Rise Time and Slew Rate Doubts

Yaakov

Joined Jan 27, 2019
1,601
I am sure there will be help.

First, a point, this belongs in the Homework forum, but the moderators will move it so don't repost there, but for the future keep that in mind.

The general way to get homework help here is to start with what you know, and get to the point of confusion, then ask about those specific things. In this case, you should tell us what you think the answer is, and where you lose the thread of the explanation so that we can help you learn the gaps.

Lots of people here want to help, so go for it.
 

Thread Starter

Saturn Globetrotter

Joined May 3, 2019
18
I am sure there will be help.

First, a point, this belongs in the Homework forum, but the moderators will move it so don't repost there, but for the future keep that in mind.

The general way to get homework help here is to start with what you know, and get to the point of confusion, then ask about those specific things. In this case, you should tell us what you think the answer is, and where you lose the thread of the explanation so that we can help you learn the gaps.

Lots of people here want to help, so go for it.
Alright, thanks for the explanation. :)
 

Thread Starter

Saturn Globetrotter

Joined May 3, 2019
18
Hi,

Here is a hint.

Look at the slope of a ramp as compared to the maximum slope of a sine wave. See what you can deduce from that.
Alright, here is what I know. The slope of a ramp has distortion with a flat peak (just like the slew rate waveform we see). The maximum slope of a sine wave is round, meaning the curve goes up and reaches the max point and then it goes down, or it can be the other way round.

Did I get the facts right? If no, may I know where did I go wrong? And what should I look for next?
Thanks
 

Thread Starter

Saturn Globetrotter

Joined May 3, 2019
18
I am sure there will be help.

First, a point, this belongs in the Homework forum, but the moderators will move it so don't repost there, but for the future keep that in mind.

The general way to get homework help here is to start with what you know, and get to the point of confusion, then ask about those specific things. In this case, you should tell us what you think the answer is, and where you lose the thread of the explanation so that we can help you learn the gaps.

Lots of people here want to help, so go for it.
What I know about slew rate is that slew rate measures the speed that the output can move to comply with the changes in amplifier’s input. The faster the output can move, the better its compliance with input changes, and the lesser the distortion to happen. I honestly do not feel that slew rate can prevent distortion. Initially, I was thinking that slew rate is a distortion but I was told by my lecturer I was wrong, since slew rate can be in triangular waveform.
Point of confusion - How does slew rate prevent distortion?
My answer to my question - Knowing slew rate will let us know how fast the output can move. If the slew rate is too low, we can work to produce a product or amplifier with a higher one, if possible, infinite slew rate.
 

WBahn

Joined Mar 31, 2012
24,850
Alright, here is what I know. The slope of a ramp has distortion with a flat peak (just like the slew rate waveform we see). The maximum slope of a sine wave is round, meaning the curve goes up and reaches the max point and then it goes down, or it can be the other way round.

Did I get the facts right? If no, may I know where did I go wrong? And what should I look for next?
Thanks
You appear to be confusing a couple of concepts. When we speak about a "slope" we are talking about the rate at which one thing (the signal) is changing with another (time). The slope of a curve at some point is the slope of the straight line that is tangent to the curve at that point. If you've had any calculus (have you?), it is the derivative of the function at that point. So the slope may change from one point in the waveform to another, but at any given point it is just a number and a number can't be "round", so to talk about the maximum slop being round is meaningless. Think of a road going across a valley and over a mountain -- the slope is the steepness of the road at any given point and the maximum slope is just the steepness at the point where the steepness happens to be steeper than anyplace else. Would it make any sense to talk about the maximum steepness along that road being "round" or any other shape?

So draw a reasonably accurate picture of a sine wave and identify where along that wave the signal is the steepest -- where the signal changes the most per unit time. What would happen if the amplifier generating that signal was incapable of changing as fast as the sine wave does at that point?
 

Thread Starter

Saturn Globetrotter

Joined May 3, 2019
18
In general terms, most of the time, you want a signal to pass thru an amplifier
with the only alteration being its magnitude.

https://en.wikipedia.org/wiki/Amplifier

So ask the question, what properties do signals have, and what do you not want
altered by the amplifier.


Regards, Dana.
Here are my answers. Do let me know if I am wrong anywhere or where I lack of.
What properties do signals have?
They can be changed in terms of magnitude.
Signals are infinite in time as it is continuous.

What do I not want altered by the amplifier?
I do not really understand the question, but here is my answer.
The properties of the amplifier such as input resistance, output resistance, input offset voltage, gain, input bias current, and input offset current.
 

Thread Starter

Saturn Globetrotter

Joined May 3, 2019
18
You appear to be confusing a couple of concepts. When we speak about a "slope" we are talking about the rate at which one thing (the signal) is changing with another (time). The slope of a curve at some point is the slope of the straight line that is tangent to the curve at that point. If you've had any calculus (have you?), it is the derivative of the function at that point. So the slope may change from one point in the waveform to another, but at any given point it is just a number and a number can't be "round", so to talk about the maximum slop being round is meaningless. Think of a road going across a valley and over a mountain -- the slope is the steepness of the road at any given point and the maximum slope is just the steepness at the point where the steepness happens to be steeper than anyplace else. Would it make any sense to talk about the maximum steepness along that road being "round" or any other shape?

So draw a reasonably accurate picture of a sine wave and identify where along that wave the signal is the steepest -- where the signal changes the most per unit time. What would happen if the amplifier generating that signal was incapable of changing as fast as the sine wave does at that point?
yup, i have learnt derivative. With your comment of "the maximum slope is just the steepness at the point where the steepness happens to be steeper than anyplace else.", I now know the meaning of max slope. The signal changes the most per unit time at the point where the value of y at the y-axis is zero [only true is the sine wave starts at the origin (0,0)] What would happen if the amplifier generating that signal was incapable of changing as fast as the sine wave does at that point? I would say slew rate occurs. Its like the signal will try to get shortcuts to comply with the changes, which is why some rise time and fall time of the slew rate is a straight line and distortion will occur.
 

WBahn

Joined Mar 31, 2012
24,850
yup, i have learnt derivative. With your comment of "the maximum slope is just the steepness at the point where the steepness happens to be steeper than anyplace else.", I now know the meaning of max slope. The signal changes the most per unit time at the point where the value of y at the y-axis is zero [only true is the sine wave starts at the origin (0,0)] What would happen if the amplifier generating that signal was incapable of changing as fast as the sine wave does at that point? I would say slew rate occurs. Its like the signal will try to get shortcuts to comply with the changes, which is why some rise time and fall time of the slew rate is a straight line and distortion will occur.
So if I have a signal that is a pure sine wave at a frequency f and with an amplitude A and I pass it through an amplifier with a gain of K, what is the minimum slew rate that my amplifier has to have in order to avoid distortion in the output?
 

MrAl

Joined Jun 17, 2014
6,610
Alright, here is what I know. The slope of a ramp has distortion with a flat peak (just like the slew rate waveform we see). The maximum slope of a sine wave is round, meaning the curve goes up and reaches the max point and then it goes down, or it can be the other way round.

Did I get the facts right? If no, may I know where did I go wrong? And what should I look for next?
Thanks
Hi,

Well, compare the slope of the ramp (slew) to the slope of a sine wave at the zero crossing as that is the maximum slope for a sine wave.
Then, deduce what the op amp would have to be able to do in order to reproduce the sine wave near that point on the sine wave.

This might be one of those things that you have to be told or you have to have read up on it already. Some things in electronics are not exactly obvious unless you really think about it for extended periods of time.
 

WBahn

Joined Mar 31, 2012
24,850
Hi,

I'd like to see where you are going with this :)

[EDIT] I see cruts also commented :)
Does this give a hint as to where I might have been going?

Question:

So if I have a signal that is a pure sine wave at a frequency f and with an amplitude A and I pass it through an amplifier with a gain of K, what is the minimum slew rate that my amplifier has to have in order to avoid distortion in the output?
Offered Answer:

Its 2 pi F V right?
My response:

What is V?

Are you saying that the amplitude of the input signal and the gain of the amplifier play no part?
Let me be more specific since apparently I wasn't sufficiently clear. The question asked what the minimum slew rate of an amplifier with a certain gain needs to be in order to avoid distorting the output given an input signal that is a pure sine wave with a certain frequency and a certain amplitude.

The offered answer does not include anything about the shape of the input signal or the gain of the amplifier, so I asked whether they were saying that those factors play no part in the answer. Where I was going was trying to drop a hint that perhaps they DO play a part. I guess I mistakenly thought that that was pretty obvious.

My understanding is that the slew rate refers to the output voltage and is not significantly affected by the gain or the input voltage (except as how they pertain to the output voltage).
I never claimed that an amplifier's slew rate is affected by the gain or the input voltage, but I most definitely was implying that the minimum slew rate that an amplifier needs to have IS impacted by the gain and the input signal and that, therefore, an answer to that question that does not involve them is most likely not correct.

Here's my (apparently highly flawed) line of reasoning -- the minimum slew rate that an amplifier has to have is determined by the maximum rate at which the output has to change in order to produce the desired output signal. Then, it seemed to me at least, that the desired output signal was somehow related to the input signal and the gain of the amplifier. Thus it seemed reasonable, to me at least, that the input signal and the gain would somehow and in some way play a role in determining the required minimum slew rate.

Since apparently I am completely mistaken in this belief, I will bow out and leave it to you to explain why the nature of the input signal and the gain of the amplifier don't matter when determining what an amplifier's minimum slew rate needs to be.
 

crutschow

Joined Mar 14, 2008
23,499
leave it to you to explain why the nature of the input signal and the gain of the amplifier don't matter when determining what an amplifier's minimum slew rate needs to be.
Of course it matters, to the extent that it affects the output voltage and required slew rate at the output.
It seemed that you were implying that the amplifier slew rate depended upon the gain, like the bandwidth is.
I now see that you were not, so peace. :)
 
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