looking for feedback cap/resistor equation..

as the subject says, I'm looking for some kind of equation that can help figure out feedback resistor/capacitor values for opamps..

thanks! :thumb:

EDIT: ah to clarify, I'm really looking for any and/or all equations i can find. i'm trying to stockpile them and possibly catalog them.

[quote author="Svart"]as the subject says, I'm looking for some kind of equation that can help figure out feedback resistor/capacitor values for opamps..[/quote]

Well, let's see...

For a non-inverting amplifier,

gain Gninv = 1 + (Rf/Rin)

where Rin is the resistor from the inverting terminal to ground, and Rf is the resistor from the inverting terminal to the output terminal (the feedback resistor).

With a capacitor Cf in parallel with Rf,

high rolloff frequency Fh = 1 / (2 * pi * Rf * Cf)

where frequency is in Hz, Rf is in ohms and Cf is in µF. The rolloff stops at Gninv * Fh Hz, where G is the gain from the previous equation.

With a capacitor Cin in series with Rin,

bass rolloff frequency Fb = 1/ (2 * pi * Rin * Cin)

where all the units are the same as before. That rolloff stops at Fb / Gninv

For an inverting amplifier, where Rin is the resistor from the signal input to the inverting terminal,

gain Ginv = Rf/Rin

The rolloff equations are the same, except that the rolloffs don't stop.

For more info and an excellent exegesis of op amp circuits, check out Walt Jung's books.

Peace,
Paul

Thanks Pstamler, that was EXACTLY what i was looking for!

i'll be checking out those books if i can find them, are they easy to come by?

Some of Walt's essays

Philbrick Applications Manual for Computing Amplifiers (whole book as one humongous 75Meg PDF)

Walt's Op Amp Cookbook is out of print, sells for US$50 and up on http://abe.com

But there has to be a more lightweight text for Svart's questions. Just don't buy anything printed by TAB books. (There is one exception but you won't find a copy....)

thanks PRR, I've saved the links and will be reading them soon.

roflol: PRR: "Just don't buy anything printed by TAB books."

They are generally real stinkers, aren't they? But what's the exception you allude to?

Nice to know the Philbrick book is online. I have a few copies of different vintages in the library. A real pleasure to read for both content and style---sic transit...

[quote author="Svart"]i'll be checking out those books if i can find them, are they easy to come by?[/quote]

The IC Op-Amp Cookbook and Audio IC Applications are out of print, but some public libraries may have them, and don't forget inter-library loan. Jung has recently edited Op-Amp Applications, published by Analog Devices. It's US$40, but looks worth it.

Peace,
Paul

IC op amp cookbook is available...I just bought it this summer through amazon.com. But, the audio applications book is out of print. Too bad, I'd prefer that one.

Oh, pick up Paul Horwitz's 'The art of Electronics' while you're there...kinda spendy, but a good long term investment.

Cheers,

Kris

I just bought IC Op-amp cookbook two weeks ago from amazon.co.uk for 28 pounds. It's really good. And I saw a new book by Jung called Op-amp applications handbook. Looks kinda interesting.

a lot of good resources it seems! I'm going by the local booksellers hopefully this weekend, I've seen the "art of electronics" there and thumbed through it once. I'll probably start with that if they have it.

thanks guys!

however..

we should do a meta of equations...

> what's the exception you allude to?

TAB was young, the rights were probably cheap, and TAB didn't even have to print it:

ELECTRONIC DESIGNER'S HANDBOOK
A Practical Guide to Transistor Circuit Design

T. K. HEMINGWAY, B.Sc.(Hons.)
Circuit Consultant and Chief of Advanced Circuit Development, Marconi Instruments Limited

TAB BOOKS - BLUE RIDGE SUMMIT, PA 17214
First Published 1966 - Second impression 1967

© Thomas Keith Hemingway 1966

LONDON
BUSINESS PUBLICATIONS LIMITED

This book has been set in 10 on 12 pt. Times New Roman, printed in Great Britain by Richard Clay (The Chaucer Press), Limited, Bungay, for the publishers Business Publications Limited (registered office: 180 Fleet Street, London, E.C.4), publishing offices: Mercury House, Waterloo Road, London, S.E.1

MADE AND PRINTED IN GREAT BRITAIN

(trying out my new OCR...)

EXCELLENT book.

Actually... it can be found: Electronic Designer's Handbook on ABE.com, US$10-$30, from both US and UK shops, in both BB Ltd and TAB imprints, over many years.

Wow---I thought I had everything, or just about, but I may not have that (if I do it's in storage). There are similar titles out there (like the nearly identically titled Electronic Designers' Handbook, Landee, Davis, and Albrecht, McGraw-Hill 1957) but the author doesn't ring a bell.

Thanks.

> I may not have that

For chuckles I OCR-ed the first few pages. Good, and it gets better as it goes along. (But this OCR is not THAT good; if I'm inspired, I may run it through an industrial-strength document-sucker.)

It may mostly be below your level. Light reading for the potty. But some excellent explanations that are worth borrowing for your own writing, and a few novel ideas I had not seen before. And there are many folks here who would find it educational, informative, and interesting.

> the author doesn't ring a bell.

He's got one other book listed on ABE.com. The Handbook is so good, I've ordered Powell's cheapest copy of the Casebook.

I buy lots of books just to see how an author handles the subject, even if I think I know it pretty well.

That excerpt is a good sign---a very good balance of quasi-theoretical results with practical observations, and no long discussions of crystallography. The remarks about surface leakage effects in reverse-bias are trenchant. I wonder though if zeners have changed enough since the writing to shift his stated zero tempco nominal voltage, as I found it (circa 1974 or so) to be zero for about a 5.1V part (actually for a nominal 5.1V part run at about 4.9V). Oddly, and probably for deep reasons, the noise seems to be about maximum there as well (as a percentage of the nominal voltage). That's my recollection anyway, but it could be impaired after 30 years.

I ordered a copy; bookfinder.com will soon show one less of them if things are kept current.

I buy lots of books.