Op Amp Calculations

RST Engineering (jw) · 2005-09-21T16:20:11+00:00

A lot of years ago (38 to be exact) a wise old "rule of thumb" engineertaught me a slick way of getting maximum gain out of an opamp withoutresorting to very high or very low values of resistors.As we all know, for an inverting opamp, the gain is given simply by Rf/Ri,where Rf is the feedback resistor from output to inverting input and Ri isthe resistor between signal and inverting input. The DC level of the outputmay be set anywhere you choose by an appropriate bias level on thenoninverting input. For AC amplifiers from a single supply, this isgenerally Vcc/2 with capacitive coupling between Ri and the signal.However, for very large AC gains, either Rf must be rather large or Rirather small. Rf being rather large makes the input voltage/current errorsbecome significant as regards quiescent DC output point and Ri being rathersmall requires large capacitors for coupling and loading errors from thesignal source.So, sez old wily rule-of-thumb, just break Rf into two reasonable sizedequal value resistors equal to Rf/2 and run them in series from output backto (-) input. And, from the midpoint tap on these two resistors run aseries RC circuit to ground. Bingo, the AC gain improves greatly.And guess what, it works. How do I calculate the R in the series RC circuitI asks old wily. The answer comes back "Tweak it until you get the gain youwant." (Assume that C can be made appropriately large to get thelow-frequency gain you want.)I haven't used that trick in an awfully long time, but I've got anapplication that needs it. And, if I want to use Diddle's constant in asimulation program I can fool around (ahem, heuristically experiment) to getthe gain I need.However, I can't convince myself that I can mathematically come up with theresistor value. I have googled the problem and come up short. Anybody gota pointer to a URL that goes through the math of how this configurationworks? And what I'm doing to my phase margin?Jim

Sep 21, 2005 210 Replies

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Ken Smith 20 years ago

In article , John Larkin wrote: [...]

That only works if you assume that the person was a conservative to start with. A liberal will become more liberal for the same reason. Each side believes that they know what is best for the future of their children.

Even the "neocons" seem to become more "neoconish" as they age. In their case, I think it because they know that they will die soon and don't really give the hind most 25% of a rat what happens after that. I'm sure some of them think we are in the "last days" and that any money left over will simply be wasted.

-- kensmith@rahul.net forging knowledge

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Winfield Hill 20 years ago

The first, of course.

Thanks, - Win

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Winfield Hill 20 years ago

Look at the wmv file, please, and see that it's clearly no more than 1 foot off the floor, then STOP all your self-righteous pontification. There simply were no "kicks to the groin."

You may not have intended a lie, but your FACTS were wrong, so it was a mean vicious lie, against the critics of George Bush. I've noticed you far right-wing types aren't so careful with your facts when you go into attack mode.

Thanks, - Win

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Winfield Hill 20 years ago

Yep, more name-calling, that's your avocation.

Thanks, - Win

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Rich, Under the Affluence 20 years ago

I believe Mr. Hill probably sees all of his hypocrisy, to whatever degree he has it, quite clearly, as do I. I see my own level of hyprocisy - gawd, what a weird word! - and I see yours and that of the rest of your reich-wing ilk, and I compare.

You guys win hands down.

Thanks, Rich

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Rich, Under the Affluence 20 years ago

[the usual keith hate fluff]

'keith' seems to be one of the ones who can't bring himself to say "crotch" or "balls". Another symptom of self-righteous blue- nosery.

And I've got him filtered, even his "professional" persona, since he bitched about _not_ being filtered, but Win, you don't _have_ to keep feeding his trolling - you can't win an argument with a zealot - all you can do is drag yourself down to their level and grunt.

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keith 20 years ago

Win, you name call with the best of 'em. I only wish I could come up to your name-calling standards. Sheesh, to think you can't even see yourself in the mirror!

Keith

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keith 20 years ago

"Yours"? You drunk dum-f*ck, you're responding to Win!

Even when toasted, I have more brains in my dingernail dirt than someone like you who can't even keep a job! ...and rely on the likes of the Wins of the world to throw you a bone once in a while.

Of course you won't read this, because you're not man enough (or perpetually stoned) to stand up for yourself.

Keith

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keith 20 years ago

Win, *I* did not say he kicked anyone in the groin. I said such humor (groin/shin ~ no difference) is *NOT* funny. Please do try to be honest, though I know it's a stretch.

*I* did not lie (I don't watch that moron). Do you really think there is a huge difference between a kick in the groin and one to the shins? Neither are funny, but I didn't say one way or the other.

Do look in a mirror Win and get your facts straight! Sheesh, I hope your books are more accurate! ...and do chill! I'd not like to be the cause of your mental-hernia.

Keith

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