Bob Pease on amplifier capacitive loading

I pointed out to a bunch of amplifier engineers Bonnie Baker’s column on the ways to fix a capacitively loaded op amp that is oscillating. Mark Thoren from Linear Tech responded:

The LT1368 is a load-compensated op amp that requires a capacitive load for stability. Of course it’s not very fast, but it makes a great ADC driver because the C load provides the sampling circuit’s transient current, and the op amp provides the microamp or two of average sampling current.

And on that note, if you just need a buffer and it only needs to source current and you can tolerate a couple of millivolts of offset, you can use an LT3080 regulator (once again, requiring a C load).

The LT3080 is the new voltage regulator invented by Bob Dobkin that is meant to be surface mounted and also is meant to share loads. Dobkin explained how the regulator has a low offset voltage (mp3). I never thought of using it as a buffer, but you know that a linear voltage regulator like capacitive load. No wonder I see Thoren at the electronic flea market every month.

Then Bob Pease wrote back with a nice note and a page from his book. Since EDN published the book, I think I am allowed to put up a page from it, if not I am sure the lawyers will be contacting me. Better yet, all you analog folks should really go out and buy a copy of the book for yourself, the page Bob refers to is page 99. Bob wrote:

I have found that LOTS OF op-amps are decent for driving CL, BUT not for all loads. Even a crappy old ‘741 can do a lot of things right, in some cases, some applications. I’ve seen datasheets that recommend an op-amp for almost any cap load. But when you look at the "Phase Margin", they ain’t got very much. They may not oscillate, but they could ring like a bell.

(A) There is no substitute for setting up your amplifier and adding the actual kind of cap load. Set it up with the right noise gain and everything else the same as you will use. If you are going to use it with electrolytic 1 UF, don’t waste your time with 1 UF of Mylar or ceramic - - nor vice versa.

(((( (A’) Needless to say SPICE is worse than useless for this. It lies. Badly. ))))

(B) Exercise the amplifier over its working range of Iout (dc) and Vout and Cl. And, to do it right, OVER TEMPERATURE… And if you are going to try to use more than one Mfr., make sure you check them carefully, and lock out all other makers.  A third source can easily be JUNK.

(C) Don’t just see if it oscillates - see how it RINGS when you tickle it with a pulse of current. In other words, BANG ON IT.

(D) As recommended in the Page Attached, a series R-C damper may help. Etc., etc…..

(E) Walt Bacharowski [National Semi applications guru] says that the LM8261 is really pretty decent. I’ll take his word on that.

(F) I wish I could recommend some of our [National Semiconductor] CMOS rail-to-rail amplifiers, that could drive a big Cl, but most of them are rated up to 120 to 240 pF. Refer to the datasheet. I wish I could name a good one; but most of ours have a miller loop around the output stages. For small Cl, this is excellent; but for large Cl’s, that loop doesn’t like the Cl. Even the normal de-coupling schemes don’t work well.

I wish I could tell all you guys, "Shout it from the Hilltops", but technically this page, as attached, is copyrighted by EDN and Cahners and Elsevier and Butterworth-Heinemann, and Jesus knows who all else.

But you can pass it around, what the hell. I don’t need ANY "Friends of Analog” to get screwed by lack of this information.

Well, consider it shouted from the hilltops Bob. We here at EDN and Cahners and Elsevier are more than glad to do the shouting, as long as you keep giving great advice for folks in the difficult but rewarding field of analog circuit design.