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So I figured I need an audio range function generator and was thinking of generating a square wave with a 555 and then using op-amps to integrate it into a triangle and sine. Since I happen to have some LM339s laying around and I was thinking, I could use these. All my searches yielded only talk about why you should not use opamps as comparators, but not the other way around.

So hence my question, what are the drawbacks of using a comparator with pull up resistor as an op-amp integrator? Should I just use 2 or 3 discrete 741s instead?

Also the design frequencies are low, some 40 to maybe a few kilo Hz, mainly for testing linearities and distortion of different circuits(I got tired of using the old computer's sound card method)

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  • \$\begingroup\$ Mad oscillation. \$\endgroup\$ – Ignacio Vazquez-Abrams Jan 6 '15 at 7:01
  • \$\begingroup\$ You mean they will start to unintentionally oscillate? if it's out of the audio range, I could probably damp it with a simple cap though, right? \$\endgroup\$ – Limiter Jan 6 '15 at 7:21
  • \$\begingroup\$ There's no internal frequency compensation, so comparator won't be unity-gain stable i.e. no idea what open-loop frequency would yield 180 degree phase shift. This is pretty much off-label usage. Maybe a better question is, why don't you have any general-purpose op-amps in stock? \$\endgroup\$ – MarkU Jan 6 '15 at 9:09
  • \$\begingroup\$ Ah I see, care to post an answer, so I can accept? As a hobbyist my stock is not large, but I do have some 741s around, figured I could do it all in one chip though. I guess not. Well time to stock up on opamps:) \$\endgroup\$ – Limiter Jan 6 '15 at 9:30
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Unlike an op amp, comparators generally have no internal frequency compensation, so therefore a comparator won't be unity-gain stable. There is no way to know what open-loop frequency would yield 180 degree phase shift. The equivalent circuit diagram for LM339 supports this generalization. ( http://www.onsemi.com/pub/Collateral/LM339-D.PDF )

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