Op-amp slew rate over frequency

Question

I did an experiment with an op-amp for measuring slew rate of a 741. After a certain frequency, with the help of an oscilloscope, I measured a slew rate which was approximately 25 V/ms. After that I was expecting that as I increase the frequency, the slew rate will also change, but it didn't changes at all. I varied my frequency up to 100 kHz, and I found the same 25 V/ms. Why is this happening?

Before I did the experiment, I was also wondering what frequency I should use to measure my slew rate. Through my experiment, I found it to be constant over frequency, but I still don't understand why this is happening. Is it due to the maximum rate of change of voltage?

Answer 1

Slew rate is the maximum dv/dt the op amp can produce at its output, and it doesn't depend on frequency.

This means that at high frequencies, the output will be distorted. The distortion happens when the slew rate is not enough to support the dv/dt required to accurately produce a sinusoidal output. Since the dv/dt of the sine wave depends on both the frequency and the amplitude, you can reduce the distortion by reducing either one.

Edit

what frequency should I use to measure my slew rate?

As you found, you can measure the slew rate at any frequency that is fast enough to exercise the slew rate limiting behavior. As discussed above, the minimum frequency to see the slew rate also depends on the input amplitude. You could also use a step input rather than a sinusoid to measure the slew rate.

Answer 2

Is it due to the maximum rate of change of voltage?

Indeed it is. The Photon has provided the correct answer. I'll expand on it.

Almost always, an op-amp has an internal compensation capacitor that effectively limits the rate of change of the output voltage. Roughly speaking, there is a maximum current available to charge the compensation capacitor and this limits the rate of change of the output voltage.

There's plenty of material available on the web to investigate this. Back in the '70s, there was quite a bit of research done on so-called "transient intermodulation distortion" in audio amplifiers. TIM is related to slew rate limiting.

In fact, Marshall Leach showed, with his "Low TIM amplifier" design, that avoiding TIM amounted to avoiding slew rate limiting.