Opamps as voltage sources (C-loading)

[George Herold]

So I've got this lmxxx (317, x05) voltage bias source..
(V bias = 5V, 5ma max. i'm loading it ) that I can
put in some corner of this circuit board. Or there's this left
over opamp, (I think it's like a 5mA max for the sensor, and I'm
loading it much less than that... (maybe I should try more R?)

Anyway, I was thinking about JL, and with more C-load it got
'better' again.
And searching for C-loding of opmap.. I got these,
https://www.electronicdesign.com/power/what-s-all-capacitive-loading-stuff-anyhow

Which says.. at high C it's the cap ESR loading the opamp.
(I built an inverting and non-inverting opamp voltage soucre
on a piece of copper clad today, I'm only up to 100nF, the
opamp's not railing but it's getting bad.

And this,
https://ww1.microchip.com/downloads/en/appnotes/00884a.pdf

(which I think I've read before. I've used both
circuits 7 and 12 in the past. (I never realized the
inverting circuit was so bad!)
So Re: 7 and 12, in both those I've hung a BFC
(BFC >1 uF ) across the feedback R...
Is that just too ugly?

There is also the circuit that uses two feedback paths
around the opamp, fast C and Rf after added R_out.

George H.

 

[George Herold]

On Monday, April 29, 2019 at 9:12:05 PM UTC-4, George Herold wrote:

So I've got this lmxxx (317, x05) voltage bias source..
(V bias = 5V, 5ma max. i'm loading it ) that I can
put in some corner of this circuit board. Or there's this left
over opamp, (I think it's like a 5mA max for the sensor, and I'm
loading it much less than that... (maybe I should try more R?)

Anyway, I was thinking about JL, and with more C-load it got
'better' again.
And searching for C-loding of opmap.. I got these,
https://www.electronicdesign.com/power/what-s-all-capacitive-loading-stuff-anyhow

Which says.. at high C it's the cap ESR loading the opamp.
(I built an inverting and non-inverting opamp voltage soucre
on a piece of copper clad today, I'm only up to 100nF, the
opamp's not railing but it's getting bad.

And this,
https://ww1.microchip.com/downloads/en/appnotes/00884a.pdf

(which I think I've read before. I've used both
circuits 7 and 12 in the past. (I never realized the
inverting circuit was so bad!)
So Re: 7 and 12, in both those I've hung a BFC
(BFC >1 uF ) across the feedback R...
Is that just too ugly?

There is also the circuit that uses two feedback paths
around the opamp, fast C and Rf after added R_out.

George H.

Sorry, the beginning of that needs some serious editing.
GH

 

[George Herold]

On Monday, April 29, 2019 at 9:18:30 PM UTC-4, George Herold wrote:

On Monday, April 29, 2019 at 9:12:05 PM UTC-4, George Herold wrote:
So I've got this lmxxx (317, x05) voltage bias source..
(V bias = 5V, 5ma max. i'm loading it ) that I can
put in some corner of this circuit board. Or there's this left
over opamp, (I think it's like a 5mA max for the sensor, and I'm
loading it much less than that... (maybe I should try more R?)

Anyway, I was thinking about JL, and with more C-load it got
'better' again.
And searching for C-loding of opmap.. I got these,
https://www.electronicdesign.com/power/what-s-all-capacitive-loading-stuff-anyhow

Which says.. at high C it's the cap ESR loading the opamp.
(I built an inverting and non-inverting opamp voltage soucre
on a piece of copper clad today, I'm only up to 100nF, the
opamp's not railing but it's getting bad.

And this,
https://ww1.microchip.com/downloads/en/appnotes/00884a.pdf

(which I think I've read before. I've used both
circuits 7 and 12 in the past. (I never realized the
inverting circuit was so bad!)
So Re: 7 and 12, in both those I've hung a BFC
(BFC >1 uF ) across the feedback R...
Is that just too ugly?

There is also the circuit that uses two feedback paths
around the opamp, fast C and Rf after added R_out.

George H.

Sorry, the beginning of that needs some serious editing.
GH

What about these... they come in 'p-dips'*
https://www.analog.com/media/en/technical-documentation/data-sheets/13556fc.pdf

George H.
* to be said in the same voice as Pippin in the movie version of
LTR, 'they come in pints!'

 

[John Larkin]

On Mon, 29 Apr 2019 18:12:01 -0700 (PDT), George Herold
<gherold@teachspin.com> wrote:

So I've got this lmxxx (317, x05) voltage bias source..
(V bias = 5V, 5ma max. i'm loading it ) that I can
put in some corner of this circuit board. Or there's this left
over opamp, (I think it's like a 5mA max for the sensor, and I'm
loading it much less than that... (maybe I should try more R?)

Anyway, I was thinking about JL, and with more C-load it got
'better' again.
And searching for C-loding of opmap.. I got these,
https://www.electronicdesign.com/power/what-s-all-capacitive-loading-stuff-anyhow

Which says.. at high C it's the cap ESR loading the opamp.
(I built an inverting and non-inverting opamp voltage soucre
on a piece of copper clad today, I'm only up to 100nF, the
opamp's not railing but it's getting bad.

And this,
https://ww1.microchip.com/downloads/en/appnotes/00884a.pdf

(which I think I've read before. I've used both
circuits 7 and 12 in the past. (I never realized the
inverting circuit was so bad!)
So Re: 7 and 12, in both those I've hung a BFC
(BFC >1 uF ) across the feedback R...
Is that just too ugly?

There is also the circuit that uses two feedback paths
around the opamp, fast C and Rf after added R_out.

George H.

This sort of explains why many RRIO opamps are c-load stable.

https://www.dropbox.com/s/u6omebxsxl14xuo/Opamp_C-load.JPG?dl=0

The load cap doesn't add a pole, it just shifts the break frequency
down.

Some RRIOs ring with modest cap loading but get very stable with more,
like several uF.

ESR doesn't do any harm here.



--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[Tim Williams]

Use this method. Most general:
https://www.seventransistorlabs.com/Images/Opamps.png
(Left side is output to connector; the "BAV99" just cutoff betrays the ESD
diodes for said connector.)

As mentioned, in large values the ESR dominates, at least when it does
(probably avoid bare ceramics, unless stated otherwise by the amp
datasheet?). That plays the role of the R57/R65 shown above. Since it's in
series with the capacitance, there's no DC error and no DC feedback path
necessary to correct its voltage drop, so the RC around the outside also
goes away. Downside, you can't compensate the amp independently in this
case, so the ESR and C must be within the range that the amp is comfortable
with.

The effect of the above circuit is to add a pole and zero to the response
(when the load is capacitive). When they coincide, the response is flat
until cutoff, otherwise there is a peak or dip before cutoff. You can't, in
general, compensate for _any_ load this way, only the worst case, and keep
the wonkiness at high enough frequencies that are hopefully out of your
passband.

Note that you can still use low-ESR capacitors, as long as the dominant
capacitance, at the critical (crossover) frequency, is lossy (ESR = 1 /
(2*pi*Fc*C)). It's not enough to throw in a few dozen 0.1 ceramics and a
10uF electrolytic of unknown ESR for any random amp, the crossover frequency
needs to be right, too.

Tim

--
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Design
Website: https://www.seventransistorlabs.com/

"George Herold" <gherold@teachspin.com> wrote in message
news:5bc9497b-3c38-4830-8aac-00ed120f78e0@googlegroups.com...

So I've got this lmxxx (317, x05) voltage bias source..
(V bias = 5V, 5ma max. i'm loading it ) that I can
put in some corner of this circuit board. Or there's this left
over opamp, (I think it's like a 5mA max for the sensor, and I'm
loading it much less than that... (maybe I should try more R?)

Anyway, I was thinking about JL, and with more C-load it got
'better' again.
And searching for C-loding of opmap.. I got these,
https://www.electronicdesign.com/power/what-s-all-capacitive-loading-stuff-anyhow

Which says.. at high C it's the cap ESR loading the opamp.
(I built an inverting and non-inverting opamp voltage soucre
on a piece of copper clad today, I'm only up to 100nF, the
opamp's not railing but it's getting bad.

And this,
https://ww1.microchip.com/downloads/en/appnotes/00884a.pdf

(which I think I've read before. I've used both
circuits 7 and 12 in the past. (I never realized the
inverting circuit was so bad!)
So Re: 7 and 12, in both those I've hung a BFC
(BFC >1 uF ) across the feedback R...
Is that just too ugly?

There is also the circuit that uses two feedback paths
around the opamp, fast C and Rf after added R_out.

George H.

 

On Tuesday, 30 April 2019 08:49:59 UTC+1, Tim Williams wrote:

As mentioned, in large values the ESR dominates, at least when it does
(probably avoid bare ceramics, unless stated otherwise by the amp
datasheet?).

Why does opamp R_out not dominate?


NT

 

[George Herold]

On Monday, April 29, 2019 at 11:07:06 PM UTC-4, John Larkin wrote:

On Mon, 29 Apr 2019 18:12:01 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

So I've got this lmxxx (317, x05) voltage bias source..
(V bias = 5V, 5ma max. i'm loading it ) that I can
put in some corner of this circuit board. Or there's this left
over opamp, (I think it's like a 5mA max for the sensor, and I'm
loading it much less than that... (maybe I should try more R?)

Anyway, I was thinking about JL, and with more C-load it got
'better' again.
And searching for C-loding of opmap.. I got these,
https://www.electronicdesign.com/power/what-s-all-capacitive-loading-stuff-anyhow

Which says.. at high C it's the cap ESR loading the opamp.
(I built an inverting and non-inverting opamp voltage soucre
on a piece of copper clad today, I'm only up to 100nF, the
opamp's not railing but it's getting bad.

And this,
https://ww1.microchip.com/downloads/en/appnotes/00884a.pdf

(which I think I've read before. I've used both
circuits 7 and 12 in the past. (I never realized the
inverting circuit was so bad!)
So Re: 7 and 12, in both those I've hung a BFC
(BFC >1 uF ) across the feedback R...
Is that just too ugly?

There is also the circuit that uses two feedback paths
around the opamp, fast C and Rf after added R_out.

George H.


This sort of explains why many RRIO opamps are c-load stable.

https://www.dropbox.com/s/u6omebxsxl14xuo/Opamp_C-load.JPG?dl=0

The load cap doesn't add a pole, it just shifts the break frequency
down.

Well your picture doesn't really help me. But it's the compensation cap
that causes the phase shift and makes the output impedance look sorta
inductive at higher frequencies...

Anyway I've got some opa2192's that are RRIO I'll see what they look like.
figure's 32 and 33 here,
http://www.ti.com/lit/ds/symlink/opa192.pdf

I can certainly add 50 ohms in series to the output.

George H.

Some RRIOs ring with modest cap loading but get very stable with more,
like several uF.

ESR doesn't do any harm here.



--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[George Herold]

On Tuesday, April 30, 2019 at 3:49:59 AM UTC-4, Tim Williams wrote:

Use this method. Most general:
https://www.seventransistorlabs.com/Images/Opamps.png
(Left side is output to connector; the "BAV99" just cutoff betrays the ESD
diodes for said connector.)
Right I've done that. As you say below it's a little tuned to the load C.

George h.

As mentioned, in large values the ESR dominates, at least when it does
(probably avoid bare ceramics, unless stated otherwise by the amp
datasheet?). That plays the role of the R57/R65 shown above. Since it's in
series with the capacitance, there's no DC error and no DC feedback path
necessary to correct its voltage drop, so the RC around the outside also
goes away. Downside, you can't compensate the amp independently in this
case, so the ESR and C must be within the range that the amp is comfortable
with.

The effect of the above circuit is to add a pole and zero to the response
(when the load is capacitive). When they coincide, the response is flat
until cutoff, otherwise there is a peak or dip before cutoff. You can't, in
general, compensate for _any_ load this way, only the worst case, and keep
the wonkiness at high enough frequencies that are hopefully out of your
passband.

Note that you can still use low-ESR capacitors, as long as the dominant
capacitance, at the critical (crossover) frequency, is lossy (ESR = 1 /
(2*pi*Fc*C)). It's not enough to throw in a few dozen 0.1 ceramics and a
10uF electrolytic of unknown ESR for any random amp, the crossover frequency
needs to be right, too.

Tim

--
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Design
Website: https://www.seventransistorlabs.com/

"George Herold" <gherold@teachspin.com> wrote in message
news:5bc9497b-3c38-4830-8aac-00ed120f78e0@googlegroups.com...
So I've got this lmxxx (317, x05) voltage bias source..
(V bias = 5V, 5ma max. i'm loading it ) that I can
put in some corner of this circuit board. Or there's this left
over opamp, (I think it's like a 5mA max for the sensor, and I'm
loading it much less than that... (maybe I should try more R?)

Anyway, I was thinking about JL, and with more C-load it got
'better' again.
And searching for C-loding of opmap.. I got these,
https://www.electronicdesign.com/power/what-s-all-capacitive-loading-stuff-anyhow

Which says.. at high C it's the cap ESR loading the opamp.
(I built an inverting and non-inverting opamp voltage soucre
on a piece of copper clad today, I'm only up to 100nF, the
opamp's not railing but it's getting bad.

And this,
https://ww1.microchip.com/downloads/en/appnotes/00884a.pdf

(which I think I've read before. I've used both
circuits 7 and 12 in the past. (I never realized the
inverting circuit was so bad!)
So Re: 7 and 12, in both those I've hung a BFC
(BFC >1 uF ) across the feedback R...
Is that just too ugly?

There is also the circuit that uses two feedback paths
around the opamp, fast C and Rf after added R_out.

George H.

 

I can certainly add 50 ohms in series to the output.

right...

and take the DC feedback after the R and the AC feedback before the R.

m

 

[John Larkin]

On Tue, 30 Apr 2019 06:17:15 -0700 (PDT), George Herold
<gherold@teachspin.com> wrote:

On Monday, April 29, 2019 at 11:07:06 PM UTC-4, John Larkin wrote:
On Mon, 29 Apr 2019 18:12:01 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

So I've got this lmxxx (317, x05) voltage bias source..
(V bias = 5V, 5ma max. i'm loading it ) that I can
put in some corner of this circuit board. Or there's this left
over opamp, (I think it's like a 5mA max for the sensor, and I'm
loading it much less than that... (maybe I should try more R?)

Anyway, I was thinking about JL, and with more C-load it got
'better' again.
And searching for C-loding of opmap.. I got these,
https://www.electronicdesign.com/power/what-s-all-capacitive-loading-stuff-anyhow

Which says.. at high C it's the cap ESR loading the opamp.
(I built an inverting and non-inverting opamp voltage soucre
on a piece of copper clad today, I'm only up to 100nF, the
opamp's not railing but it's getting bad.

And this,
https://ww1.microchip.com/downloads/en/appnotes/00884a.pdf

(which I think I've read before. I've used both
circuits 7 and 12 in the past. (I never realized the
inverting circuit was so bad!)
So Re: 7 and 12, in both those I've hung a BFC
(BFC >1 uF ) across the feedback R...
Is that just too ugly?

There is also the circuit that uses two feedback paths
around the opamp, fast C and Rf after added R_out.

George H.


This sort of explains why many RRIO opamps are c-load stable.

https://www.dropbox.com/s/u6omebxsxl14xuo/Opamp_C-load.JPG?dl=0

The load cap doesn't add a pole, it just shifts the break frequency
down.
Well your picture doesn't really help me. But it's the compensation cap
that causes the phase shift and makes the output impedance look sorta
inductive at higher frequencies...

Sorry, too cryptic.

In a classic opamp, there is an internal compensation cap that adds a
-6 dB/octave frequency rolloff, namely 90 degrees of phase lag. That's
stable when you close a feedback loop. An external load cap adds
another 90 degrees of lag, which makes 180, which turns negative
feedback positive at some frequency, so it oscillates.

In a c-load amp, the external load c is *in parallel* with the opamp's
compensation cap(s) so the frequency rolloff just gets slower as you
add load caps, but remains single-pole, -6 dB/octave, still 90 degrees
of phase shift, so still stable. Net output impedance goes down at all
frequencies as load cap is added.

There are some great parts in the OPA1xx family.


--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[John Larkin]

On Mon, 29 Apr 2019 19:00:28 -0700 (PDT), George Herold
<gherold@teachspin.com> wrote:

On Monday, April 29, 2019 at 9:18:30 PM UTC-4, George Herold wrote:
On Monday, April 29, 2019 at 9:12:05 PM UTC-4, George Herold wrote:
So I've got this lmxxx (317, x05) voltage bias source..
(V bias = 5V, 5ma max. i'm loading it ) that I can
put in some corner of this circuit board. Or there's this left
over opamp, (I think it's like a 5mA max for the sensor, and I'm
loading it much less than that... (maybe I should try more R?)

Anyway, I was thinking about JL, and with more C-load it got
'better' again.
And searching for C-loding of opmap.. I got these,
https://www.electronicdesign.com/power/what-s-all-capacitive-loading-stuff-anyhow

Which says.. at high C it's the cap ESR loading the opamp.
(I built an inverting and non-inverting opamp voltage soucre
on a piece of copper clad today, I'm only up to 100nF, the
opamp's not railing but it's getting bad.

And this,
https://ww1.microchip.com/downloads/en/appnotes/00884a.pdf

(which I think I've read before. I've used both
circuits 7 and 12 in the past. (I never realized the
inverting circuit was so bad!)
So Re: 7 and 12, in both those I've hung a BFC
(BFC >1 uF ) across the feedback R...
Is that just too ugly?

There is also the circuit that uses two feedback paths
around the opamp, fast C and Rf after added R_out.

George H.

Sorry, the beginning of that needs some serious editing.
GH

What about these... they come in 'p-dips'*
https://www.analog.com/media/en/technical-documentation/data-sheets/13556fc.pdf

George H.
* to be said in the same voice as Pippin in the movie version of
LTR, 'they come in pints!'

LM8261 is cheaper; it's an official c-load opamp.

OPA197 is even better to have around. It's happy with a few (or many)
uF of ceramic or film caps from output to ground. It will oscillate as
a follower with 47 nF.

Pdip? Like this?

https://www.tubesandmore.com/sites/default/files/uc_products/p-sp8-476.png




--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[John Larkin]

On Tue, 30 Apr 2019 08:56:41 -0700 (PDT), George Herold
<gherold@teachspin.com> wrote:

On Tuesday, April 30, 2019 at 10:37:42 AM UTC-4, John Larkin wrote:
On Tue, 30 Apr 2019 06:17:15 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

On Monday, April 29, 2019 at 11:07:06 PM UTC-4, John Larkin wrote:
On Mon, 29 Apr 2019 18:12:01 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

So I've got this lmxxx (317, x05) voltage bias source..
(V bias = 5V, 5ma max. i'm loading it ) that I can
put in some corner of this circuit board. Or there's this left
over opamp, (I think it's like a 5mA max for the sensor, and I'm
loading it much less than that... (maybe I should try more R?)

Anyway, I was thinking about JL, and with more C-load it got
'better' again.
And searching for C-loding of opmap.. I got these,
https://www.electronicdesign.com/power/what-s-all-capacitive-loading-stuff-anyhow

Which says.. at high C it's the cap ESR loading the opamp.
(I built an inverting and non-inverting opamp voltage soucre
on a piece of copper clad today, I'm only up to 100nF, the
opamp's not railing but it's getting bad.

And this,
https://ww1.microchip.com/downloads/en/appnotes/00884a.pdf

(which I think I've read before. I've used both
circuits 7 and 12 in the past. (I never realized the
inverting circuit was so bad!)
So Re: 7 and 12, in both those I've hung a BFC
(BFC >1 uF ) across the feedback R...
Is that just too ugly?

There is also the circuit that uses two feedback paths
around the opamp, fast C and Rf after added R_out.

George H.


This sort of explains why many RRIO opamps are c-load stable.

https://www.dropbox.com/s/u6omebxsxl14xuo/Opamp_C-load.JPG?dl=0

The load cap doesn't add a pole, it just shifts the break frequency
down.
Well your picture doesn't really help me. But it's the compensation cap
that causes the phase shift and makes the output impedance look sorta
inductive at higher frequencies...

Sorry, too cryptic.

In a classic opamp, there is an internal compensation cap that adds a
-6 dB/octave frequency rolloff, namely 90 degrees of phase lag. That's
stable when you close a feedback loop. An external load cap adds
another 90 degrees of lag, which makes 180, which turns negative
feedback positive at some frequency, so it oscillates.

In a c-load amp, the external load c is *in parallel* with the opamp's
compensation cap(s) so the frequency rolloff just gets slower as you
So looking back at your previous pic, for the RRIO opamp the load C is
in parallel, because the one side (of C-comp) is common,
and the other side is a base-emitter junction away from the power rail...
(which is the same as ground for HF.)

Right. Looking into the output pin, the opamp looks like a Miller cap.
The added Cload is in parallel with that. Sort of.

add load caps, but remains single-pole, -6 dB/octave, still 90 degrees
of phase shift, so still stable. Net output impedance goes down at all
frequencies as load cap is added.

There are some great parts in the OPA1xx family.

The OPA192 worked fine at all C-loads... I messed around with the
circuit and got it to ring.. worse was with ~0.1uF and then a ten
ohm series R on the output killed that and made it sweet.
'scope shot
https://www.dropbox.com/s/av6lav5cwe4ll3d/Opamp-pwr.bmp?dl=0

Try a bigger cap. That will often stabilize an rrio opamp nicely.


--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[George Herold]

On Tuesday, April 30, 2019 at 10:37:42 AM UTC-4, John Larkin wrote:

On Tue, 30 Apr 2019 06:17:15 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

On Monday, April 29, 2019 at 11:07:06 PM UTC-4, John Larkin wrote:
On Mon, 29 Apr 2019 18:12:01 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

So I've got this lmxxx (317, x05) voltage bias source..
(V bias = 5V, 5ma max. i'm loading it ) that I can
put in some corner of this circuit board. Or there's this left
over opamp, (I think it's like a 5mA max for the sensor, and I'm
loading it much less than that... (maybe I should try more R?)

Anyway, I was thinking about JL, and with more C-load it got
'better' again.
And searching for C-loding of opmap.. I got these,
https://www.electronicdesign.com/power/what-s-all-capacitive-loading-stuff-anyhow

Which says.. at high C it's the cap ESR loading the opamp.
(I built an inverting and non-inverting opamp voltage soucre
on a piece of copper clad today, I'm only up to 100nF, the
opamp's not railing but it's getting bad.

And this,
https://ww1.microchip.com/downloads/en/appnotes/00884a.pdf

(which I think I've read before. I've used both
circuits 7 and 12 in the past. (I never realized the
inverting circuit was so bad!)
So Re: 7 and 12, in both those I've hung a BFC
(BFC >1 uF ) across the feedback R...
Is that just too ugly?

There is also the circuit that uses two feedback paths
around the opamp, fast C and Rf after added R_out.

George H.


This sort of explains why many RRIO opamps are c-load stable.

https://www.dropbox.com/s/u6omebxsxl14xuo/Opamp_C-load.JPG?dl=0

The load cap doesn't add a pole, it just shifts the break frequency
down.
Well your picture doesn't really help me. But it's the compensation cap
that causes the phase shift and makes the output impedance look sorta
inductive at higher frequencies...

Sorry, too cryptic.

In a classic opamp, there is an internal compensation cap that adds a
-6 dB/octave frequency rolloff, namely 90 degrees of phase lag. That's
stable when you close a feedback loop. An external load cap adds
another 90 degrees of lag, which makes 180, which turns negative
feedback positive at some frequency, so it oscillates.

In a c-load amp, the external load c is *in parallel* with the opamp's
compensation cap(s) so the frequency rolloff just gets slower as you

So looking back at your previous pic, for the RRIO opamp the load C is
in parallel, because the one side (of C-comp) is common,
and the other side is a base-emitter junction away from the power rail...
(which is the same as ground for HF.)

add load caps, but remains single-pole, -6 dB/octave, still 90 degrees
of phase shift, so still stable. Net output impedance goes down at all
frequencies as load cap is added.

There are some great parts in the OPA1xx family.

The OPA192 worked fine at all C-loads... I messed around with the
circuit and got it to ring.. worse was with ~0.1uF and then a ten
ohm series R on the output killed that and made it sweet.
'scope shot
https://www.dropbox.com/s/av6lav5cwe4ll3d/Opamp-pwr.bmp?dl=0

Hey as always, thanks for the help.

George H.

--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[George Herold]

On Tuesday, April 30, 2019 at 12:18:30 PM UTC-4, John Larkin wrote:

On Tue, 30 Apr 2019 08:56:41 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

On Tuesday, April 30, 2019 at 10:37:42 AM UTC-4, John Larkin wrote:
On Tue, 30 Apr 2019 06:17:15 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

On Monday, April 29, 2019 at 11:07:06 PM UTC-4, John Larkin wrote:
On Mon, 29 Apr 2019 18:12:01 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

So I've got this lmxxx (317, x05) voltage bias source..
(V bias = 5V, 5ma max. i'm loading it ) that I can
put in some corner of this circuit board. Or there's this left
over opamp, (I think it's like a 5mA max for the sensor, and I'm
loading it much less than that... (maybe I should try more R?)

Anyway, I was thinking about JL, and with more C-load it got
'better' again.
And searching for C-loding of opmap.. I got these,
https://www.electronicdesign.com/power/what-s-all-capacitive-loading-stuff-anyhow

Which says.. at high C it's the cap ESR loading the opamp.
(I built an inverting and non-inverting opamp voltage soucre
on a piece of copper clad today, I'm only up to 100nF, the
opamp's not railing but it's getting bad.

And this,
https://ww1.microchip.com/downloads/en/appnotes/00884a.pdf

(which I think I've read before. I've used both
circuits 7 and 12 in the past. (I never realized the
inverting circuit was so bad!)
So Re: 7 and 12, in both those I've hung a BFC
(BFC >1 uF ) across the feedback R...
Is that just too ugly?

There is also the circuit that uses two feedback paths
around the opamp, fast C and Rf after added R_out.

George H.


This sort of explains why many RRIO opamps are c-load stable.

https://www.dropbox.com/s/u6omebxsxl14xuo/Opamp_C-load.JPG?dl=0

The load cap doesn't add a pole, it just shifts the break frequency
down.
Well your picture doesn't really help me. But it's the compensation cap
that causes the phase shift and makes the output impedance look sorta
inductive at higher frequencies...

Sorry, too cryptic.

In a classic opamp, there is an internal compensation cap that adds a
-6 dB/octave frequency rolloff, namely 90 degrees of phase lag. That's
stable when you close a feedback loop. An external load cap adds
another 90 degrees of lag, which makes 180, which turns negative
feedback positive at some frequency, so it oscillates.

In a c-load amp, the external load c is *in parallel* with the opamp's
compensation cap(s) so the frequency rolloff just gets slower as you
So looking back at your previous pic, for the RRIO opamp the load C is
in parallel, because the one side (of C-comp) is common,
and the other side is a base-emitter junction away from the power rail...
(which is the same as ground for HF.)

Right. Looking into the output pin, the opamp looks like a Miller cap.
The added Cload is in parallel with that. Sort of.


add load caps, but remains single-pole, -6 dB/octave, still 90 degrees
of phase shift, so still stable. Net output impedance goes down at all
frequencies as load cap is added.

There are some great parts in the OPA1xx family.

The OPA192 worked fine at all C-loads... I messed around with the
circuit and got it to ring.. worse was with ~0.1uF and then a ten
ohm series R on the output killed that and made it sweet.
'scope shot
https://www.dropbox.com/s/av6lav5cwe4ll3d/Opamp-pwr.bmp?dl=0


Try a bigger cap. That will often stabilize an rrio opamp nicely.

Oh I did, 1 uF and 10 uF film caps... they still ring when
I step the input. Hmm, just tried a 4.7 uF tant. It looks fine
(a little over shoot)
Here's the 10 uF film (no output R)
https://www.dropbox.com/s/qpngwy5dynkccnx/10uF%20film.bmp?dl=0

and 4.7 uF tant.

https://www.dropbox.com/s/j8vxghhy5w9rdnb/4.7uF%20tant.bmp?dl=0

I think we have to point to cap ESR as the difference...
film has ~0.03 ohms
tant ~0.7 ohms

George H.

--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[John Larkin]

On Tue, 30 Apr 2019 09:50:17 -0700 (PDT), George Herold
<gherold@teachspin.com> wrote:

On Tuesday, April 30, 2019 at 12:18:30 PM UTC-4, John Larkin wrote:
On Tue, 30 Apr 2019 08:56:41 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

On Tuesday, April 30, 2019 at 10:37:42 AM UTC-4, John Larkin wrote:
On Tue, 30 Apr 2019 06:17:15 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

On Monday, April 29, 2019 at 11:07:06 PM UTC-4, John Larkin wrote:
On Mon, 29 Apr 2019 18:12:01 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

So I've got this lmxxx (317, x05) voltage bias source..
(V bias = 5V, 5ma max. i'm loading it ) that I can
put in some corner of this circuit board. Or there's this left
over opamp, (I think it's like a 5mA max for the sensor, and I'm
loading it much less than that... (maybe I should try more R?)

Anyway, I was thinking about JL, and with more C-load it got
'better' again.
And searching for C-loding of opmap.. I got these,
https://www.electronicdesign.com/power/what-s-all-capacitive-loading-stuff-anyhow

Which says.. at high C it's the cap ESR loading the opamp.
(I built an inverting and non-inverting opamp voltage soucre
on a piece of copper clad today, I'm only up to 100nF, the
opamp's not railing but it's getting bad.

And this,
https://ww1.microchip.com/downloads/en/appnotes/00884a.pdf

(which I think I've read before. I've used both
circuits 7 and 12 in the past. (I never realized the
inverting circuit was so bad!)
So Re: 7 and 12, in both those I've hung a BFC
(BFC >1 uF ) across the feedback R...
Is that just too ugly?

There is also the circuit that uses two feedback paths
around the opamp, fast C and Rf after added R_out.

George H.


This sort of explains why many RRIO opamps are c-load stable.

https://www.dropbox.com/s/u6omebxsxl14xuo/Opamp_C-load.JPG?dl=0

The load cap doesn't add a pole, it just shifts the break frequency
down.
Well your picture doesn't really help me. But it's the compensation cap
that causes the phase shift and makes the output impedance look sorta
inductive at higher frequencies...

Sorry, too cryptic.

In a classic opamp, there is an internal compensation cap that adds a
-6 dB/octave frequency rolloff, namely 90 degrees of phase lag. That's
stable when you close a feedback loop. An external load cap adds
another 90 degrees of lag, which makes 180, which turns negative
feedback positive at some frequency, so it oscillates.

In a c-load amp, the external load c is *in parallel* with the opamp's
compensation cap(s) so the frequency rolloff just gets slower as you
So looking back at your previous pic, for the RRIO opamp the load C is
in parallel, because the one side (of C-comp) is common,
and the other side is a base-emitter junction away from the power rail...
(which is the same as ground for HF.)

Right. Looking into the output pin, the opamp looks like a Miller cap.
The added Cload is in parallel with that. Sort of.


add load caps, but remains single-pole, -6 dB/octave, still 90 degrees
of phase shift, so still stable. Net output impedance goes down at all
frequencies as load cap is added.

There are some great parts in the OPA1xx family.

The OPA192 worked fine at all C-loads... I messed around with the
circuit and got it to ring.. worse was with ~0.1uF and then a ten
ohm series R on the output killed that and made it sweet.
'scope shot
https://www.dropbox.com/s/av6lav5cwe4ll3d/Opamp-pwr.bmp?dl=0


Try a bigger cap. That will often stabilize an rrio opamp nicely.

Oh I did, 1 uF and 10 uF film caps... they still ring when
I step the input. Hmm, just tried a 4.7 uF tant. It looks fine
(a little over shoot)
Here's the 10 uF film (no output R)
https://www.dropbox.com/s/qpngwy5dynkccnx/10uF%20film.bmp?dl=0

and 4.7 uF tant.

https://www.dropbox.com/s/j8vxghhy5w9rdnb/4.7uF%20tant.bmp?dl=0

I think we have to point to cap ESR as the difference...
film has ~0.03 ohms
tant ~0.7 ohms

Tants have just the right amount of ESR for many apps, including
switchers.

Not all RRIO amps have the topology I referenced. You've got to do
what you did, try some load cap and a pulsed load to see if it
oscillates or rings.


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

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

 

[George Herold]

On Tuesday, April 30, 2019 at 2:45:07 PM UTC-4, John Larkin wrote:

On Tue, 30 Apr 2019 09:50:17 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

On Tuesday, April 30, 2019 at 12:18:30 PM UTC-4, John Larkin wrote:
On Tue, 30 Apr 2019 08:56:41 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

On Tuesday, April 30, 2019 at 10:37:42 AM UTC-4, John Larkin wrote:
On Tue, 30 Apr 2019 06:17:15 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

On Monday, April 29, 2019 at 11:07:06 PM UTC-4, John Larkin wrote:
On Mon, 29 Apr 2019 18:12:01 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

So I've got this lmxxx (317, x05) voltage bias source..
(V bias = 5V, 5ma max. i'm loading it ) that I can
put in some corner of this circuit board. Or there's this left
over opamp, (I think it's like a 5mA max for the sensor, and I'm
loading it much less than that... (maybe I should try more R?)

Anyway, I was thinking about JL, and with more C-load it got
'better' again.
And searching for C-loding of opmap.. I got these,
https://www.electronicdesign.com/power/what-s-all-capacitive-loading-stuff-anyhow

Which says.. at high C it's the cap ESR loading the opamp.
(I built an inverting and non-inverting opamp voltage soucre
on a piece of copper clad today, I'm only up to 100nF, the
opamp's not railing but it's getting bad.

And this,
https://ww1.microchip.com/downloads/en/appnotes/00884a.pdf

(which I think I've read before. I've used both
circuits 7 and 12 in the past. (I never realized the
inverting circuit was so bad!)
So Re: 7 and 12, in both those I've hung a BFC
(BFC >1 uF ) across the feedback R...
Is that just too ugly?

There is also the circuit that uses two feedback paths
around the opamp, fast C and Rf after added R_out.

George H.


This sort of explains why many RRIO opamps are c-load stable.

https://www.dropbox.com/s/u6omebxsxl14xuo/Opamp_C-load.JPG?dl=0

The load cap doesn't add a pole, it just shifts the break frequency
down.
Well your picture doesn't really help me. But it's the compensation cap
that causes the phase shift and makes the output impedance look sorta
inductive at higher frequencies...

Sorry, too cryptic.

In a classic opamp, there is an internal compensation cap that adds a
-6 dB/octave frequency rolloff, namely 90 degrees of phase lag. That's
stable when you close a feedback loop. An external load cap adds
another 90 degrees of lag, which makes 180, which turns negative
feedback positive at some frequency, so it oscillates.

In a c-load amp, the external load c is *in parallel* with the opamp's
compensation cap(s) so the frequency rolloff just gets slower as you
So looking back at your previous pic, for the RRIO opamp the load C is
in parallel, because the one side (of C-comp) is common,
and the other side is a base-emitter junction away from the power rail...
(which is the same as ground for HF.)

Right. Looking into the output pin, the opamp looks like a Miller cap.
The added Cload is in parallel with that. Sort of.


add load caps, but remains single-pole, -6 dB/octave, still 90 degrees
of phase shift, so still stable. Net output impedance goes down at all
frequencies as load cap is added.

There are some great parts in the OPA1xx family.

The OPA192 worked fine at all C-loads... I messed around with the
circuit and got it to ring.. worse was with ~0.1uF and then a ten
ohm series R on the output killed that and made it sweet.
'scope shot
https://www.dropbox.com/s/av6lav5cwe4ll3d/Opamp-pwr.bmp?dl=0


Try a bigger cap. That will often stabilize an rrio opamp nicely.

Oh I did, 1 uF and 10 uF film caps... they still ring when
I step the input. Hmm, just tried a 4.7 uF tant. It looks fine
(a little over shoot)
Here's the 10 uF film (no output R)
https://www.dropbox.com/s/qpngwy5dynkccnx/10uF%20film.bmp?dl=0

and 4.7 uF tant.

https://www.dropbox.com/s/j8vxghhy5w9rdnb/4.7uF%20tant.bmp?dl=0

I think we have to point to cap ESR as the difference...
film has ~0.03 ohms
tant ~0.7 ohms


Tants have just the right amount of ESR for many apps, including
switchers.

Not all RRIO amps have the topology I referenced. You've got to do
what you did, try some load cap and a pulsed load to see if it
oscillates or rings.

I was pulsing the input. I'm not sure how to pulse the load.
Well, simply with a function generator.
How about a 10 k ohm load R and I toggle the bottom from gnd to +1 V.
(it's a 5V supply)

I'll see if that looks the same.

GH

--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

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

 

[John Larkin]

On Tue, 30 Apr 2019 11:57:00 -0700 (PDT), George Herold
<gherold@teachspin.com> wrote:

On Tuesday, April 30, 2019 at 2:45:07 PM UTC-4, John Larkin wrote:
On Tue, 30 Apr 2019 09:50:17 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

On Tuesday, April 30, 2019 at 12:18:30 PM UTC-4, John Larkin wrote:
On Tue, 30 Apr 2019 08:56:41 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

On Tuesday, April 30, 2019 at 10:37:42 AM UTC-4, John Larkin wrote:
On Tue, 30 Apr 2019 06:17:15 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

On Monday, April 29, 2019 at 11:07:06 PM UTC-4, John Larkin wrote:
On Mon, 29 Apr 2019 18:12:01 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

So I've got this lmxxx (317, x05) voltage bias source..
(V bias = 5V, 5ma max. i'm loading it ) that I can
put in some corner of this circuit board. Or there's this left
over opamp, (I think it's like a 5mA max for the sensor, and I'm
loading it much less than that... (maybe I should try more R?)

Anyway, I was thinking about JL, and with more C-load it got
'better' again.
And searching for C-loding of opmap.. I got these,
https://www.electronicdesign.com/power/what-s-all-capacitive-loading-stuff-anyhow

Which says.. at high C it's the cap ESR loading the opamp.
(I built an inverting and non-inverting opamp voltage soucre
on a piece of copper clad today, I'm only up to 100nF, the
opamp's not railing but it's getting bad.

And this,
https://ww1.microchip.com/downloads/en/appnotes/00884a.pdf

(which I think I've read before. I've used both
circuits 7 and 12 in the past. (I never realized the
inverting circuit was so bad!)
So Re: 7 and 12, in both those I've hung a BFC
(BFC >1 uF ) across the feedback R...
Is that just too ugly?

There is also the circuit that uses two feedback paths
around the opamp, fast C and Rf after added R_out.

George H.


This sort of explains why many RRIO opamps are c-load stable.

https://www.dropbox.com/s/u6omebxsxl14xuo/Opamp_C-load.JPG?dl=0

The load cap doesn't add a pole, it just shifts the break frequency
down.
Well your picture doesn't really help me. But it's the compensation cap
that causes the phase shift and makes the output impedance look sorta
inductive at higher frequencies...

Sorry, too cryptic.

In a classic opamp, there is an internal compensation cap that adds a
-6 dB/octave frequency rolloff, namely 90 degrees of phase lag. That's
stable when you close a feedback loop. An external load cap adds
another 90 degrees of lag, which makes 180, which turns negative
feedback positive at some frequency, so it oscillates.

In a c-load amp, the external load c is *in parallel* with the opamp's
compensation cap(s) so the frequency rolloff just gets slower as you
So looking back at your previous pic, for the RRIO opamp the load C is
in parallel, because the one side (of C-comp) is common,
and the other side is a base-emitter junction away from the power rail...
(which is the same as ground for HF.)

Right. Looking into the output pin, the opamp looks like a Miller cap.
The added Cload is in parallel with that. Sort of.


add load caps, but remains single-pole, -6 dB/octave, still 90 degrees
of phase shift, so still stable. Net output impedance goes down at all
frequencies as load cap is added.

There are some great parts in the OPA1xx family.

The OPA192 worked fine at all C-loads... I messed around with the
circuit and got it to ring.. worse was with ~0.1uF and then a ten
ohm series R on the output killed that and made it sweet.
'scope shot
https://www.dropbox.com/s/av6lav5cwe4ll3d/Opamp-pwr.bmp?dl=0


Try a bigger cap. That will often stabilize an rrio opamp nicely.

Oh I did, 1 uF and 10 uF film caps... they still ring when
I step the input. Hmm, just tried a 4.7 uF tant. It looks fine
(a little over shoot)
Here's the 10 uF film (no output R)
https://www.dropbox.com/s/qpngwy5dynkccnx/10uF%20film.bmp?dl=0

and 4.7 uF tant.

https://www.dropbox.com/s/j8vxghhy5w9rdnb/4.7uF%20tant.bmp?dl=0

I think we have to point to cap ESR as the difference...
film has ~0.03 ohms
tant ~0.7 ohms


Tants have just the right amount of ESR for many apps, including
switchers.

Not all RRIO amps have the topology I referenced. You've got to do
what you did, try some load cap and a pulsed load to see if it
oscillates or rings.
I was pulsing the input. I'm not sure how to pulse the load.
Well, simply with a function generator.
How about a 10 k ohm load R and I toggle the bottom from gnd to +1 V.
(it's a 5V supply)

I'll see if that looks the same.

Input is just as good. I'd go from, say, +2 to +3 and not start at
ground. The amp will slew into a big cap, and current limit, which can
be more interesting. So play with the square wave amplitude on top of
some DC offset.

You can AC couple a square wave, from a 50 ohm fungen, into the output
too.

GH


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

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

--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

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

 

[Tim Williams]

<tabbypurr@gmail.com> wrote in message
news:0aa3f735-4e20-4187-922f-0f76ce041ab9@googlegroups.com...

Why does opamp R_out not dominate?

That was in context of C only (and whatever ESR it has), no explicit R_out.

Do you mean the amp's output (source) impedance? The problem there is, it's
usually inductive (a modest impedance, in the 10s or 100s of ohms, but
typically rising with frequency), and that, plus some excess phase shift
inside the amp, causes the pole to split, resonate and continue rightwards
into the right half plane (i.e. pushing it into oscillation).

Tim

 

On Wednesday, 1 May 2019 00:46:30 UTC+1, Tim Williams wrote:

tabbypurr> wrote in message
news:0aa3f735-4e20-4187-922f-0f76ce041ab9@googlegroups.com...

Why does opamp R_out not dominate?


That was in context of C only (and whatever ESR it has), no explicit R_out.

not sure I'm following you there.

> Do you mean the amp's output (source) impedance?

yes

The problem there is, it's
usually inductive (a modest impedance, in the 10s or 100s of ohms, but
typically rising with frequency), and that, plus some excess phase shift
inside the amp, causes the pole to split, resonate and continue rightwards
into the right half plane (i.e. pushing it into oscillation).

Tim

yes - and R_out dominates massively over C_ESR in that situation.


NT

 

[George Herold]

On Tuesday, April 30, 2019 at 8:29:56 PM UTC-4, tabb...@gmail.com wrote:

On Wednesday, 1 May 2019 00:46:30 UTC+1, Tim Williams wrote:
tabbypurr> wrote in message
news:0aa3f735-4e20-4187-922f-0f76ce041ab9@googlegroups.com...

Why does opamp R_out not dominate?


That was in context of C only (and whatever ESR it has), no explicit R_out.

not sure I'm following you there.

Do you mean the amp's output (source) impedance?

yes

The problem there is, it's
usually inductive (a modest impedance, in the 10s or 100s of ohms, but
typically rising with frequency), and that, plus some excess phase shift
inside the amp, causes the pole to split, resonate and continue rightwards
into the right half plane (i.e. pushing it into oscillation).

Tim

yes - and R_out dominates massively over C_ESR in that situation.

Yabut, R_out is inside the opamp feedback loop, and C_esr isn't.
(If I'm following correctly)
GH

NT