OpAmp Feedback Resistance vs. Stability

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

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.

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

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.

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.

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

George H.

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 - 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:

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

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.

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

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

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

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

Familiar numbers. Nice to see the correct ones.

Hul

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