Can Slew Rate Limiting Cause Triangle Wave Output in Op-Amp Comparators?

[hayze2728]

Hi, I'm sorry I can't follow the recommended structure for this, as it's not a problem as such, more of a 'what should I have got?'

I have to write up a lab report in which we were using Op-Amps as summing amplifiers, comparators and integrators. We used an open loop 741 as the comparator, referenced to ground. A sine wave input signal was passed through the non-inverting input, and the output resembled a square wave. Now, we were meant to increase the frequency of the input signal to see what would happen to the output, but we didn't get this far in the experiment. I know a little about slew rate limiting, and I'm assuming this is where the answer lies. Am I right in saying that at a certain frequency the output would resemble a triangle wave, as there is not enough time for the output to saturate before the input crosses the reference again?

Thank you for your help.
Stefan

 

[jbsteele]

Yes, you are correct. The output of the comparator will naturally begin to resemble a triangle wave (or a sawtooth wave) as the frequency of the input signal increases and approaches the slew rate. This is because the comparator is unable to switch the output fast enough to keep up with the input signal, and so the output does not have enough time to saturate before the input crosses the reference again. As the frequency of the input signal increases further, the output waveform becomes increasingly distorted until it eventually resembles a square wave again.

 

[thomate1]

Yes, you are correct in your assumption that at a certain frequency, the output of the op-amp comparator would resemble a triangle wave due to slew rate limiting. Slew rate refers to the maximum rate of change of the output voltage of an op-amp, and it is limited by the internal circuitry of the op-amp. When the input signal has a high frequency, the output voltage of the op-amp cannot change quickly enough to keep up with the input signal, resulting in a distorted output. This is known as slew rate limiting.

In the case of an op-amp being used as a comparator, the output voltage will either be at the positive supply voltage or the negative supply voltage, depending on whether the input voltage is above or below the reference voltage. When the input signal has a high frequency, the output voltage will not have enough time to reach the supply voltage before the input signal crosses the reference voltage again, resulting in a triangular output.

It is important to take into consideration the slew rate of an op-amp when using it as a comparator, as it can affect the accuracy and reliability of the output signal. In your experiment, it would have been interesting to see the effect of increasing the frequency on the output signal and observe the transition from a square wave to a triangle wave.

I hope this helps clarify the concept of slew rate limiting in op-amps used as comparators. If you have any further questions, please feel free to ask. Good luck with your lab report!