Composite amps

[Phil Hobbs]

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

 

[George Herold]

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

 

[John Larkin]

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

 

[bitrex]

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.

 

[Steve Wilson]

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.

 

[George Herold]

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

 

[George Herold]

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

 

[George Herold]

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

 

[Jim Thompson]

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>

(*) 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 | |
| Voice480)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

 

[Phil Hobbs]

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

 

[Phil Hobbs]

On 05/28/18 13:49, 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.
Have you peaked at the in between opamp signal?
GH

It's generally well behaved until the output amp rails, the reason being
that the input amp is the slow one.

I've been playing with a CPH3910/ADA4899/THS3091 combination that is
starting to look reasonable in the spherical-cow universe. It's a
little gain-of-50 amp with about 120 MHz bandwidth, Zin of 10M // <<1pF,
and flatband 1-Hz noise of about 1.5 nV. (The capacitance of the
connector will dominate.)

I seriously doubt it'll work that well in real life.

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

 

[Phil Hobbs]

On 05/28/18 14:07, 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.

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.

Overcast here--has been all weekend.

Hey a shoutout to any vet's on Memorial Day.
Thanks!

+1

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

 

[Phil Hobbs]

On 05/27/18 20:05, 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?

I had a look at the article, and it does have some points of interest,
thanks. The author distinguishes between his technique and feedforward,
but I'd argue that for circuit purposes it's a species of feedforward,
whose basic principle is to sense just the error of an amplifier and
apply the appropriate correction to the _output_ and not the _input_.

I didn't know that FF was invented by Harold Black.

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

 

[bitrex]

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...

 

[bitrex]

On 05/28/2018 03:38 PM, Phil Hobbs wrote:

On 05/28/18 13:49, 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.
Have you peaked at the in between opamp signal?
GH

It's generally well behaved until the output amp rails, the reason being
that the input amp is the slow one.

I've been playing with a CPH3910/ADA4899/THS3091 combination that is
starting to look reasonable in the spherical-cow universe.  It's a
little gain-of-50 amp with about 120 MHz bandwidth, Zin of 10M // <<1pF,
and flatband 1-Hz noise of about 1.5 nV. (The capacitance of the
connector will dominate.)

I seriously doubt it'll work that well in real life.

Cheers

Phil Hobbs

Have you tried designing a composite-amp oscillator instead? If my
theory is correct it should make a wonderful amplifier.

 

[bitrex]

On 05/28/2018 01:28 PM, John Larkin wrote:

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.

Ah, helpfully the Wiki seems to never mention that it's a feed-forward
and not feedback topology.

 

[George Herold]

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.

 

[bitrex]

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.

 

[bitrex]

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.

I'm the youngest of four kids, I got more time with him than most guys
whose fathers were in their early 50s when they were born. He remained
pretty much independent until he died but required a few hours a week of
help from me and a nurse the past few years that I was glad to assist
with, we were still going out for lunch about once a week until last month.

About a week before he died he'd got some test results back that didn't
look too good. He was rarely a pessimist, about the closest he came was
that day when he looked very tired and said that he missed his friends,
all the people who he used to know who by now were all gone.

I could understand that, one son who visits regularly is nice but can
hardly make up for all the rest. He became very ill the next week and
passed away about 48 hours after arrival at the hospital, just enough
time for the rest of the family to make it to be there with us.

It went about as "well" as it could and at that age and I'd known for
quite a while the day would come sooner or later, but I'm still thinking
it's him when the phone rings. It's all still very strange.

 

[George Herold]

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.