Author Topic: Slew Rate of an Op-amp  (Read 1008 times)

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Freshman

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Slew Rate of an Op-amp
« 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?

Zero999

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Re: Slew Rate of an Op-amp
« Reply #1 on: April 17, 2024, 07:33:22 am »
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.

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Terry Bites

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Re: Slew Rate of an Op-amp
« Reply #2 on: April 17, 2024, 08:54:33 am »
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.

A capacitor is an unwelcome load for most opamps. An isolation resisistor, compensation newtork or a buffer would generally be needed.

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

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MrAl

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Re: Slew Rate of an Op-amp
« Reply #3 on: April 17, 2024, 09:52:46 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?

"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

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Re: Slew Rate of an Op-amp
« Reply #4 on: April 17, 2024, 11:18:22 am »

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?

Kleinstein

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Re: Slew Rate of an Op-amp
« Reply #5 on: April 17, 2024, 06:12:55 pm »
For most OP-amps the loading of the output does not make a big difference, unless the OP-amp hits the current limits. I would expect a small the effect at the transition, e.g. when starting to slew and maybe at the output cross over range (the current changing sign).  Usually the slew rate is limited by the input stage and internal compensation. The linke from the compensation to the output is usually relative direct.

The supply voltage and temperature can effect the slew rate. With a few older types also the offset adjustment part can have an effect. Of cause external compensation can also make a big difference, but not many types support this.

MrAl

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Re: Slew Rate of an Op-amp
« Reply #6 on: April 18, 2024, 10:38:25 am »

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?

Well I think it is time we started to outline the clear difference between the op amp slew rate specification SR and the circuit "slew rate" which I think would be better thought of as the circuit response (CR).

The SR is really not changeable.   That's because it is how the op amp itself behaves without any circuitry connected yet.

The CR is completely changeable and this should be obvious because different electrical parts have different characteristics of their own, such as inductors and capacitors.  For a simple example, say we connect the output of an op amp with SR around 10v/us to the input of an op amp with an SR of only 1v/us.  The output of the second op amp is only going to slew at 1v/us even though the first one is able to slew at 10v/us, and thus we altered the slew rate of the first op amp with the second op amp.
A more typical setup though is to create a filter of some kind.  In that case, the slew will be limited to whatever the added circuit does, not the op amp, provided the op amp selection fit the application decently to begin with.

Now maybe we can talk about the questions 1 and 2.

[1]
Any components with a reactive element such as a capacitor or inductor may alter the CR but it would not alter the SR because again, the SR is a specification of the op amp itself.  If we connected a capacitor from the output to ground that could make the SR look slower, but that would really still be part of the circuit response CR because then we no longer have JUST an op amp we have an entire circuit, even though it's a small circuit.
A diode, with internal capacitance, may or may not alter the CR (it does not alter the SR) depending on how fast the op amp is.  If the op amp is a general purpose 0.5v/us SR op amp, a diode probably is not going to affect it much.  If the op amp had a very fast slew rate spec, then the diode capacitance could alter the CR somewhat.  It depends on the op amp slew rate, the drive capability, and the diode.

[2]
If you increase the cable length on the output of the op amp, or even use a short cable, then we have to look at transmission line effects, and that could have a massive impact on the CR when you go to measure the output of the cable, and it may even have an impact on the input to the cable, but that would still be considered the CR not the SR.

In short, the SR comes with the op amp and pretty much stays the same all the time.  The CR depends on circuit components and can therefore vary widely, and possibly make the op amp output itself look slower depending on how it loads the op amp.

The input can also have an effect on the CR of course because there could be reactive components there too.  Now the output of the op amp may appear to slew much slower, but that's not due to the op amp itself it is due to the components connected to the input.  Maybe we could call this the "apparent slew rate" but it's just part of the CR really, and the SR of the op amp itself did not change.

This is just like anything else.  When we look at one component by itself it had certain characteristics, then when we connect it to other components we see some different responses come about.  That does not mean that that one component changed itself.  It still, usually, maintains its own behavior it is just that the circuit response changes due to the mix of parts.

Smf