Slew rate and rise time

Sean, your analogies are very colorful, one might say even fiery, but I am afraid they are not exactly right. Given the fact that part of what you say is correct and some of it isn't makes it hard to follow.

"Think of how fast the signal can make it through the hose ( circuit ) from one end to the other on demand. This is a measure of accelleration and is equivalent to the rise time." No, how fast it makes through the hose would be propogation delay. The signal could take all day to make it from one end to the other and still have a very fast rise time once it got to the output. The two are unrelated.

"Slew rate is equivalent to how much flow the hose ( circuit ) is capable of." You also refer to it as "volume of water." No, this would be the equivalent of electrical current.

Slew rate, by definition, is simply the maximum rate of change of the output voltage for all possible inputs. In simple terms, how fast the voltage can change.

Mathematically it is the slope of the line on a graph plotting voltage versus time. Take the amount the voltage has changed, divide it by the time it took to change, and you have the slew rate, usually expresed in volts per microsecond.

Since music is made up of sine waves, and higher frequency sine waves have faster rates of change (the fastest being where it crosses zero) at some frequency this rate of change will exceed the slew rate of the amplifier. It will distort this frequency and all of those higher.

Therefore, we can say that the bandwidth of an amplifier is slew rate limited.

Rise time is how long it takes the voltage to get from 10 to 90 percent of it's peak as I stated above. You are correct that there is also a fall time and it can be different.

I admit I may have oversimplified the relationship between risetime and slew rate. Even though you can calculate a rate of change based on the risetime it is not the same as the slew rate. It is possible that a circuit can have a high enough slew rate to handle a signal but still be unable to do so because it is limited by the rise time, and vice versa.

From a practical point of view it makes more sense to talk about rise time with digital circuits since they are always switching between 2 voltage levels. For instance, a zero represented by zero volts and a one represented by 5 volts. The rise time would be how long it takes to get from .5V to 4.5V. The falltime how long it takes to get from 4.5V to .5V. In an analog circuit that has an infinite number of different levels it probably makes more sense to describe it in terms of slew rate.

I could go into a long winded mathematical explanation of the two and show how they are both related to the RC time constants of a circuit, and therefore both are limiting the bandwidth of a circuit, but that is probably more than is needed and it is very difficult to do mathematical equations on this forum.

Sean, I think Herman is right: http://www.amplifier.cd/Tutorial/Slew_Rate/SlewRate.htm.
But I also understand your analogy, Sean. Is slew rate analogous to a car's acceleration speed and rise time to the time a car needs to go from 0 to 90 mph? The slew rate and rise time are a bit related to each other, but if plotted a device with a certain rise time can have another slew rate if compared to another device with the same rise time.

The term acceleration is not used correctly in Sean's analogy. The correct term would be velocity, but the analogy is still wrong. Velocity is how fast something is moving, how much distance you can travel in a given period of time. Acceleration is how fast the velocity is changing, you are moving but either speeding up or slowing down. Neither slew rate nor rise time have anything to do with speeding up or slowing down, only moving at a constant rate.

Velocity is a rate of change.
Acceleration is a changing rate of change.

If you want to use a speed analogy then slew rate is the velocity of the voltage. It is not related to the "volume" of the signal, at least I don't think it is because in 25 years I've never heard anyone talk about the volume of a signal except in terms of current.

These numbers won't tell you squat about how it sounds.

Herman: Iin the grander scheme of things, the hose analogy was not perfect and your comments pertaining to propogation delay were correct. I think that most people that lacked such understanding would get the basic idea though, or at least i think that they would. For sake of reference, i did try to explain things further when i said that rise time referred to the transition time from small to large signal flow. I'm glad that you pointed this out though for sake of clarity and better understanding.

Herman's clarification about propogation of delay from the beginning to the end of the circuit ( i referred to it as Td or Time Delay ) vs rise time also brings another matter to light. When trying to discuss technical matters in a non-technical manner, it is easy to make things more confusing to the lay person. As i think that most of you know, that is not my goal at all. It is quite evident that i'm not a professional teacher though, so my wording or ability to convey some ideas may not be the best that could have been chosen for any given subject.

As such, i would encourage questions, comments and clarification as one feels the need. I would rather make sure that everyone feels comfortable and has a grasp on what we've covered. While we can't expect to cover every aspect of operation in a simplistic forum like this, i don't want to gloss over what might be important details and leave one confused about the subject for life. As such, those having questions and / or corrections should PLEASE post them in a timely basis. The longer that someone believes "mis-information" to be correct, the harder it is to get them to unlearn it. On top of that, further clarification and / or correction will typically lead to an even wider coverage of the subject, furthering the educational value of this thread and the resultant questions asked.

I ran into this problem with my business partner many years ago. His college professor used an extremely poor analogy that caused him to stumble when trying to understand a certain part of circuit design & troubleshooting. This analogy, and therefore this stumbling block, has stuck with him for life. I have had a helluva hard time trying to get him to understand why it was wrong and a better way to look at things. The fact that i'm "self-educated" had made it hard for him to believe that i could know more about the subject or how to teach it than his college professor did. None the less, the only way that i found out about this analogy was because he couldn't figure out what was wrong with a circuit. As such, i tried to explain how to find the problem using a different analogy than what he had been taught, which is what caused us to get into the semantics of the analogy and why he couldn't track the problem.

With that in mind, i don't want to be the one that teaches you folks the wrong thing that you have to "unlearn" at a later date, so please ask questions / add comments as different subjects arise. I and the others that typically contribute to such threads will do our best to try and explain things in an easy to understand but correct manner. So long as we stay on course and avoid personal conflict, i think that this thread could be very enlightening. Sean
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