OpAmp as Schmitt-trigger

RE: OpAmp as Schmitt-trigger

jc2048 — Sun, 26 Jan 2025 22:16:54 GMT

I've never owned a copy of tAoE. Perhaps e14 could offer one or two as a prize in a P14 competition.

Good blog, but...

"it immediately goes to the rail voltage"

not with a 741 it doesn't. The slew rate is less than a volt per microsecond, and that's slower than the recommended input transition rate even for a slow logic like HCT. In practice it would probably happily work anyway (for a personal project), but there's a danger the input might oscillate during the transition.

RE: OpAmp as Schmitt-trigger

scottiebabe — Sun, 26 Jan 2025 18:00:52 GMT

Asked the smallest deepseek model what a Schmitt trigger is, the thinking tokens are amusing

[embed:dc8ab71f-3b98-42d9-b0f6-e21e02a0f8e2:2bb6a5e3-23d7-46e5-89a5-fbb6b6ea6e23:type=text&text=%3Cthink%3E%0D%0AOkay%2C%20so%20I%27m%20trying%20to%20understand%20what%20a%20Schmitt%20trigger%20is.%20From%20what%20I%20gather%2C%20it%27s%20something%20to%20do%20with%0D%0Aamplifiers%20or%20electronic%20circuits%2C%20but%20I%27m%20not%20entirely%20sure%20about%20all%20the%20details.%20Let%20me%20see%20if%20I%20can%20piece%20this%0D%0Atogether.%0D%0A%0D%0AI%20remember%20hearing%20terms%20like%20%22non-linear%2C%22%20%22hysteresis%2C%22%20and%20%22op%20amp%22%20in%20my%20electronics%20class%2C%20but%20I%27m%20a%20bit%0D%0Afuzzy%20on%20what%20exactly%20those%20mean%20and%20how%20they%20relate%20to%20the%20Schmitt%20trigger.%20Maybe%20I%20should%20start%20by%20breaking%20down%0D%0Aeach%20of%20these%20words%20and%20see%20how%20they%20fit%20together.%0D%0A%0D%0AFirst%20off%2C%20an%20op%20amp%E2%80%94oh%20yeah%2C%20that%27s%20the%20operational%20amplifier.%20It%27s%20a%20voltage%20comparator%20with%20input%20buffers%2C%0D%0Aright%3F%20And%20I%20think%20it%27s%20used%20for%20its%20ability%20to%20amplify%20signals%20without%20introducing%20distortion.%20But%20wait%2C%20isn%27t%0D%0Athere%20something%20about%20negative%20feedback%20in%20op%20amps%3F%20That%20must%20be%20related%20to%20hysteresis.]

RE: OpAmp as Schmitt-trigger

shabaz — Sat, 25 Jan 2025 21:45:50 GMT

Excellent post, and very clear-to-follow explanation. I'm not sure I've ever seen an explanation that showed the set point in the 'scope traces, which helps understand it a lot.

It's so easy to accidentally assume that an op-amp as basic comparator is all that is needed, whereas the hysteresis can greatly improve it.

RE: OpAmp as Schmitt-trigger

genebren — Sat, 25 Jan 2025 20:02:39 GMT

I remember learning OpAmp circuits back in the mid 70’s and the comparator with hysteresis was one my favorite circuits. On my first job out of college, my first improvement to an existing design had to do with a poorly designed comparator circuit with insufficient hysteresis. It real felt good to solve an issue so early in my career (within the first month out of school and on the job).

RE: OpAmp as Schmitt-trigger

DAB — Sat, 25 Jan 2025 19:50:01 GMT

Great post Jan.