Composite amps

On Mon, 28 May 2018 15:26:58 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

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

I have a paper somewhere that discusses GBW-product and optimization
of the two-OpAmp composite. Quite nicely done, as I recall...
unfortunately I haven't re-found it :-(

...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| STV, Queen Creek, AZ 85142 Skype: skypeanalog | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

Thinking outside the box... producing elegant solutions,
by understanding what nature is hiding.

"It is not in doing what you like, but in liking what you do that
is the secret of happiness." -James Barrie
 
On 05/29/2018 11:11 AM, George Herold wrote:
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.

No warning feels extremely unfair, but having a couple last
conversations is extremely hard too. There's so much you want to say but
gosh you don't want to be too emotional or look like you're coming
unglued yourself! It feels "unfair" any way you slice it I suppose.

We mostly talked about his grandkids, some old movies he wanted to see
again, girls, and boats. Seems as reasonable an assortment of topics as
any other :)
 
On Tuesday, May 29, 2018 at 12:36:02 PM UTC-4, bitrex wrote:
On 05/29/2018 11:11 AM, George Herold wrote:
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.



No warning feels extremely unfair, but having a couple last
conversations is extremely hard too. There's so much you want to say but
gosh you don't want to be too emotional or look like you're coming
unglued yourself! It feels "unfair" any way you slice it I suppose.

We mostly talked about his grandkids, some old movies he wanted to see
again, girls, and boats. Seems as reasonable an assortment of topics as
any other :)

Yeah, life's not fair. :^) I keep telling my kids this, but I think
you only learn it through hard knocks. Me mum passed away about 1 year ago
now. Alzheimer's, which is a particularly icky way to go. When I heard,
after shock, my next emotion was relief.

George H.
 
On Tue, 29 May 2018 11:49:22 -0700 (PDT), George Herold
<gherold@teachspin.com> wrote:

On Tuesday, May 29, 2018 at 12:36:02 PM UTC-4, bitrex wrote:
On 05/29/2018 11:11 AM, George Herold wrote:
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.



No warning feels extremely unfair, but having a couple last
conversations is extremely hard too. There's so much you want to say but
gosh you don't want to be too emotional or look like you're coming
unglued yourself! It feels "unfair" any way you slice it I suppose.

We mostly talked about his grandkids, some old movies he wanted to see
again, girls, and boats. Seems as reasonable an assortment of topics as
any other :)

Yeah, life's not fair. :^) I keep telling my kids this, but I think
you only learn it through hard knocks. Me mum passed away about 1 year ago
now. Alzheimer's, which is a particularly icky way to go. When I heard,
after shock, my next emotion was relief.

Ouch! That is probably the hardest way to go, at least for the
family. My mother was getting pretty bad at the end but it wasn't
Alzheimer's. She was 95, with heart and kidney problems that were
taking their toll on the brain. I understand your comment about
"relief".
 
On Tue, 29 May 2018 08:11:42 -0700 (PDT), George Herold
<gherold@teachspin.com> wrote:

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.

My dad died suddenly at age 52, shoveling snow. I was 12 at the time.
 
On Tue, 29 May 2018 09:21:19 -0700, Jim Thompson wrote:

On Mon, 28 May 2018 15:26:58 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

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

I have a paper somewhere that discusses GBW-product and optimization of
the two-OpAmp composite. Quite nicely done, as I recall...
unfortunately I haven't re-found it :-(

...Jim Thompson

That sounds like something I would have saved if I had seen it, so I
searched my SED notes folder without luck but found some old app notes
and a post from Win Hill that might be of interest to someone. (Probably
old hat to Phil):

Analyzing feedback loops containing secondary amplifiers:
www.ti.com/lit/an/slyt103/slyt103.pdf

Burr Brown Application Bulletin
FEEDBACK PLOTS DEFINE OP AMP AC PERFORMANCE
www.ti.com/lit/an/sboa015/sboa015.pdf
(composite amp discussion pp 9-13)


From: Winfield Hill <Winfield_member@newsguy.com>
Newsgroups: sci.electronics.design
Subject: Re: Photodiode wich is fast enough to detect +50Mhz analog
(sinus) signal??
Date: 11 Aug 2004 03:31:18 -0700

Yannick wrote...
Winfield wrote...
Careful, using a sufficiently-wideband opamp can insure the summing-
junction impedance will be low compared to the total shunt
capacitance.
Resistors have 0.05pF to 0.1pF of self capacitance, this should be
your
total feedback capacitance. With 3k resistor you'd have a -3dB
rolloff
at 530MHz. You want high R for low noise, so we'll try 100k, yielding
a 16MHz rolloff. Then we can apply the standard R-C-R trick (this is
more than 30 years old) to get a flat frequency response to 75MHz, or
whatever you decide your bandwidth should be.

R-C-R trick to get a flat frequency responce, i never heard of this
before, can you explain how it works?

I posted the circuit previously in this thread (20 July) and described
in detail how it works. Here's the circuit again:

| Rf R2 adjustable
| ,---/\/\---+---/\/\--/\/\----,
| | '--||--' | C2 R3 | nA-sensitivity wideband
| | Cf '--||--/\/\-- gnd | transresistance amplifier
| | |
| | __ ,-||--/\/\--+ correction network details
| input O--+---|+ \ | __ | R2 C2 = Rf Cf
| | >-+-/\/\-+-|- \ | R3 C2 sets bandwidth
| ,-|-_/ | | >-----+---
| | | gnd --|+_/
| gnd --/\/\--+-/\/\--' composite amplifier

The undesired Rf capacitance Cf is canceled by the R2 C2 network.
R3 is used to limit the upper frequency of this cancellation.

R2 C2 and R3 constitute the standard R-C-R trick. I thought of this
about 18 years ago, and have used it with great success since then.
Later I learned that it had been described in an old Keithley manual,
and probably in many other places years before that.

BTW, the circuit above will outperform (sensitivity, bandwidth, SNR,
phase accuracy) any of the resonant schemes you've been contemplating
here. It's not true that feedback makes things more noisy. In this
circuit feedback (and a high-performance composite amplifier) insures
that all of the signal current is used by the amplifier, rather than
becoming uselessly drained away by the input-node capacitance. Thus
feedback actually improves the SNR. Using a resonant input doesn't
solve the capacitance problem because if a high enough Q is used for
a solution, it simply creates insurmountable phase-error problems.



--
Thanks,
- Win
 
On Mon, 28 May 2018 11:58:52 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

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

Well, it's refreshing to, once in a while, swap a screen for the
underside of a bathroom sink.

But really, most consumer products, including plumbing, are such
trash.


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com
 
Zero DC offset low noise composite TIA for high capacitance photodides
http://ixbt.photo/?id=photo:1329265

2200pF input capacitance
http://ixbt.photo/?id=photo:1329266

Zero-drift OPA189 composite TIA
http://ixbt.photo/?id=photo:1329269

Inductance LB3218T102K (1207 smd)
 
On Fri, 21 Feb 2020 12:37:03 -0800 (PST), plastcontrol.ru@gmail.com
wrote:

Zero DC offset low noise composite TIA for high capacitance photodides
http://ixbt.photo/?id=photo:1329265

2200pF input capacitance
http://ixbt.photo/?id=photo:1329266

Zero-drift OPA189 composite TIA
http://ixbt.photo/?id=photo:1329269

Inductance LB3218T102K (1207 smd)

Just a note: a photodiode has two leads.

--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com
 
On Fri, 21 Feb 2020 13:11:52 -0800 (PST), plastcontrol.ru@gmail.com
wrote:

>Capacitor 2200pF is my photodiode.

That's a lot. You could drive a cascode emitter to keep the load
impedance way down on the photodiode. The transistor collector, much
lower capacitance, can drive the TIA.

The problem there is that you should have a standing current in the
cascode transistor, to keep the emitter impedance low. That makes an
offset and maybe messes up the TIA.

Hence the notion that a photodiode has two leads. Take the fast stuff
from one end and the slow signal from the other. It's the Tektronix
"feed-beside" idea.

I'm expecting the first article of my GHz o/e converter any day now.
It uses the feed-beside idea.

--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com
 
Internal input capacitance OPA140 :
differential Cd = 10 pF
common mode Ccm = 7 pF

Cascode transistor or voltage follower useless in this case.

Zero bias voltage photodiode boosted TIA :

http://ixbt.photo/?id=photo:1329322

OPA140 with internal compensation cap - amplifier like integrator.
 
Metamorphoses :

http://ixbt.photo/?id=photo:1329419

Bootstrapping TIA grounded photodiode
 
On Saturday, February 22, 2020 at 4:56:56 PM UTC-5, plastco...@gmail.com wrote:
Metamorphoses :

http://ixbt.photo/?id=photo:1329419

Bootstrapping TIA grounded photodiode

Dmitriy, What's the advantage of this? At one point in your
circuit evolution I thought you were going to move the non-inverting
input away from ground to bias the photodiode (PD).
(You can reduce PD capacitance ~factor of 3 or so with reverse bias.)
Have you read Phil Hobb's book? Most of his ideas can be found
for free online... check his website.

George H.
 
1) grounded detector, grounded source/emitter HF transistor
2) true zero-bias operation of detector
3) my circuit is simpler
I like to read books))
 
On Monday, February 24, 2020 at 6:55:54 AM UTC-5, plastco...@gmail.com wrote:
1) grounded detector, grounded source/emitter HF transistor
2) true zero-bias operation of detector
3) my circuit is simpler
I like to read books))

OK, what do you find better about zero bias operation?
I should admit that for many years I ran all my PD's at
zero bias. I thought this gave me better 'zero' light detection.
(No DC offset with no light... but the dark current from
PDs is generally pretty low.)
Running with some bias has two main advantages.
1.) reduced C.. faster
2.) Higher saturation current (light intensity) without bias the
electrons build up in the junction and it saturates.. more light
gives no more electrons.

George H.
(who is addicted to reading... I need to find a few new fiction writers)
 
On 2020-02-24 11:47, George Herold wrote:
On Monday, February 24, 2020 at 6:55:54 AM UTC-5, plastco...@gmail.com wrote:
1) grounded detector, grounded source/emitter HF transistor
2) true zero-bias operation of detector
3) my circuit is simpler
I like to read books))

OK, what do you find better about zero bias operation?
I should admit that for many years I ran all my PD's at
zero bias. I thought this gave me better 'zero' light detection.
(No DC offset with no light... but the dark current from
PDs is generally pretty low.)
Running with some bias has two main advantages.
1.) reduced C.. faster
2.) Higher saturation current (light intensity) without bias the
electrons build up in the junction and it saturates.. more light
gives no more electrons.

George H.
(who is addicted to reading... I need to find a few new fiction writers)

Zero bias is better in one respect: you can get zero leakage current.
For jobs such as very wide range, very slow photometers, that's a win.
Garry Epeldauer et al. wrote a beautiful paper about getting 14 orders
of magnitude in photocurrent, if you don't mind being stuck with
millihertz bandwidths:

<https://electrooptical.net/www/optics/eppeldauer14decadephotocurrent.pdf>

Crappy PN photodiodes and solar cells don't respond well to large
reverse bias either.

For just about anything else, zero bias is a complete crock.

With almost any PIN diode, APD, MPPC, (etc) zero bias is a disaster.
Applying reverse bias to a PIN diode can reduce its capacitance by a
factor of 7 or so, which reduces the high frequency noise by the same
factor.

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
 
On Mon, 24 Feb 2020 14:47:33 -0500, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2020-02-24 11:47, George Herold wrote:
On Monday, February 24, 2020 at 6:55:54 AM UTC-5, plastco...@gmail.com wrote:
1) grounded detector, grounded source/emitter HF transistor
2) true zero-bias operation of detector
3) my circuit is simpler
I like to read books))

OK, what do you find better about zero bias operation?
I should admit that for many years I ran all my PD's at
zero bias. I thought this gave me better 'zero' light detection.
(No DC offset with no light... but the dark current from
PDs is generally pretty low.)
Running with some bias has two main advantages.
1.) reduced C.. faster
2.) Higher saturation current (light intensity) without bias the
electrons build up in the junction and it saturates.. more light
gives no more electrons.

George H.
(who is addicted to reading... I need to find a few new fiction writers)


Zero bias is better in one respect: you can get zero leakage current.
For jobs such as very wide range, very slow photometers, that's a win.
Garry Epeldauer et al. wrote a beautiful paper about getting 14 orders
of magnitude in photocurrent, if you don't mind being stuck with
millihertz bandwidths:

https://electrooptical.net/www/optics/eppeldauer14decadephotocurrent.pdf

Crappy PN photodiodes and solar cells don't respond well to large
reverse bias either.

For just about anything else, zero bias is a complete crock.

With almost any PIN diode, APD, MPPC, (etc) zero bias is a disaster.
Applying reverse bias to a PIN diode can reduce its capacitance by a
factor of 7 or so, which reduces the high frequency noise by the same
factor.

Cheers

Phil Hobbs

I know of a large organization that has wasted about a million dollars
a year, since 2002, by running a lot of very expensive Hamamatsu
photodiodes at zero bias.

https://www.dropbox.com/s/wm3a3cpxa8tcarg/S8551_1.JPG?raw=1

--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com
 
On Monday, February 24, 2020 at 2:47:45 PM UTC-5, Phil Hobbs wrote:
On 2020-02-24 11:47, George Herold wrote:
On Monday, February 24, 2020 at 6:55:54 AM UTC-5, plastco...@gmail.com wrote:
1) grounded detector, grounded source/emitter HF transistor
2) true zero-bias operation of detector
3) my circuit is simpler
I like to read books))

OK, what do you find better about zero bias operation?
I should admit that for many years I ran all my PD's at
zero bias. I thought this gave me better 'zero' light detection.
(No DC offset with no light... but the dark current from
PDs is generally pretty low.)
Running with some bias has two main advantages.
1.) reduced C.. faster
2.) Higher saturation current (light intensity) without bias the
electrons build up in the junction and it saturates.. more light
gives no more electrons.

George H.
(who is addicted to reading... I need to find a few new fiction writers)


Zero bias is better in one respect: you can get zero leakage current.
For jobs such as very wide range, very slow photometers, that's a win.
Garry Epeldauer et al. wrote a beautiful paper about getting 14 orders
of magnitude in photocurrent, if you don't mind being stuck with
millihertz bandwidths:

https://electrooptical.net/www/optics/eppeldauer14decadephotocurrent.pdf

Crappy PN photodiodes and solar cells don't respond well to large
reverse bias either.

For just about anything else, zero bias is a complete crock.
(Well thanks for trashing the first ~20 years of my PD career. :^)

In defense of zero bias it's got great simplicity, doesn't
break if put in backwards*. And if your DMM has a 200uA scale
a ~10mm^2 PD makes an easy light detector. Lots of applications
don't care so much about speed.
(a bow to the lm324, I never used one, but enjoyed the schematics.)

With almost any PIN diode, APD, MPPC, (etc) zero bias is a disaster.
Applying reverse bias to a PIN diode can reduce its capacitance by a
factor of 7 or so, which reduces the high frequency noise by the same
factor.

George H.
*I've never tried breaking a PD by forward biasing hard.
my bpw34 looks about the same volume as a 1/4 watt resistor,
so ~1/4 W, 0.6V and 0.4A

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
 
On 2020-02-24 15:13, John Larkin wrote:
On Mon, 24 Feb 2020 14:47:33 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2020-02-24 11:47, George Herold wrote:
On Monday, February 24, 2020 at 6:55:54 AM UTC-5, plastco...@gmail.com wrote:
1) grounded detector, grounded source/emitter HF transistor
2) true zero-bias operation of detector
3) my circuit is simpler
I like to read books))

OK, what do you find better about zero bias operation?
I should admit that for many years I ran all my PD's at
zero bias. I thought this gave me better 'zero' light detection.
(No DC offset with no light... but the dark current from
PDs is generally pretty low.)
Running with some bias has two main advantages.
1.) reduced C.. faster
2.) Higher saturation current (light intensity) without bias the
electrons build up in the junction and it saturates.. more light
gives no more electrons.

George H.
(who is addicted to reading... I need to find a few new fiction writers)


Zero bias is better in one respect: you can get zero leakage current.
For jobs such as very wide range, very slow photometers, that's a win.
Garry Epeldauer et al. wrote a beautiful paper about getting 14 orders
of magnitude in photocurrent, if you don't mind being stuck with
millihertz bandwidths:

https://electrooptical.net/www/optics/eppeldauer14decadephotocurrent.pdf

Crappy PN photodiodes and solar cells don't respond well to large
reverse bias either.

For just about anything else, zero bias is a complete crock.

With almost any PIN diode, APD, MPPC, (etc) zero bias is a disaster.
Applying reverse bias to a PIN diode can reduce its capacitance by a
factor of 7 or so, which reduces the high frequency noise by the same
factor.

Cheers

Phil Hobbs

I know of a large organization that has wasted about a million dollars
a year, since 2002, by running a lot of very expensive Hamamatsu
photodiodes at zero bias.

https://www.dropbox.com/s/wm3a3cpxa8tcarg/S8551_1.JPG?raw=1
I suspect I know the organization. ;)
They have some very good folks though.

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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