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Slew Rate of an Op-amp

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Freshman:
I read that the slew rate of an op-amp the rate at which the output voltage changes with time with respect to a step change in input.

So my question is will the slew rate of an op-amp depend on external circuit components?

Suppose,

1. I have a capacitor at the output of an op-amp. In this case, the slew rate will not be the same as mentioned in the op-amp datasheet correct?
(I think that adding a capacitor at the output of an op-amp is not a good idea as its purpose is not to charge a capacitor or moreover I haven't seen a requirement where there would be a capacitor requirement at the output of the op-amp. Please correct me if I am wrong).

2. What impact would a diode create on the slew rate of the op-amp if it is placed at the output?


3. On what external factors does the slew rate of an op-amp depend upon?

Zero999:
1) Of course a capacitor will slow it down. It'll probably also result in oscillation.

2) It probably won't make any difference, but it depends.

3) Too much of a vague question.

Terry Bites:
The slew rate of a particular circuit and the slew rate of an opamp are not the same thing.
The SR values in the datasheet specify a set of conditions under which you get the advertised slew rate.
All loads will impact the dv/dt at the output.
Always read the datasheet.

A capacitor is an unwelcome load for most opamps. An isolation resisistor, compensation newtork or a buffer would generally be needed.
https://www.analog.com/en/resources/analog-dialogue/articles/ask-the-applications-engineer-25.html

I'm guessing you're looking to build a peak detector.

MrAl:

--- Quote from: Freshman on April 17, 2024, 07:15:01 am ---I read that the slew rate of an op-amp the rate at which the output voltage changes with time with respect to a step change in input.

So my question is will the slew rate of an op-amp depend on external circuit components?

Suppose,

1. I have a capacitor at the output of an op-amp. In this case, the slew rate will not be the same as mentioned in the op-amp datasheet correct?
(I think that adding a capacitor at the output of an op-amp is not a good idea as its purpose is not to charge a capacitor or moreover I haven't seen a requirement where there would be a capacitor requirement at the output of the op-amp. Please correct me if I am wrong).

2. What impact would a diode create on the slew rate of the op-amp if it is placed at the output?


3. On what external factors does the slew rate of an op-amp depend upon?

--- End quote ---

"Slew rate" is a very general term just like "frequency", but when we talk about slew rate of an op amp we are usually talking about the FASTEST the output can change and this is a specification shown on the data sheet.

For example, if we had an op amp that has a slew rate of 1v per microsecond with the non inverting input terminal grounded, when we apply a step change on the inverting input the fastest the output could rise (or fall) would be 1v every microsecond.  That's an example of the slew rate spec of an op amp.
If we keep the ground connection on the non inverting terminal and connect a 10uf capacitor from the output to the inverting input, and a 100k resistor from the input signal to the inverting input, we create an integrator circuit.  Now when we apply a step change to the input (which is now at the open end of the 100k resistor) the fastest the output could change would be 1v per second.
So we went from the fastest (1v per microsecond) to a much slower rate (1v per second) due to the external components.

It's possible to slow down the rate of change of the output, but it is not possible to speed it up faster than the specification of the op amp which is called the slew rate of the op amp, and can be usually found on the data sheet.  It will be a little bit of an approximation but serves to characterize the output as the maximum change obtainable with that particular op amp.

If we were to look at other op amps, we may find some that are faster and some that are slower.  That would mean some would fit an application better than others.  For example, the fast slew rate op amps are usually used when we need a really fast change on the output with high bandwidth, while the slower ones are usually for circuits that need a higher degree of DC accuracy or are just older op amp models.

Take a look at some op amp data sheets and look for the slew rate specifications and compare them.

Freshman:
Thank you for the answer!

To clarify:

1. On what external "components" would the slew rate of the op-amp depend upon?In what conditions, diode will affect the slew rate? Junction capacitance of the diode?

2. Also, if I increase the length of the output of the op-amp signal line (say I connect the output of op-amp to a cable), the slew rate will get impacted for the worse, right?

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