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 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 28, 20182018-05-28
Reply by ●May 28, 20182018-05-28
John Larkin <jjlarkin@highlandtechnology.com> wrote:> 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.Compund, not composite. Thanks.
Reply by ●May 28, 20182018-05-28
On 05/28/2018 11:42 AM, 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. >What is this "predistortion" discussed in the article but itself a form of "regular" negative feedback? It says "In essence, 'inverse distortion' is introduced into the input of the amplifier, thereby cancelling any non-linearity the amplifier might have." Yes, that's what negative feedback is.
Reply by ●May 28, 20182018-05-28
On Mon, 28 May 2018 13:09:38 -0400, bitrex <user@example.net> wrote:>On 05/28/2018 11:42 AM, 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. >> > >What is this "predistortion" discussed in the article but itself a form >of "regular" negative feedback? > >It says "In essence, 'inverse distortion' is introduced into the input >of the amplifier, thereby cancelling any non-linearity the amplifier >might have." Yes, that's what negative feedback is.Except that predistortion is not negative feedback; all the signals are going in the same direction. It's hard to do feedback on a GHz power amp. -- John Larkin Highland Technology, Inc trk jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●May 28, 20182018-05-28
On Sunday, May 27, 2018 at 4:28:33 PM UTC-4, 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. > > Tek called this "feed-beside", a brutally fast but ugly signal path, > and slow stuff in parallel to make it clean.I broke the feedback loop with an RC lowpass. (int-amp reading the R, there was at least one gain adjustment in each leg.) But I don't think that's what Phil is asking about. George H.> > > > > > -- > > John Larkin Highland Technology, Inc trk > > jlarkin att highlandtechnology dott com > http://www.highlandtechnology.com
Reply by ●May 28, 20182018-05-28
On Sunday, May 27, 2018 at 5:45:21 PM UTC-4, 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.Right, I've never done that, but as long as one can be tuned down to match the slowest (time-wise). George H.> > 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 28, 20182018-05-28
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.Have you peaked at the in between opamp signal? GH> > 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 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.comOh 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
Reply by ●May 28, 20182018-05-28
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 don't have time (*) to join the discussion, but read this... <http://www.analog-innovations.com/SED/CompositeAmplifiers.pdf> (*) Pursuing a new trick for behavioral modeling ;-) ...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
Reply by ●May 28, 20182018-05-28
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 -- 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
