Hi folks, I’ve been trying to use LM311 to compare for a simple project, and it is depressing (to say the least) that my friends and I have not been able to get the comparator to work, I suspect that it is due to a knowledge gap, but our lab instructors keep saying, well the equipments here are for Educational Purposes and no one certifies them, and the LM311’s are cheap and unreliable so it’s not a big deal to not be able to compare voltages properly, for your project get IC’s which are reliable and more expensive I personally hate this mindset, what are your thoughts, and if the problem indeed is knowledge gap (which it certainly is according to me, as I don’t fully understand why the heck you need a pull up resistor with the thing) then do please tell me what I should be doing to catch up.
Thanks a lot for going through this post… All answers are appreciated.
P.S. the project involves comparing a sine wave to get a square wave which is then fed to a T flip flop (made using JK flops that we have) to generate phase shifted waves as shown in the image attached, which is an LTspice simulation with the LM741s
Not what you were asking about, but you reminded me of this video arguing that the LM741 is a relic and not a good choice for a modern circuit. IIRC the video also shortly explains some standard characteristics of an opamp, which could be interesting to you if you suspect a knowledge gap.
I seem to recall people saying some 311’s are fake. Search YT for people doing die shots.
I think there are lots of situations where rejects are sold on the grey market. Try characterizing them to verify they meet the datasheet specification.
There is an IC tester in our lab and it says GOOD everytime I’ve plugged in. By die shots you mean like opening up the IC and viewing it under the microscope and testing it?
You would need to know how the IC tester works. I’ve built my own op amp tester before. All I did was check that some basic inputs yield the correct polarity of output capable of lighting an LED. Characterization involves verifying the actual specifications such as quiescence.
Look through old datasheets and databooks. Back in the 1980s and older, it was common for nearly all ICs to have full documentation. Many are all the way down to the transistor level of stuff like op amps. These often have example circuits you should save and keep for reference. You are likely to encounter test circuits for characterization in some of them. BitSavers.org is a repo of such things. This place is hard to navigate for anything specific. Their stuff is on archive.org too. While it will not help you in this instance, here is a list of logic databooks on archive. It is not complete. https://github.com/Upcycle-Electronics/ChipLabels/blob/master/ListOfLogicDatabooksOnArchive.md (Aside: Pay special attention to the 1996 Motorola databook on that list. Motorola tried to defy the 4k and 74 series logic nomenclature/syntax that originated with Texas Instruments. Motorola had a history of trying to out quality and document the competition in exchange for exploitive brand locking and manipulation so you had to always buy Motorola exclusively. Around the mid to late 1990s they shifted documentation to focus more on compatibility as the company was floundering. This results in an outstanding source of useful information.)
Search the eevblog forum for information and circuits for characterizing.
Keep in mind that the English world is often stupid prejudice. Many times people do not look past the part number. Many of the old part numbers are based on expired patents. So anyone with an ancient fab sitting around is able to make these parts. The datasheet specifications from one manufacturer is not relevant to any other. The part number is just a general thing with no regulatory body or requirement for maintaining some fictional standard. Even with the original part from ages ago when it was patented, had a distribution curve of quality and behavior with a published failure rate. Many of the specifications show typical values on the datasheet without a minimum or maximum because of the potential failure rate and distribution curve of parts that pass initial automated inspection.
Lastly, yes, search YT for channels like ElectronUpdate that use sulfuric acid to dissolve the epoxy packaging and photograph the die under a microscope. Ignore anyone that simply claims parts are fake but does nothing to prove their claims through rigorous testing and shows evidence.
What’s your schematic with the lm311 ? Input impedances, output impedance, voltages, etc
Made this two hours back, going to go and simulate this in the lab now, will get back with results later.
Looking at the datasheet for the lm311, the output is a transistor (I think it’s shown on your schematic), the reason you need a pull-up on it is because it has no bias or power. That will let it pull the voltage down. If you’re noting that it doesn’t rise fast enough, try making that 100k smaller, give it some more strength.
. The 741 on the other hand has a pair of transistors on the output stage, attached to +v and -v so it can push and pull the output
