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 -- 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
Composite amps
Started by ●May 27, 2018
Reply by ●May 27, 20182018-05-27
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 HobbsI'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. Tek called this "feed-beside", a brutally fast but ugly signal path, and slow stuff in parallel to make it clean. -- John Larkin Highland Technology, Inc trk jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●May 27, 20182018-05-27
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 -- 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
Reply by ●May 27, 20182018-05-27
On Sun, 27 May 2018 17:45:11 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> 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.So that would take the frequency response curve of the precision amp and just shift it up 20 dB? That should work, as long as the first amp keeps rolling off -6 dB/octave past its normal Ft. You could tweak a bit if not. Make the first amp limit clean at +-2 volts or whatever, to not wind up too much, and let the second, fast amp clip however it likes. Or, when the second amp hits some swing limit, stuff current into the input of the first one. Some common-base things maybe. This would need some Spicing but shouldn't be too bad.> >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-- John Larkin Highland Technology, Inc trk jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●May 27, 20182018-05-27
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 HobbsThis Wireless World article from '74 might be of interest: <http://www.keith-snook.info/wireless-world-magazine/Wireless-World-1974/Reducing%20Amplifier%20Distortion%20-%20Sandman.pdf>
Reply by ●May 27, 20182018-05-27
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 HobbsAnd this one from Burr-Brown: <http://www.ti.com/lit/an/sboa002/sboa002.pdf>
Reply by ●May 27, 20182018-05-27
On 05/27/18 19:31, 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 > > This Wireless World article from '74 might be of interest: > > <http://www.keith-snook.info/wireless-world-magazine/Wireless-World-1974/Reducing%20Amplifier%20Distortion%20-%20Sandman.pdf>How so? 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
Reply by ●May 27, 20182018-05-27
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
Reply by ●May 27, 20182018-05-27
On 05/27/2018 08:10 PM, 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 >Guessing this stool-design problem probably only has three legs, the labor will in be picking the appropriate length of the legs to meet the requirements without ending up with the other kind of stool.
Reply by ●May 27, 20182018-05-27
On 05/27/2018 08:05 PM, Phil Hobbs wrote:> On 05/27/18 19:31, 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 >> >> This Wireless World article from '74 might be of interest: >> >> <http://www.keith-snook.info/wireless-world-magazine/Wireless-World-1974/Reducing%20Amplifier%20Distortion%20-%20Sandman.pdf> > > > How so? > > Cheers > > Phil Hobbs > >Sorry hadn't read the follow-up yet
