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Phil Hobbs
Guest

Sun May 27, 2018 8:45 pm   



Hi, all,

I'm generally prejudiced against composite amplifiers (two op amps
inside one feedback loop) because they're generally squirrelly, with
poor settling performance and weird transient response.

On the other hand, my aversion to them means that I don't have as much
experience with them as do composite-amp fans. So what do you folks say
about them?

Orchids? Onions? Actual expertise?

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

John Larkin
Guest

Sun May 27, 2018 9:45 pm   



On Sun, 27 May 2018 15:09:33 -0400, Phil Hobbs
<pcdhSpamMeSenseless_at_electrooptical.net> wrote:

Quote:
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.





--

John Larkin Highland Technology, Inc trk

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

Phil Hobbs
Guest

Sun May 27, 2018 11:45 pm   



On 05/27/18 16:28, John Larkin wrote:
Quote:
On Sun, 27 May 2018 15:09:33 -0400, Phil Hobbs
pcdhSpamMeSenseless_at_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!

Quote:

Tek called this "feed-beside", a brutally fast but ugly signal path,
and slow stuff in parallel to make it clean.


Plus a lot a lot of hand work to get them to match. I've benefited
greatly from their labours, but I have no interest in doing that myself!

What I'm mostly talking about is using a nice quiet accurate amp such as
an ADA4898 plus a faster but less accurate thing such as a THS3091 or
LM6171. The output amp is run at some fixed gain like 10, and the input
amp is run at high enough gain that the combination is stable at
quiescent conditions.

Other composite amps such as the one you mention or the common case of
using a chopamp to control the offset voltage of some fast-but-ugly
amplifier have a different set of problems.

Cheers

Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

John Larkin
Guest

Sun May 27, 2018 11:45 pm   



On Sun, 27 May 2018 17:45:11 -0400, Phil Hobbs
<pcdhSpamMeSenseless_at_electrooptical.net> wrote:

Quote:
On 05/27/18 16:28, John Larkin wrote:
On Sun, 27 May 2018 15:09:33 -0400, Phil Hobbs
pcdhSpamMeSenseless_at_electrooptical.net> wrote:

Hi, all,

I'm generally prejudiced against composite amplifiers (two op amps
inside one feedback loop) because they're generally squirrelly, with
poor settling performance and weird transient response.

On the other hand, my aversion to them means that I don't have as much
experience with them as do composite-amp fans. So what do you folks say
about them?

Orchids? Onions? Actual expertise?

Cheers

Phil Hobbs

I've only done it a little, in very special cases, but if the intent
is to apply a slow DC offset correction, and the main amp and the DC
trim amp don't overlap in frequency response, it seems to work fine.

If you want to make a general -6 dB/octave amp as a composite, the
risk is probably saturating one of the amps in large-signal/slewing
cases, or at leasy doing goofy things. A composite that clips clean
would be a challenge.

A sorta similar case is where a fast signal needs to be DC coupled
across a big DC offset. A capacitor is the fast path and some slow
opamp thing does the DC part before the AC path decays. The gains have
to both be the same, about 1.00 usually, and the frequency responses
need to be matched, to get clean step response and no ISI.

Not so easy!


Tek called this "feed-beside", a brutally fast but ugly signal path,
and slow stuff in parallel to make it clean.

Plus a lot a lot of hand work to get them to match. I've benefited
greatly from their labours, but I have no interest in doing that myself!

What I'm mostly talking about is using a nice quiet accurate amp such as
an ADA4898 plus a faster but less accurate thing such as a THS3091 or
LM6171. The output amp is run at some fixed gain like 10, and the input
amp is run at high enough gain that the combination is stable at
quiescent conditions.


So that would take the frequency response curve of the precision amp
and just shift it up 20 dB?

That should work, as long as the first amp keeps rolling off -6
dB/octave past its normal Ft. You could tweak a bit if not.

Make the first amp limit clean at +-2 volts or whatever, to not wind
up too much, and let the second, fast amp clip however it likes.

Or, when the second amp hits some swing limit, stuff current into the
input of the first one. Some common-base things maybe.

This would need some Spicing but shouldn't be too bad.

Quote:

Other composite amps such as the one you mention or the common case of
using a chopamp to control the offset voltage of some fast-but-ugly
amplifier have a different set of problems.

Cheers

Phil Hobbs


--

John Larkin Highland Technology, Inc trk

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

bitrex
Guest

Mon May 28, 2018 12:45 am   



On 05/27/2018 05:45 PM, Phil Hobbs wrote:
Quote:
On 05/27/18 16:28, John Larkin wrote:
On Sun, 27 May 2018 15:09:33 -0400, Phil Hobbs
pcdhSpamMeSenseless_at_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>

bitrex
Guest

Mon May 28, 2018 12:45 am   



On 05/27/2018 05:45 PM, Phil Hobbs wrote:
Quote:
On 05/27/18 16:28, John Larkin wrote:
On Sun, 27 May 2018 15:09:33 -0400, Phil Hobbs
pcdhSpamMeSenseless_at_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>

bitrex
Guest

Mon May 28, 2018 1:45 am   



On 05/27/2018 08:05 PM, Phil Hobbs wrote:
Quote:
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_at_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

bitrex
Guest

Mon May 28, 2018 1:45 am   



On 05/27/2018 08:10 PM, Phil Hobbs wrote:
Quote:
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_at_electrooptical.net> wrote:

Hi, all,

I'm generally prejudiced against composite amplifiers (two op amps
inside one feedback loop) because they're generally squirrelly, with
poor settling performance and weird transient response.

On the other hand, my aversion to them means that I don't have as much
experience with them as do composite-amp fans.  So what do you
folks say
about them?

Orchids? Onions? Actual expertise?

Cheers

Phil Hobbs

I've only done it a little, in very special cases, but if the intent
is to apply a slow DC offset correction, and the main amp and the DC
trim amp don't overlap in frequency response, it seems to work fine.

If you want to make a general -6 dB/octave amp as a composite, the
risk is probably saturating one of the amps in large-signal/slewing
cases, or at leasy doing goofy things. A composite that clips clean
would be a challenge.

A sorta similar case is where a fast signal needs to be DC coupled
across a big DC offset. A capacitor is the fast path and some slow
opamp thing does the DC part before the AC path decays. The gains have
to both be the same, about 1.00 usually, and the frequency responses
need to be matched, to get clean step response and no ISI.

Not so easy!


Tek called this "feed-beside", a brutally fast but ugly signal path,
and slow stuff in parallel to make it clean.

Plus a lot a lot of hand work to get them to match.  I've benefited
greatly from their labours, but I have no interest in doing that myself!

What I'm mostly talking about is using a nice quiet accurate amp such
as an ADA4898 plus a faster but less accurate thing such as a THS3091
or LM6171.  The output amp is run at some fixed gain like 10, and the
input amp is run at high enough gain that the combination is stable
at quiescent conditions.

Other composite amps such as the one you mention or the common case
of using a chopamp to control the offset voltage of some
fast-but-ugly amplifier have a different set of problems.

Cheers

Phil Hobbs

And this one from Burr-Brown:

http://www.ti.com/lit/an/sboa002/sboa002.pdf

Yeah, that's the idea.  Problem is that it has horrible transient response.

Cheers

Phil Hobbs


Guessing this stool-design problem probably only has three legs, the
labor will in be picking the appropriate length of the legs to meet the
requirements without ending up with the other kind of stool.

Phil Hobbs
Guest

Mon May 28, 2018 1:45 am   



On 05/27/18 19:33, bitrex wrote:
Quote:
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_at_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

Phil Hobbs
Guest

Mon May 28, 2018 1:45 am   



On 05/27/18 19:31, bitrex wrote:
Quote:
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_at_electrooptical.net> wrote:

Hi, all,

I'm generally prejudiced against composite amplifiers (two op amps
inside one feedback loop) because they're generally squirrelly, with
poor settling performance and weird transient response.

On the other hand, my aversion to them means that I don't have as much
experience with them as do composite-amp fans.  So what do you folks
say
about them?

Orchids? Onions? Actual expertise?

Cheers

Phil Hobbs

I've only done it a little, in very special cases, but if the intent
is to apply a slow DC offset correction, and the main amp and the DC
trim amp don't overlap in frequency response, it seems to work fine.

If you want to make a general -6 dB/octave amp as a composite, the
risk is probably saturating one of the amps in large-signal/slewing
cases, or at leasy doing goofy things. A composite that clips clean
would be a challenge.

A sorta similar case is where a fast signal needs to be DC coupled
across a big DC offset. A capacitor is the fast path and some slow
opamp thing does the DC part before the AC path decays. The gains have
to both be the same, about 1.00 usually, and the frequency responses
need to be matched, to get clean step response and no ISI.

Not so easy!


Tek called this "feed-beside", a brutally fast but ugly signal path,
and slow stuff in parallel to make it clean.

Plus a lot a lot of hand work to get them to match.  I've benefited
greatly from their labours, but I have no interest in doing that myself!

What I'm mostly talking about is using a nice quiet accurate amp such
as an ADA4898 plus a faster but less accurate thing such as a THS3091
or LM6171.  The output amp is run at some fixed gain like 10, and the
input amp is run at high enough gain that the combination is stable at
quiescent conditions.

Other composite amps such as the one you mention or the common case of
using a chopamp to control the offset voltage of some fast-but-ugly
amplifier have a different set of problems.

Cheers

Phil Hobbs

This Wireless World article from '74 might be of interest:

http://www.keith-snook.info/wireless-world-magazine/Wireless-World-1974/Reducing%20Amplifier%20Distortion%20-%20Sandman.pdf


How so?

Cheers

Phil Hobbs


--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

Phil Hobbs
Guest

Mon May 28, 2018 1:45 am   



On 05/27/18 20:30, bitrex wrote:
Quote:
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_at_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

--
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

John Larkin
Guest

Mon May 28, 2018 3:45 pm   



On Sun, 27 May 2018 20:40:17 -0400, Phil Hobbs
<pcdhSpamMeSenseless_at_electrooptical.net> wrote:

Quote:
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_at_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


--

John Larkin Highland Technology, Inc trk

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

John Larkin
Guest

Mon May 28, 2018 4:45 pm   



On Mon, 28 May 2018 11:16:24 -0400, Phil Hobbs
<pcdhSpamMeSenseless_at_electrooptical.net> wrote:

Quote:
On 05/28/18 10:05, John Larkin wrote:
On Sun, 27 May 2018 20:40:17 -0400, Phil Hobbs
pcdhSpamMeSenseless_at_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_at_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.


--

John Larkin Highland Technology, Inc trk

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

Phil Hobbs
Guest

Mon May 28, 2018 4:45 pm   



On 05/28/18 11:25, John Larkin wrote:
Quote:
On Mon, 28 May 2018 14:59:11 GMT, Steve Wilson <no_at_spam.com> wrote:

John Larkin <jjlarkin_at_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

Phil Hobbs
Guest

Mon May 28, 2018 4:45 pm   



On 05/28/18 10:59, Steve Wilson wrote:
Quote:
John Larkin <jjlarkin_at_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

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