Comparator as op amp in oscillator circuits? - Page 1
Forum section: electronics / beginners

author: Infraviolet
author_rank: Super Contributor
timestamp: April 15, 2023, 05:00:09 pm
content: For circuits where an op-amp is usually used, but in a positive feedback scenario such as an oscillator (such as this sort of thinghttps://circuitdigest.com/sites/default/files/inlineimages/u1/Op-Amp-Based-Colpitts-Oscillator.png) can a comparator be used instead? particualrly in circumstances where you need the speed of a comparator as the LC tank is tuned for frequencies higher than a typical op amp will happily handle.I was thinking mainly in the case of an oscillator with a high enough gain to give only a square wave output rather than a sine wave, but I'd be intersted to know if such a substitution of comparator for op amp could work for sine wave generating lower gain oscillators.Thanks

author: Kleinstein
author_rank: Super Contributor
timestamp: April 15, 2023, 05:14:54 pm
content: The linked circuit still has the OP-amp working in linear mode. For comparators the behavior in linear modes is not well defined and it depends on the model. Some behave like a not compensated (or only for high gain) OP-amp and may work for high gain. Others can have hysteresis and would behave odd if tried to use them as linear.

author: Zero999
author_rank: Super Contributor
timestamp: April 16, 2023, 12:04:27 pm
content: Just one question: why not use a crystal oscillator IC such as the 74LVC1404 or 74LVC1GX04?https://www.ti.com/lit/ds/symlink/sn74lvc1404.pdfhttps://assets.nexperia.com/documents/data-sheet/74LVC1GX04.pdf

author: dobsonr741
author_rank: Frequent Contributor
timestamp: April 16, 2023, 02:08:33 pm
content: Try it and let us know what happens.

author: David Hess
author_rank: Super Contributor
timestamp: April 16, 2023, 03:36:16 pm
content: Your example has the operational amplifier operating in its linear region, however comparators are useful at higher frequencies in oscillator circuits, including crystal oscillators, although it is not very common.

author: Infraviolet
author_rank: Super Contributor
timestamp: April 17, 2023, 12:17:10 am
content: Thanks, I'll check if I've got a comparator IC lying around in DIP format and breadboard it. With luck I might even have a comparator with the same pinout as the op amps I've breadboarded this with before, swapping back and forth will make differences in behavious very clear, if the comparator version will behave at all.Why not a crystal? This is for an application where I'm using colpitts frequency as a meaure of inductance and eddy current sensing.

author: Zero999
author_rank: Super Contributor
timestamp: April 17, 2023, 12:48:54 pm
content: Quote from: Infraviolet on April 17, 2023, 12:17:10 amI ICs I suggested in my previous post, should work in a Colpitts LC oscillator circuit.

author: Infraviolet
author_rank: Super Contributor
timestamp: April 18, 2023, 09:21:32 pm
content: Thank you, I thought you were meaning a crystal as an alternative to a colpitts. Now you say it like that though I can see what you're suggesting.

author: Zero999
author_rank: Super Contributor
timestamp: April 18, 2023, 10:25:19 pm
content: You could try the 4007 or 4069.https://wiki.analog.com/university/courses/alm1k/alm-lab-21