On 05/27/18 20:30, bitrex wrote:> 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 yetNo worries, I'm mainly interested in actual experiences. Spice we can all do. ;) 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
Reply by ●May 28, 20182018-05-28
On Sun, 27 May 2018 20:40:17 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>On 05/27/18 20:30, bitrex wrote: >> 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 > >No worries, I'm mainly interested in actual experiences. Spice we can >all do. ;) > >Cheers > >Phil HobbsNot 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 -- John Larkin Highland Technology, Inc trk jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●May 28, 20182018-05-28
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=1Not 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?
Reply by ●May 28, 20182018-05-28
On 05/28/18 10:05, John Larkin wrote:> On Sun, 27 May 2018 20:40:17 -0400, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> On 05/27/18 20:30, bitrex wrote: >>> 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 >> >> No worries, I'm mainly interested in actual experiences. Spice we can >> all do. ;) >> >> Cheers >> >> Phil Hobbs > > 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 > >I've used emitter followers for similar things, most recently in a super low noise laser driver. It uses MC33078 op amps and 330-ohm feedback resistors in a 12-V system. It's basically a PNP current source with a two-pole decoupling network between base and 10-V supply. (The +10 rail is made by a couple of diodes from +12, so that the op amp + emitter follower has enough output swing.) 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 28, 20182018-05-28
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. -- John Larkin Highland Technology, Inc trk jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●May 28, 20182018-05-28
On 05/28/18 10:59, Steve Wilson 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?Dunno. I cordially dislike architectures like that on account of their weird settling behaviour. It's super hard to get the nonlinearities to match, so it's generally much better to put bandaids on the RF amp to make it behave properly. I've only done that once or twice, so I don't have too many specifics to contribute. 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 28, 20182018-05-28
On Mon, 28 May 2018 11:16:24 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>On 05/28/18 10:05, John Larkin wrote: >> On Sun, 27 May 2018 20:40:17 -0400, Phil Hobbs >> <pcdhSpamMeSenseless@electrooptical.net> wrote: >> >>> On 05/27/18 20:30, bitrex wrote: >>>> 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 >>> >>> No worries, I'm mainly interested in actual experiences. Spice we can >>> all do. ;) >>> >>> Cheers >>> >>> Phil Hobbs >> >> 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 >> >> > >I've used emitter followers for similar things, most recently in a super >low noise laser driver. It uses MC33078 op amps and 330-ohm feedback >resistors in a 12-V system. It's basically a PNP current source with a >two-pole decoupling network between base and 10-V supply. (The +10 rail >is made by a couple of diodes from +12, so that the op amp + emitter >follower has enough output swing.) > >Cheers > >Phil HobbsOpamp data sheets and appnotes are pretty good at avoiding any mention of transient thermal effects, or static effects of output stage dissipation on input offset. I know it is a serious consideration in opamp chip layout. They can measure their DC specs and AC/noise specs and large-signal swing specs with different setups. Sneaky devils. A compound amp can avoid some of these problems. -- John Larkin Highland Technology, Inc trk jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●May 28, 20182018-05-28
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 -- 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 28, 20182018-05-28
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 HobbsNot 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. -- John Larkin Highland Technology, Inc trk jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●May 28, 20182018-05-28
On 05/28/18 11:35, John Larkin wrote:> On Mon, 28 May 2018 11:16:24 -0400, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> On 05/28/18 10:05, John Larkin wrote: >>> On Sun, 27 May 2018 20:40:17 -0400, Phil Hobbs >>> <pcdhSpamMeSenseless@electrooptical.net> wrote: >>> >>>> On 05/27/18 20:30, bitrex wrote: >>>>> 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 >>>> >>>> No worries, I'm mainly interested in actual experiences. Spice we can >>>> all do. ;) >>>> >>>> Cheers >>>> >>>> Phil Hobbs >>> >>> 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 >>> >>> >> >> I've used emitter followers for similar things, most recently in a super >> low noise laser driver. It uses MC33078 op amps and 330-ohm feedback >> resistors in a 12-V system. It's basically a PNP current source with a >> two-pole decoupling network between base and 10-V supply. (The +10 rail >> is made by a couple of diodes from +12, so that the op amp + emitter >> follower has enough output swing.) >> >> Cheers >> >> Phil Hobbs > > Opamp data sheets and appnotes are pretty good at avoiding any mention > of transient thermal effects, or static effects of output stage > dissipation on input offset. I know it is a serious consideration in > opamp chip layout. > > They can measure their DC specs and AC/noise specs and large-signal > swing specs with different setups. Sneaky devils. > > A compound amp can avoid some of these problems.For temperature controllers you normally want the front end amp on the cold plate, but you don't want its dissipation to vary. I often put a lead-lag integrator inside the loop but off the cold plate. That forces the input amp's bias to stay very very stable, so there's no change in dissipation. 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
