OpAmp Feedback Resistance vs. Stability

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.

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Tim Wescott 
Wescott Design Services 
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Tim Wescott
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Den onsdag den 29. marts 2017 kl. 00.12.54 UTC+2 skrev Tim Wescott:

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Reply to
Lasse Langwadt Christensen

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.

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Tim Wescott 
Wescott Design Services 
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Tim Wescott

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

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Dr Philip C D Hobbs 
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Reply to
Phil Hobbs

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.

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John Larkin         Highland Technology, Inc 
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Reply to
John Larkin

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.

Reply to
bitrex

Not rail to rail, but I've put 500 ohms in the OPA227's following figure 37 here,

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as current limiting resistors.

George H.

Reply to
George Herold

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.

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Tim Wescott 
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Tim Wescott

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:

Reply to
Hul Tytus

[snip]

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

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

The military love it... absolutely stable, high-slew-rate for the era, indefinite-time short-circuit proof. ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Reply to
Jim Thompson

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.

Reply to
George Herold

Oh, I didn't see you gave me all the equations too! GH.

Reply to
George Herold

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

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Tim Wescott 
Control systems, embedded software and circuit design 
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Reply to
Tim Wescott

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

Reply to
tabbypurr

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

Jim Thomps> > >Tim - the early opamps (lm707??) required "compensation" so that the loop

Reply to
Hul Tytus

Familiar numbers. Nice to see the correct ones.

Hul

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Hul Tytus

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