Op Amps and freq/duty. - Page 1
Forum section: electronics / beginners

author: nsled
author_rank: Contributor
timestamp: June 03, 2024, 07:15:08 am
content: What I need,Change the following simulation to achieve 0-100% duty and frequency ranges of 50-500Hz, 500Hz-5000Hz, 5000-20000, 20000-50000Hz.https://www.falstad.com/circuit/circuitjs.html?ctz=CQAgjCCsCmC0DsIDMAOATAOngNjZdYALEgJwkAM2hU5UNUcYYAUIeRLCduDuN2LwgoAOgGcwkMbAliw5eQrHYMSMCXgo2hEtsiQ8Y+Bkg6USbPp1kJaWQvkhyYtE-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-RRLZdm-P9EPA81F0YRo5GJSBSXJEhKWpdg91eEMfTKOUMS1Loq3A5CAK2CZEMYpFkFNFoAHtwA7OCYnIMh-z3eR1AsXAOxiTjkCBEIgmQZgONICiYjkRBYnIDB7CEyo8HALpkBIGSgAI cannot follow this well enough to change it, I can get it to output 50-500Hz or close enough, but I cannot maintain the triangle wave to give a good duty cycle as it tends to turn square wave in the centre of its range. I'm wondering if someone with a better understanding of this circuit can show me which components need to be changed to change the freq. range and shape of the triangle waveform.Attaching the problem in pic below, maybe it's just not possible with the component placements available. 50Hz nice triangle, 138Hz crappy squarish waveform, 7.5kHz acceptable wavey waveform.

author: Terry Bites
author_rank: Super Contributor
timestamp: June 03, 2024, 12:53:22 pm
content: The devcies you have used are comparators not opamps.Try it with an LM324 opamp. R18 is shorted out.I suggest you redraw it. Strange circuit.

author: MrAl
author_rank: Super Contributor
timestamp: June 03, 2024, 02:00:30 pm
content: Hello there,Unfortunately, the power bandwidth of the LM324, LM358, etc., is too limited if he wants to get all the way to 50kHz even with only 10 volt peaks (that's +10v and -10v). The slew rate needs to be higher to get that high in frequency and get a reasonable triangle wave output.You can calculate the slope pretty easy because it's just a triangle that goes up in 1/4 cycle and then down to zero in the next 1/4 cycle, then goes negative for the next 1/4 cycle and then back to zero in the final 1/4 cycle. To get to 10v in 1/4 cycle at 50kHz, the slope has to be 2e6 volts per second, or 2v/us, and the above mentioned op amps will only do about 0.5v/us.Of course if the amplitude of the peaks are reduced to around plus and minus 2.5 volts, it may work with those types of op amps.At the current peak values shown in the waveforms, it looks like he is trying to do around 14 volts peak, and that's going to require an even faster op amp than 2v/us. Since 14 is very roughly 1.5 times 10, an op amp that can do 3v/us could work. Remains to be tested though.

author: Infraviolet
author_rank: Super Contributor
timestamp: June 03, 2024, 06:07:43 pm
content: Falsatd won't simulate it, it only lets you simulate a few specific op amps or an idealised one, but the MCP6024 or MCP6294 series would have a fast enough slew rate for you (7V/us), though they can only take 5V max. The MCP6004 may also be fast enough on slew, if I remember rightly, though not near the top end of your frequency requirements, but again 5V max. You might generate your triangle wave at lower voltage then use a final transistor stage to give the higher voltage output you eventually want?