OpAmp Feedback Resistance vs. Stability

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So, about a decade and a half ago, I worked with some very ill-tempered  
rail-rail opamps.  They were among the first-generation rail-rail amps,  
and they had the "nice" feature that in spite of their specified  
behavior, they'd go unstable as a voltage follower if you just connected  
the output to the negative input -- but they'd work fine if you did so  
through a 1k-ohm resistor.

I got into the habit of putting a resistor there in my voltage followers.

I just discovered an error in a circuit of mine that uses TSV912 op-amps,  
connected as voltage followers, with 22K-ohm resistors from output to  
input.  I'm having a very "d'oh" moment, because I'm thinking that if you  
DO want resistors there at all, you really want something in the 200-ohm  
to 1k-ohm range.  Habit, I think, kneecapped me.

So, I'm going to experiment around with the best value for this, but does  
anyone have any suggestions as to how to figure out what to use?  Was  
that old ill-tempered op-amp that I used Back Then a fluke, or is it  
generally advantageous to use a resistor in the feedback path of a  
voltage follower?

Suggestions welcome.

--  

Tim Wescott
Wescott Design Services
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Re: OpAmp Feedback Resistance vs. Stability
Den onsdag den 29. marts 2017 kl. 00.12.54 UTC+2 skrev Tim Wescott:
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https://e2e.ti.com/blogs_/b/precisionhub/archive/2014/07/15/resistors-in-the-feedback-of-a-buffer-ask-why

Re: OpAmp Feedback Resistance vs. Stability
On Tue, 28 Mar 2017 15:17:14 -0700, Lasse Langwadt Christensen wrote:

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in-the-feedback-of-a-buffer-ask-why

OK.  That explains why it should be zero (easy enough, there's other zero-
ohm resistors on the board).

I still wonder why that way-back-when amp needed one.  I wish I knew the  
part number -- all I remember is that it was one of the first wave of  
rail-rail op-amps, and we were all pretty bitter about it by the time the  
product shipped.

--  

Tim Wescott
Wescott Design Services
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Re: OpAmp Feedback Resistance vs. Stability
On Tue, 28 Mar 2017 15:17:14 -0700, Lasse Langwadt Christensen wrote:

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in-the-feedback-of-a-buffer-ask-why

Thanks.  Zeroing out the resistance solves the problem, although it does  
leave me with "d'oh" stamped on my forehead.

Unfortunately, it doesn't solve the problem that my customer is fighting  
in Baltimore, at the same time that it _does_ solve the symptoms that I  
was seeing in my test setup here on the west coast.  We've been plagued  
by this all along, alas.

--  
Tim Wescott
Control systems, embedded software and circuit design
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Re: OpAmp Feedback Resistance vs. Stability
Tim - the early opamps (lm707??) required "compensation" so that the loop  
gain was less than unity at the frequency where the phase shift was 180  
degrees. The first opamp that completely compensated itself, ie could be  
used at unity gain with no weird behavior, was, I think, the single  
version of the lm747.  
   Those versions that required compensation had special pins for
capacitors. But the circuitry between the output and the negative input  
could also be used for compensation, that is to reduce the loop gain with a  
low frequency rc filter. That is probably what you were doing with the  
rail to rail opamp. A 1K resister and 20 pf creates a 3 db point around 8  
mhz or so.

Hul

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Re: OpAmp Feedback Resistance vs. Stability
wrote:

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[snip]

My first OpAmp, MC1530/31, early '60's...

<http://www.analog-innovations.com/SED/MC1530-DataSheet.pdf>

was like that, external compensation, sliding-class-A output stage,
still being sold by Lansdale (licensed from Motorola), 53-years later
;-)

<http://www.analog-innovations.com/SED/MC1530-TeachingExercise.pdf>

The military love it... absolutely stable, high-slew-rate for the era,
indefinite-time short-circuit proof.
        
                                        ...Jim Thompson
--  
| James E.Thompson                                 |    mens     |
| Analog Innovations                               |     et      |
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Re: OpAmp Feedback Resistance vs. Stability
On Wednesday, March 29, 2017 at 7:31:31 PM UTC-4, Jim Thompson wrote:
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Huh!!, I was going to ask what "sliding Class A was".  
(I first try to be a "class A" kind of guy... )
Thanks for the exercise.. I'm going to have to think about it.  
  
George H.  
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Re: OpAmp Feedback Resistance vs. Stability
On Wednesday, March 29, 2017 at 9:30:04 PM UTC-4, George Herold wrote:
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Oh, I didn't see you gave me all the equations too!  
GH.
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Re: OpAmp Feedback Resistance vs. Stability
Jim - I was familiar with the Motorola devices in the late 60's; those and  
RCA's but mostly the Fairchild opamp's, but that was mostly due to  
available purchasing channels.
   Fun times those. If one device was announced with break through  
features in the spring, there'd be a better one in the fall.  
Congratulations on a device that lasted 53 years. That's no mean  
accomplishment.    

Hul


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Re: OpAmp Feedback Resistance vs. Stability
On Wed, 29 Mar 2017 22:54:13 +0000, Hul Tytus wrote:

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I got out of college just as those weren't being used much any more.

--  
Tim Wescott
Control systems, embedded software and circuit design
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Re: OpAmp Feedback Resistance vs. Stability
On Thursday, 30 March 2017 04:18:18 UTC+1, Tim Wescott  wrote:
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The 709 was the popular uncompensated one, and the 741 the number 1 unity stable opamp for a long time. There were others of course before & after, eg the 702. And even valve opamps.


NT

Re: OpAmp Feedback Resistance vs. Stability
Familiar numbers. Nice to see the correct ones.

Hul

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Re: OpAmp Feedback Resistance vs. Stability
On 03/28/2017 06:12 PM, Tim Wescott wrote:
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There are lots of good op amps out there.  Just find one that doesn't  
need the resistor. ;)

Seriously, I've never ever done that in a follower.  (Well, once long  
ago I was in a jam and used a decompensated op amp as a follower by  
jacking up the noise gain.  But not recently.)

Seriously, apart from some weirdness with input protection diodes and  
fast transients, I can't imagine an amp needing such a resistor.  It  
just adds noise and high frequency peaking.

Cheers

Phil Hobbs

--  
Dr Philip C D Hobbs
Principal Consultant
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Re: OpAmp Feedback Resistance vs. Stability
On 03/28/2017 06:27 PM, Phil Hobbs wrote:
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A unity-gain-stable op amp that's laid out correctly but can't remain  
stable in a basic follower configuration in 2k17 is a garbage-tier amp,  
throw it in the garbage.

Re: OpAmp Feedback Resistance vs. Stability
On Tue, 28 Mar 2017 17:12:46 -0500, Tim Wescott

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I don't do that, unless it's a current-feedback opamp.

In theory, balancing the input resistances can cancel Ib-driven
errors, but I seldom do that either.

OPA197 is a nice gumdrop 32-volt RRIO amp. It clips/recovers cleanly
and it's stable with a big cap from output to ground.







--  

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement  

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Re: OpAmp Feedback Resistance vs. Stability
On Tuesday, March 28, 2017 at 6:12:54 PM UTC-4, Tim Wescott wrote:
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Not rail to rail, but I've put 500 ohms in the OPA227's following  
figure 37 here,
http://www.ti.com/lit/ds/symlink/opa227.pdf
as current limiting resistors.  

George H.  


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