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Re: OpAmp Feedback Resistance vs. Stability

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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

Tim Wescott <tim@seemywebsite.com> wrote:
> On Tue, 28 Mar 2017 15:17:14 -0700, Lasse Langwadt Christensen wrote:

> > Den onsdag den 29. marts 2017 kl. 00.12.54 UTC+2 skrev Tim Wescott:
> >> 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.
> > 
> > https://e2e.ti.com/blogs_/b/precisionhub/archive/2014/07/15/resistors-
> 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
> I'm looking for work!  See my website if you're interested
> http://www.wescottdesign.com

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OpAmp Feedback Resistance vs. Stability Tim Wescott <seemywebsite@myfooter.really> - 2017-03-28 17:12 -0500
  Re: OpAmp Feedback Resistance vs. Stability Lasse Langwadt Christensen <langwadt@fonz.dk> - 2017-03-28 15:17 -0700
    Re: OpAmp Feedback Resistance vs. Stability Tim Wescott <seemywebsite@myfooter.really> - 2017-03-28 17:21 -0500
    Re: OpAmp Feedback Resistance vs. Stability Tim Wescott <tim@seemywebsite.com> - 2017-03-29 11:57 -0500
      Re: OpAmp Feedback Resistance vs. Stability Hul Tytus <ht@panix.com> - 2017-03-29 22:54 +0000
        Re: OpAmp Feedback Resistance vs. Stability Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> - 2017-03-29 16:31 -0700
          Re: OpAmp Feedback Resistance vs. Stability George Herold <gherold@teachspin.com> - 2017-03-29 18:29 -0700
            Re: OpAmp Feedback Resistance vs. Stability George Herold <gherold@teachspin.com> - 2017-03-29 18:46 -0700
          Re: OpAmp Feedback Resistance vs. Stability Hul Tytus <ht@panix.com> - 2017-03-30 06:02 +0000
        Re: OpAmp Feedback Resistance vs. Stability Tim Wescott <tim@seemywebsite.com> - 2017-03-29 22:18 -0500
          Re: OpAmp Feedback Resistance vs. Stability tabbypurr@gmail.com - 2017-03-29 21:24 -0700
            Re: OpAmp Feedback Resistance vs. Stability Hul Tytus <ht@panix.com> - 2017-03-30 06:09 +0000
  Re: OpAmp Feedback Resistance vs. Stability Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2017-03-28 18:27 -0400
    Re: OpAmp Feedback Resistance vs. Stability bitrex <bitrex@de.lete.earthlink.net> - 2017-03-28 18:47 -0400
  Re: OpAmp Feedback Resistance vs. Stability John Larkin <jjlarkinxyxy@highlandtechnology.com> - 2017-03-28 15:37 -0700
  Re: OpAmp Feedback Resistance vs. Stability George Herold <gherold@teachspin.com> - 2017-03-29 05:41 -0700

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