Path: csiph.com!1.us.feeder.erje.net!3.us.feeder.erje.net!feeder.erje.net!news.linkpendium.com!news.linkpendium.com!panix!not-for-mail From: Hul Tytus Newsgroups: sci.electronics.design Subject: Re: OpAmp Feedback Resistance vs. Stability Date: Wed, 29 Mar 2017 22:54:13 +0000 (UTC) Organization: PANIX Public Access Internet and UNIX, NYC Lines: 59 Message-ID: References: <772ddd19-f896-480a-804c-b1fbd98d9c3e@googlegroups.com> NNTP-Posting-Host: panix2.panix.com X-Trace: reader1.panix.com 1490828053 5079 166.84.1.2 (29 Mar 2017 22:54:13 GMT) X-Complaints-To: abuse@panix.com NNTP-Posting-Date: Wed, 29 Mar 2017 22:54:13 +0000 (UTC) Xref: csiph.com sci.electronics.design:454151 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 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