Calculating Attenuation points

[Johnson777717]

Good day folks!
I'm doing a little reading on Op Amps, and slew rates, and I have a question about how to calculate if a slew rate is quick enough to keep from attenuation, given a certain gain.

What is the mathematical relationship between slew rate, gain and the point of attenuation?

I'm reading this particular point in an article, which may help to describe what I'm looking for:
"One side effect of high gains is that sometimes the Op-Amp is not fast enough to keep up with the voltage swings. Slew rate defines how "fast" an Op-Amp is. A common 741 Op Amp is pretty slow with a 1/2 volt per micro second slew rate. This is fine for voltage followers, but if you push a 10X gain through a 741, you'll find that it can alter the tonal quality of the signal, usually attenuating the high frequencies." -- in effect from Shavano Music Online 3/2000 Jens Moller.

Thanks for any assistance that you can provide.

 

[zevon8]

there's a pretty good series of articles on the Jameco site, its in 4 parts, direct links below

this link gives the index page for the aritcles:

**broken link removed**

the 4 articles are thus:

**broken link removed**
**broken link removed**
**broken link removed**
**broken link removed**

 

[Johnson777717]

WOW! Thanks a million Zevon! You're awesome.

 

[Optikon]

Good day folks!
I'm doing a little reading on Op Amps, and slew rates, and I have a question about how to calculate if a slew rate is quick enough to keep from attenuation, given a certain gain.

What is the mathematical relationship between slew rate, gain and the point of attenuation?

I'm reading this particular point in an article, which may help to describe what I'm looking for:
"One side effect of high gains is that sometimes the Op-Amp is not fast enough to keep up with the voltage swings. Slew rate defines how "fast" an Op-Amp is. A common 741 Op Amp is pretty slow with a 1/2 volt per micro second slew rate. This is fine for voltage followers, but if you push a 10X gain through a 741, you'll find that it can alter the tonal quality of the signal, usually attenuating the high frequencies." -- in effect from Shavano Music Online 3/2000 Jens Moller.

Thanks for any assistance that you can provide.

I think these statements are a little misleading. I can take a 741 and set it up for a gain of 10 and still swing say 10V at the 0.5 V/us rate.

I'm not sure what you mean by the point of attenuation.

For internally compensated Opamps,

Gain* bandwidth = constant
SR is related to bandwidth due to internal current source & Cc (comp cap)

Look for op amp macromodel.

 

[Johnson777717]

I'm not sure what you mean by the point of attenuation.

Apologies, I'm still learning the proper terminology that revolves around Op-Amps. What I mean is: Attentuation begins at a specific point, relative to the gain and slew rate of the Op-amp? Correct? Thus, my statement "...the point of attenuation" refers to the point where attenuation begins (at least, where measurable attenuation begins).

I hope this time around makes sense.

I think these statements are a little misleading. I can take a 741 and set it up for a gain of 10 and still swing say 10V at the 0.5 V/us rate.

I see what you're saying here. So another factor to consider is the input voltage. This makes complete sense to me. I might as well smack myself in the forehead for the oversight. DUH!

Look for op amp macromodel.

Is this the basic block diagram that you're referring to?

Thanks to all for the help!

 

[Optikon]

I'm not sure what you mean by the point of attenuation.

Apologies, I'm still learning the proper terminology that revolves around Op-Amps. What I mean is: Attentuation begins at a specific point, relative to the gain and slew rate of the Op-amp? Correct? Thus, my statement "...the point of attenuation" refers to the point where attenuation begins (at least, where measurable attenuation begins).

I hope this time around makes sense.

I think these statements are a little misleading. I can take a 741 and set it up for a gain of 10 and still swing say 10V at the 0.5 V/us rate.

I see what you're saying here. So another factor to consider is the input voltage. This makes complete sense to me. I might as well smack myself in the forehead for the oversight. DUH!

Look for op amp macromodel.

Is this the basic block diagram that you're referring to?

Thanks to all for the help!

Here is some rather humorous reading on op amp basics.
https://www.electro-tech-online.com/custompdfs/2004/04/2-Op-Amp_Concepts.pdf

This one contains a boyle macro model but it was not the more simplified version that I was thinking of.

**broken link removed**

I was thinking of the model that shows the simplified diff input stage with gain and then a compensation capacitor that gets charged with a current source and the output stage is just an ideal x1 buffer. This model shows the relationship among the comp cap and slew rate / bandwidth.

I don't have time now, but if you would still like to see all the relationships and how they are derived, I can look some stuff up.

 

[Johnson777717]

Thanks Optikon! I appreciate your help and follow up.

I've added your links to my reading list. I found some pretty good tutorials on www.web-ee.com as well, so I have about 7 different reference articles to read through. If I don't get a better understanding of the concepts, I think I'll need to evaluate my reading capabilities!!! :wink:
Thus, I don't want to waste your time by asking you to do some research. Thank you for the offer though. That's very kind of you.

As a side note: I have some lessons about Op-amps comming up in class, and I've decided to get a head start, so I'm sure the relationships will be provided me when the time is right. :shock:

Again, thank you for your time, and assistance!