 20182018-05-29
On Tue, 29 May 2018 11:49:22 -0700 (PDT), George Herold <gherold@teachspin.com> wrote:>On Tuesday, May 29, 2018 at 12:36:02 PM UTC-4, bitrex wrote: >> On 05/29/2018 11:11 AM, George Herold wrote: >> > On Tuesday, May 29, 2018 at 10:26:04 AM UTC-4, bitrex wrote: >> >> On 05/29/2018 08:53 AM, George Herold wrote: >> >>> On Monday, May 28, 2018 at 7:07:51 PM UTC-4, bitrex wrote: >> >>>> On 05/28/2018 02:07 PM, George Herold wrote: >> >>>>> On Sunday, May 27, 2018 at 8:10:21 PM UTC-4, Phil Hobbs wrote: >> >>>>>> On 05/27/18 19:33, bitrex wrote: >> >>>>>>> On 05/27/2018 05:45 PM, Phil Hobbs wrote: >> >>>>>>>> On 05/27/18 16:28, John Larkin wrote: >> >>>>>>>>> On Sun, 27 May 2018 15:09:33 -0400, Phil Hobbs >> >>>>>>>>> <pcdhSpamMeSenseless@electrooptical.net> wrote: >> >>>>>>>>> >> >>>>>>>>>> Hi, all, >> >>>>>>>>>> >> >>>>>>>>>> I'm generally prejudiced against composite amplifiers (two op amps >> >>>>>>>>>> inside one feedback loop) because they're generally squirrelly, with >> >>>>>>>>>> poor settling performance and weird transient response. >> >>>>>>>>>> >> >>>>>>>>>> On the other hand, my aversion to them means that I don't have as much >> >>>>>>>>>> experience with them as do composite-amp fans.� So what do you folks >> >>>>>>>>>> say >> >>>>>>>>>> about them? >> >>>>>>>>>> >> >>>>>>>>>> Orchids? Onions? Actual expertise? >> >>>>>>>>>> >> >>>>>>>>>> Cheers >> >>>>>>>>>> >> >>>>>>>>>> Phil Hobbs >> >>>>>>>>> >> >>>>>>>>> I've only done it a little, in very special cases, but if the intent >> >>>>>>>>> is to apply a slow DC offset correction, and the main amp and the DC >> >>>>>>>>> trim amp don't overlap in frequency response, it seems to work fine. >> >>>>>>>>> >> >>>>>>>>> If you want to make a general -6 dB/octave amp as a composite, the >> >>>>>>>>> risk is probably saturating one of the amps in large-signal/slewing >> >>>>>>>>> cases, or at leasy doing goofy things. A composite that clips clean >> >>>>>>>>> would be a challenge. >> >>>>>>>>> >> >>>>>>>>> A sorta similar case is where a fast signal needs to be DC coupled >> >>>>>>>>> across a big DC offset. A capacitor is the fast path and some slow >> >>>>>>>>> opamp thing does the DC part before the AC path decays. The gains have >> >>>>>>>>> to both be the same, about 1.00 usually, and the frequency responses >> >>>>>>>>> need to be matched, to get clean step response and no ISI. >> >>>>>>>> >> >>>>>>>> Not so easy! >> >>>>>>>> >> >>>>>>>>> >> >>>>>>>>> Tek called this "feed-beside", a brutally fast but ugly signal path, >> >>>>>>>>> and slow stuff in parallel to make it clean. >> >>>>>>>> >> >>>>>>>> Plus a lot a lot of hand work to get them to match.� I've benefited >> >>>>>>>> greatly from their labours, but I have no interest in doing that myself! >> >>>>>>>> >> >>>>>>>> What I'm mostly talking about is using a nice quiet accurate amp such >> >>>>>>>> as an ADA4898 plus a faster but less accurate thing such as a THS3091 >> >>>>>>>> or LM6171.� The output amp is run at some fixed gain like 10, and the >> >>>>>>>> input amp is run at high enough gain that the combination is stable at >> >>>>>>>> quiescent conditions. >> >>>>>>>> >> >>>>>>>> Other composite amps such as the one you mention or the common case of >> >>>>>>>> using a chopamp to control the offset voltage of some fast-but-ugly >> >>>>>>>> amplifier have a different set of problems. >> >>>>>>>> >> >>>>>>>> Cheers >> >>>>>>>> >> >>>>>>>> Phil Hobbs >> >>>>>>> >> >>>>>>> And this one from Burr-Brown: >> >>>>>>> >> >>>>>>> <http://www.ti.com/lit/an/sboa002/sboa002.pdf> >> >>>>>> >> >>>>>> Yeah, that's the idea. Problem is that it has horrible transient response. >> >>>>>> >> >>>>>> Cheers >> >>>>>> >> >>>>>> Phil Hobbs >> >>>>>> >> >>>>>> -- >> >>>>>> Dr Philip C D Hobbs >> >>>>>> Principal Consultant >> >>>>>> ElectroOptical Innovations LLC / Hobbs ElectroOptics >> >>>>>> Optics, Electro-optics, Photonics, Analog Electronics >> >>>>>> Briarcliff Manor NY 10510 >> >>>>>> >> >>>>>> http://electrooptical.net >> >>>>>> http://hobbs-eo.com >> >>>>> >> >>>>> Oh you've got some time delay front to back, >> >>>>> maybe feed back around the 1st opamp... >> >>>>> an RC in series? at some intermediate freq./time. >> >>>>> George H. >> >>>>> Inside cooling off, I'm grilling more meat later. >> >>>>> Hey a shoutout to any vet's on Memorial Day. >> >>>>> Thanks! >> >>>>> (I should give some buddies a call.) >> >>>>> >> >>>>> GH >> >>>>> >> >>>> >> >>>> I'm taking today to remember my Dad, who passed about two and a half >> >>>> weeks ago just shy of 92. S/Sgt 10th Mountain, served in Italy late 1944 >> >>>> to 1946. >> >>>> >> >>>> Thanks, Dad! >> >>>> >> >>>> And also sort through all his stuff, he was of the generation that never >> >>>> really liked to throw anything away that looked like it would come in >> >>>> handy someday. Sigh... >> >>> >> >>> I'm sorry for your loss. My dad was part of the 'greatest' generation too, >> >>> but he passed away in his 70's. Years ago. He served in the South Pacific. >> >>> >> >>> George H. >> >>> >> >> >> >> And thank you! Dad was kind of hit-or-miss as a father (like most >> >> fathers I suppose) but it was remarkable hearing the stories about the >> >> time he came from which was both so different and so much the same as th >> >> time I grew up in, I am happy to say he was a man I was proud to have >> >> had a chance to know. >> > >> > Sure, my dad died one night, massive heart attack, he was set to go >> > hunting the next day with buddies. It was totally unexpected from my >> > point of view, but we learned after the fact (from his girlfriend) >> > that he wasn't taking his high blood pressure meds. Some type of >> > advanced warning is mostly a good thing, let's you sorta prepare... >> > emotionally. >> > >> > George H. >> > >> >> >> No warning feels extremely unfair, but having a couple last >> conversations is extremely hard too. There's so much you want to say but >> gosh you don't want to be too emotional or look like you're coming >> unglued yourself! It feels "unfair" any way you slice it I suppose. >> >> We mostly talked about his grandkids, some old movies he wanted to see >> again, girls, and boats. Seems as reasonable an assortment of topics as >> any other :) > >Yeah, life's not fair. :^) I keep telling my kids this, but I think >you only learn it through hard knocks. Me mum passed away about 1 year ago >now. Alzheimer's, which is a particularly icky way to go. When I heard, >after shock, my next emotion was relief.Ouch! That is probably the hardest way to go, at least for the family. My mother was getting pretty bad at the end but it wasn't Alzheimer's. She was 95, with heart and kidney problems that were taking their toll on the brain. I understand your comment about "relief".
Reply by ●May 30, 20182018-05-30
On Tue, 29 May 2018 09:21:19 -0700, Jim Thompson wrote:> On Mon, 28 May 2018 15:26:58 -0400, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > >>On 05/28/18 14:56, Jim Thompson wrote: >>> On Sun, 27 May 2018 15:09:33 -0400, Phil Hobbs >>> <pcdhSpamMeSenseless@electrooptical.net> wrote: >>> >>>> Hi, all, >>>> >>>> I'm generally prejudiced against composite amplifiers (two op amps >>>> inside one feedback loop) because they're generally squirrelly, with >>>> poor settling performance and weird transient response. >>>> >>>> On the other hand, my aversion to them means that I don't have as >>>> much experience with them as do composite-amp fans. So what do you >>>> folks say about them? >>>> >>>> Orchids? Onions? Actual expertise? >>>> >>>> Cheers >>>> >>>> Phil Hobbs >>> >>> I don't have time (*) to join the discussion, but read this... >>> >>> <http://www.analog-innovations.com/SED/CompositeAmplifiers.pdf> >> >>I have that one, thanks. Lots of the circuits are summing-junction >>snoopers, i.e. circuits wih a slow-but-accurate amp looking at the >>time-averaged input error of a fast-but-cruder amp, and nulling it out. >>That's a useful trick sometimes, and are examples of "putting a bandaid >>on the fast circuit", which I was talking about upthread. >> >>There are other sorts of bandaids, e.g. the White cathode follower and >>many sorts of local feedback. Often the key is to figure out a way that >>the bandaid can be much slower than the main amplifier, as in the >>snooper circuits. >> >>I often use op amps to force JFETs to run at exactly I_DSS, for >>instance--the problem then is to keep the low-frequency noise from going >>nuts. >> >>Cheers >> >>Phil Hobbs > > I have a paper somewhere that discusses GBW-product and optimization of > the two-OpAmp composite. Quite nicely done, as I recall... > unfortunately I haven't re-found it :-( > > ...Jim ThompsonThat sounds like something I would have saved if I had seen it, so I searched my SED notes folder without luck but found some old app notes and a post from Win Hill that might be of interest to someone. (Probably old hat to Phil): Analyzing feedback loops containing secondary amplifiers: www.ti.com/lit/an/slyt103/slyt103.pdf Burr Brown Application Bulletin FEEDBACK PLOTS DEFINE OP AMP AC PERFORMANCE www.ti.com/lit/an/sboa015/sboa015.pdf (composite amp discussion pp 9-13) From: Winfield Hill <Winfield_member@newsguy.com> Newsgroups: sci.electronics.design Subject: Re: Photodiode wich is fast enough to detect +50Mhz analog (sinus) signal?? Date: 11 Aug 2004 03:31:18 -0700 Yannick wrote...>> Winfield wrote...>> Careful, using a sufficiently-wideband opamp can insure the summing->> junction impedance will be low compared to the total shuntcapacitance.>> Resistors have 0.05pF to 0.1pF of self capacitance, this should beyour>> total feedback capacitance. With 3k resistor you'd have a -3dBrolloff>> at 530MHz. You want high R for low noise, so we'll try 100k, yielding>> a 16MHz rolloff. Then we can apply the standard R-C-R trick (this is>> more than 30 years old) to get a flat frequency response to 75MHz, or>> whatever you decide your bandwidth should be.>> R-C-R trick to get a flat frequency responce, i never heard of this> before, can you explain how it works?I posted the circuit previously in this thread (20 July) and described in detail how it works. Here's the circuit again: | Rf R2 adjustable | ,---/\/\---+---/\/\--/\/\----, | | '--||--' | C2 R3 | nA-sensitivity wideband | | Cf '--||--/\/\-- gnd | transresistance amplifier | | | | | __ ,-||--/\/\--+ correction network details | input O--+---|+ \ | __ | R2 C2 = Rf Cf | | >-+-/\/\-+-|- \ | R3 C2 sets bandwidth | ,-|-_/ | | >-----+--- | | | gnd --|+_/ | gnd --/\/\--+-/\/\--' composite amplifier The undesired Rf capacitance Cf is canceled by the R2 C2 network. R3 is used to limit the upper frequency of this cancellation. R2 C2 and R3 constitute the standard R-C-R trick. I thought of this about 18 years ago, and have used it with great success since then. Later I learned that it had been described in an old Keithley manual, and probably in many other places years before that. BTW, the circuit above will outperform (sensitivity, bandwidth, SNR, phase accuracy) any of the resonant schemes you've been contemplating here. It's not true that feedback makes things more noisy. In this circuit feedback (and a high-performance composite amplifier) insures that all of the signal current is used by the amplifier, rather than becoming uselessly drained away by the input-node capacitance. Thus feedback actually improves the SNR. Using a resonant input doesn't solve the capacitance problem because if a high enough Q is used for a solution, it simply creates insurmountable phase-error problems. -- Thanks, - Win
Reply by ●May 30, 20182018-05-30
On Mon, 28 May 2018 11:58:52 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>On 05/28/18 11:42, John Larkin wrote: >> On Mon, 28 May 2018 11:35:37 -0400, Phil Hobbs >> <pcdhSpamMeSenseless@electrooptical.net> wrote: >> >>> On 05/28/18 11:25, John Larkin wrote: >>>> On Mon, 28 May 2018 14:59:11 GMT, Steve Wilson <no@spam.com> wrote: >>>> >>>>> John Larkin <jjlarkin@highlandtechnology.com> wrote: >>>>> >>>>>> Not to change the subject (I'd never do that) but I have made a >>>>>> compound amp just to shift the power dissipation away from the >>>>>> front-end diff pair, off to another chip, to avoid nanovolt thermal >>>>>> hooks. I had to keep the feedback network low impedance to minimize >>>>>> Johnson noise, which required a lot of feedback current. >>>>> >>>>>> https://www.dropbox.com/s/nieqrj2um62pdu5/L700_Shunt_Amp.jpg?raw=1 >>>>> >>>>> Not to change the subject, but I have a question. There is another type of >>>>> amplifier that splits the signal into two paths - a high frequency path for >>>>> an RF amplifier with poor DC drift and small DC offset capability, and a low >>>>> frequency path for an amplifier with good DC characteristics and wide offset >>>>> capability. I thought this was a compound amplifier, and once read an article >>>>> in the HP Journal that described it. >>>>> >>>>> But I can't find the article, and google is no help. Do you know the name of >>>>> this kind of amplifier? >>>> >>>> This is usually called a compound amplifier. Tektronix called >>>> something similar to this "feed-beside." >>>> >>>> There are two ways to do this: >>>> >>>> 1. Split the signal with RC or bias tee circuits, amplify the AC and >>>> DC parts with separate amps, and combine at the output. >>>> >>>> 2. Build a compound amp, with optimized AC and DC paths, but treat it >>>> as a black-box opamp, and close a feedback loop around it. >>>> >>>> I don't know of they have specific names. As Phil noted at the start >>>> here, it's tricky to manage the overlap with precision. >>>> >>>> There is an RF power amp configuration that has a high-power amp with >>>> some distortion, and a paralleled low-power amp with correcting >>>> distortion behavior. That probably has a name. I think cell towers use >>>> that. >>> >>> Feedforward. >>> >>> Cheers >>> >>> Phil Hobbs >> >> Not to be confused with predistortion, I guess. >> >> https://en.wikipedia.org/wiki/Predistortion >> >> I'd play with the compound amp thing, but I need to force myself to do >> less interesting grunt work. Like revising proposals and replacing >> faucets. Hard to decide which is less appealing. > >Do the proposal, then you can pay somebody to do the faucet. ;) > >Cheers > >Phil HobbsWell, it's refreshing to, once in a while, swap a screen for the underside of a bathroom sink. But really, most consumer products, including plumbing, are such trash. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●February 21, 20202020-02-21
Zero DC offset low noise composite TIA for high capacitance photodides http://ixbt.photo/?id=photo:1329265 2200pF input capacitance http://ixbt.photo/?id=photo:1329266 Zero-drift OPA189 composite TIA http://ixbt.photo/?id=photo:1329269 Inductance LB3218T102K (1207 smd)
Reply by ●February 21, 20202020-02-21
On Fri, 21 Feb 2020 12:37:03 -0800 (PST), plastcontrol.ru@gmail.com wrote:>Zero DC offset low noise composite TIA for high capacitance photodides >http://ixbt.photo/?id=photo:1329265 > >2200pF input capacitance >http://ixbt.photo/?id=photo:1329266 > >Zero-drift OPA189 composite TIA >http://ixbt.photo/?id=photo:1329269 > >Inductance LB3218T102K (1207 smd)Just a note: a photodiode has two leads. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●February 21, 20202020-02-21
