Question on cap loading on op-amp used for buffering DC

Most definitely.

"Or"? Yes, you should still drive it through a resistor in the feedback loop.

There are op-amps now advertised as being able to drive capacitive loads cleanly; they are arranged so that the output capacitance becomes part of the compensating capacitance. That's about all I know, but if you want you now know what to search for on manufacturer web sites.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com

Thanks - that was what I thought, but I couldn't fine anything that specifically referred to DC. Thanks again!

Hal

One might get away with it if the ESR of the cap were right.

LM8261.

John

John Larkin a écrit :

Yup. Or even without if you really know what you're doing. We won a 10s of megabucks business against a competitor for that who still wonders how it can work. It was the only possible option given all the constraints and works reliably on a few hundreds of $5M equipments.

--
Thanks,
Fred.

I'm not sure what the OP was thinking, but I was thinking of a circuit like this:

___ .-------|___|-------. | Rf | | || Cf | o----||--. | | || | | | | | ___ | |\ | o----------o Vout o---|___|----o--|-\ | ___ | Ri | >--o--|___|---o .---|+/ Ris | | |/ | | --- | --- big honkin' | | | | === === GND GND (created by AACircuit v1.28.6 beta 04/19/05

Ris isolates the op-amp from the load, Cf stabilizes it, and Ri and Rf give the correct DC gain. The positive-gain version is obvious.

--
www.wescottdesign.com

Right, that works. The thing has to be tuned, and Ris limits swing/current a bit.

I like lots of ceramics, or a polymer aluminum cap, for big honkin'. The low ESR lets the cap handle fast transient loads, so the opamp loop can relax a little.

The LM8261 saves a lot of thinking, and it can drive pretty hard rail-to-rail.

John

That's the classic structure, and it doesn't take much Ris & Cf to make it work quite well. ...Jim Thompson

[On the Road, in New York]

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
I love to cook with wine.     Sometimes I even put it in the food.

"h snipped-for-privacy@gmail.com"

** The slew rate will be limited to about 0.2 volts per *millisecond* due to current limiting.

..... Phil

It's a classic structure, but oft neglected for some reason.

As John L. points out Ris & Cf do need to be tuned. But the tuning can be made pretty broad if you're not up against a performance wall.

--
www.wescottdesign.com

Note that this emphatic and self-confident declaration is only valid if you happen to live in the same universe that Phil does, where all possible operational amplifiers have a current drive capability of only

20mA.

If you live in the same universe as National Semiconductor, TI, Fairchild, On, ST, etc., then you will be able easily find op-amps that can drive hundreds of milliamps, and for more money yet (and possibly boutique suppliers), amps. Or if you're a tightwad you can augment a wimpy amplifier's output stage with a few discrete transistors and get the same effect for more fussing (but lower BOM cost).

--
www.wescottdesign.com

What is wrong with you, Phil?

Bob

-- == All google group posts are automatically deleted due to spam ==

"Bob Wanker"

** Look what f****it got alerted !!

ROTFL

... Phil

Interestingly enough, Phil never seems to see the irony when he accuses someone else of being a nutcase.

Take those little pills your doctor prescribes, Phil -- you'll be happier.

--
www.wescottdesign.com

At least she'll be quiet, and everyone else will be happy

--
It's easy to think outside the box, when you have a cutting torch.

It's both funny and sad. That's part of his mental illness. He certainly would benefit from professional help.

Bob

--
== All google group posts are automatically deleted due to spam ==

Maybe he should claim stress, and go to the happy-smoke doctor! ;-P

Cheers! Rich

You are more offensive than Phil, with your pathetic jabs from your pathetic peanut gallery, with your pathetic, peanut sized brain.

Even sadder still is that you are unaware of the price you'll pay for those transgressions. Your soul, as you are far too stupid to repent for your life's history of ills before the one ill that counts come calling.

Says the peanut from the left gallery.

Speaking of pots, Mr. Kettle,...

No. It doesn't need done at all.

OK. Be a sicko.