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I've designed an astable multivibrator for square wave generation using the LM741 op-amp, as showen in the following image:

astable multivibraotr Lm741 op-amp credit to https://www.electronics-tutorials.ws/opamp/op-amp-multivibrator.html

Now I've played a little with the frequencies it can output. As shown in the image I've got 1kHz both in calculations and in simulation.

If, for example, I needed a frequency of 30kHz I've changed parameters accordingly but my output simulation first of all doesn't get close to the 30khz and it starts to look more triangular than square wave, which it should be.

I've tried adding some Rs=100kΩ at the negative and positive for precautionary reasons, but it didn't do much.

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LM741 has 0.5V/µs slew rate.

30kHz half period is 16µs, so in this half period the output of LM741 can slew up or down 8V, so the maximum triangle wave amplitude it can output would be +/-4V.

So at 30kHz if the power supply voltage is more than +/- 6V or so, the output will be a triangle with no flat tops. If power supply voltage is lower, it will clip and limit the amplitude, so you will get flat tops.

Solution: when you need a comparator, use a comparator (that has no slew rate limit), not an opamp (that does).

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  • \$\begingroup\$ But still if I would want to use an opamp, what would be a good pick that has lower slew rate? \$\endgroup\$ Oct 14 at 10:02
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    \$\begingroup\$ If you want square instead of triangle, you need higher slew rate not lower. Anything is better than 741 lol. \$\endgroup\$
    – bobflux
    Oct 14 at 10:56
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    \$\begingroup\$ @PenescuVlad, The very common TL081 and TL071 have 13V/µs slew rate. Almost anything is better than a 741, and the parametric search facilities on Mouser or Digikey will enable you to find a device with whatever specs you need in a few seconds. \$\endgroup\$ Oct 14 at 13:03
  • \$\begingroup\$ A comparator like LM339, LM393 or one of its many clones will give you a nice square wave output for 30 cents. If you want a nice symmetric square wave, pick one with push-pull output (or just use a 555). \$\endgroup\$
    – bobflux
    Oct 14 at 13:24
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    \$\begingroup\$ OK! For an opamp, the compensation cap limits the slew rate. Comparators don't need to be stable in close loop, so they don't have a compensation cap, so you get much faster output switching. \$\endgroup\$
    – bobflux
    Oct 14 at 18:20

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