Opamps as voltage sources (C-loading)

George Herold · 2019-04-30T01:12:01+00:00

So I've got this lmxxx (317, x05) voltage bias source.. (V bias = 5V, 5ma max. i'm loading it ) that I can put in some corner of this circuit board. Or there's this left over opamp, (I think it's like a 5mA max for the sensor, and I'm loading it much less than that... (maybe I should try more R?) Anyway, I was thinking about JL, and with more C-load it got 'better' again. And searching for C-loding of opmap.. I got these, Which says.. at high C it's the cap ESR loading the opamp. (I built an inverting and non-inverting opamp voltage soucre on a piece of copper clad today, I'm only up to 100nF, the opamp's not railing but it's getting bad. And this, (which I think I've read before. I've used both circuits 7 and 12 in the past. (I never realized the inverting circuit was so bad!) So Re: 7 and 12, in both those I've hung a BFC (BFC >1 uF ) across the feedback R... Is that just too ugly? There is also the circuit that uses two feedback paths around the opamp, fast C and Rf after added R_out. George H.

Apr 30, 2019 23 Replies

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George Herold 7 years ago

Yabut, R_out is inside the opamp feedback loop, and C_esr isn't. (If I'm following correctly) GH

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Tim Williams 7 years ago

But L_out dominates even more, that's the problem. (The goal of the RCRC network, of course, is to dominate both, so the amp is stable by itself, /and/ the output is kept accurate.)

Tim

Seven Transistor Labs, LLC Electrical Engineering Consultation and Design Website: https://www.seventransistorlabs.com/

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tabbypurr 7 years ago

Yes, and it's the whole feedback loop that determines stability. R_out is part of that.

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tabbypurr 7 years ago

I guess just treating it as (increasing) R_out was my error.

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