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enter image description here

I made this triangular wave generator, the left op-amp is an integrator and the right one a non-inverting comparator with hysteris.

I require a 1khz signal with Vpp=5V and average value 2.5V. My circuit works fine and i get the expected results.enter image description here I wanted to decrease the capacitor value and increase the frequency as they are related by this formula
f=(R_1+R_4)/(4R_3 x C_1 x R_2).
But as i went on decreasing the capacitor value the amplitude of the triangular wave kept getting altered even though the frequency was increasing. This did not make sense to me as the amplitude of the triangular wave is given by the difference in threshold voltages of the non inverting comparator which are only a functions of R_2,R_1,R_4 and the output voltage dynamic of the op-amp which i assumed to be symmetrical.
amplitude of the triangular wave(without offset/average)=
Vs1-Vs2=2(R_2/(R_1+R_4))V_oh.
Soon enough the triangular wave started looking like a square wave because it was getting clipped at the top (yellow signal). enter image description here I have been thinking about this for a while , i considered the slew rate of the op-amp to be a limiting factor, but i cant explain the amplitude modulation as a result of the limited slew rate of the op-amp.
But it seems clear to me that the saturation voltages of the comparator are getting affected by the capacitor after a limit ~250pF and then they start to grow in magnitude. I also studied the behavior of the amplitude of the triangular wave and have plotted with respect to capacitors(pF).
This is the graph of the lower threshold voltage of the comparator (lowest value of triangular wave). enter image description here
This is the graph of the upper threshold voltage of the comparator (highest value of triangular wave).
enter image description here
And here is the ratio of the measured frequency with respect to the calculated frequency.
enter image description here

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but I cant explain the amplitude modulation as a result of the limited slew rate of the op-amp.

The problem is that the TL082 is quite slow and, when used as a comparator, it takes several micro seconds to change output state. In the short time, the triangle wave is still going to carry on charging in the same direction and produce a higher amplitude.

Then when the output comparator does start to change the limited slew rate exacerbates the problem. In effect, your comparator needs to be much-much faster.

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  • \$\begingroup\$ which parameter tells us if its slow ? how can i account for the amplitude modulation. \$\endgroup\$
    – BonPi
    Commented Nov 28, 2022 at 14:01
  • \$\begingroup\$ From answer: the triangle wave is still going to carry on charging in the same direction and produce a higher amplitude <-- that is the amplitude modulation (your words) but, calling it modulation is missing the point. \$\endgroup\$
    – Andy aka
    Commented Nov 28, 2022 at 14:03
  • \$\begingroup\$ oh yes . but how can i know through a datasheet on how fast an op-amp is as a comparator ? \$\endgroup\$
    – BonPi
    Commented Nov 28, 2022 at 14:04
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    \$\begingroup\$ @BonPi all comparator data sheets will tell you the propagation delay. That's the key parameter for consideration. However, running from +/- 12 volts will limit your choices. Better to find a single supply design, use a rail-to-rail op-amp and something like a MAX999 as comparator. \$\endgroup\$
    – Andy aka
    Commented Nov 28, 2022 at 14:07
  • \$\begingroup\$ I can't seem to find the propagation delay in the TL082 datasheet. Is the propagation delay related to slew rate that is the time it take to switch from V_oh to V_ol or is it the delay in comparison between the threshold voltage of the comparator and input to the comparator ? \$\endgroup\$
    – BonPi
    Commented Nov 28, 2022 at 14:24

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