TL081 weird glitch  - Page 1
Forum section: electronics / beginners

author: sheisthedarkness
author_rank: Newbie
timestamp: November 14, 2019, 04:29:40 pm
content: Hello, I bought a bunch of TL081CP from eBay and I got a different revision from the ones I already have. Making a simple negative gain circuit with any of the 15 I bought causes these glitches in both the input and output signal as pictured in the attachment. The exact same circuit with only the opamp swapped out for one of the older ones I have does not exhibit this problem, signal is clear as it should be. When the opamps are powered off the signal also gets through perfectly. The older revision has the code "86AEP2M" while the new ones have "18MDSCH". At first I thought they were low quality chinese parts but I found it weird that they'd all exhibit the same imperfection.Does anybody have a clue as to what might cause this or what could be done to correct the problem ? Thank you in advance.

author: magic
author_rank: Super Contributor
timestamp: November 14, 2019, 04:34:19 pm
content: Quote from: sheisthedarkness on November 14, 2019, 04:29:40 pmQuote from: sheisthedarkness on November 14, 2019, 04:29:40 pmAnother onehttps://www.eevblog.com/forum/beginners/opamp-input-offsets-working-in-the-opposite-direction-to-what-i-expect/And yeah, welcome to the forum, I guess. We have those threads every other month nowMaybe eBay should pay Dave to put up a banner ad informing that opamps from auction sites are fake

author: wraper
author_rank: Unknown
timestamp: November 14, 2019, 04:34:53 pm
content: You got not TL081 but something completely different. There is little chance getting genuine op amps on ebay, even piss-poor LM358. Especially when price is on lower side.

author: sheisthedarkness
author_rank: Newbie
timestamp: November 14, 2019, 04:43:50 pm
content: Quote from: wraper on November 14, 2019, 04:34:53 pmOh, well that's unfortunate. I didn't think something like that would be worth faking. How bad is it that it can't reliably give me a gain of 2 at 1kHz ? Or am I using whatever it is wrong by using it like I would a TL081 ?Quote from: magic on November 14, 2019, 04:34:19 pmThanks, I searched "TL081" but couldn't find anything relevant so I made the thread. The link you sent is surprisingly similar lol.

author: magic
author_rank: Super Contributor
timestamp: November 14, 2019, 04:56:18 pm
content: It apparently is worth fakingSomebody just buys those LM358/LM324 dies and has them packaged as single, dual or quad and prints whatever markings that are in demand given week on top.Gain of 2 at 1kHz shouldn't be a problem for LM358. But do you have a problem with gain or is your only complaint the distortion near zero crossing? The distortion is crossover distortion, it's kinda expected in this opamp regardless of gain.Not sure what to think about the input side glitches. Perhaps it's capacitive coupling from the output (what's the source impedance driving the amplifier) or perhaps it is somehow caused by input stage overload when the output stage is switching polarity and completely unresponsive. Try some legit LM358 from a reputable manufacturer if you have one.

author: sheisthedarkness
author_rank: Newbie
timestamp: November 14, 2019, 05:02:45 pm
content: Quote from: magic on November 14, 2019, 04:56:18 pmAh, you're correct, gain is as expected, it's really just crossover distortion. It's most likely the LM358 then ? Thanks, that explains it perfectly. I guess that rules out eBay for any future component purchasesQuote from: magic on November 14, 2019, 04:56:18 pmI don't, but I will remember to if I get my hands on one

author: wraper
author_rank: Unknown
timestamp: November 14, 2019, 05:04:35 pm
content: Quote from: magic on November 14, 2019, 04:56:18 pmThey are not LM358/LM324, too expensive. They are some sort of cheapest Chinese opamps. And LM358/LM324 are counterfeited too.https://www.eevblog.com/forum/reviews/chinese-fake-op-amps/https://zeptobars.com/en/read/Ti-NE5532-real-vs-fake-opamp

author: magic
author_rank: Super Contributor
timestamp: November 14, 2019, 05:08:06 pm
content: They are the cheapest Chinese LM358/LM324Classic chips like 709/741/071/358/74xx have been cloned all over the former commie bloc for half a century. You probably know.

author: wraper
author_rank: Unknown
timestamp: November 14, 2019, 05:09:54 pm
content: Quote from: magic on November 14, 2019, 05:08:06 pmThey are cheap but not LM358/LM324, at least most of them.

author: iMo
author_rank: Super Contributor
timestamp: November 14, 2019, 05:21:50 pm
content: Try to wire a few kohm resistor from opamp's output against Vcc. It could be the glitch disappears..

author: magic
author_rank: Super Contributor
timestamp: November 14, 2019, 05:23:27 pm
content: And what's cheaper than LM358?Probably not even recycled chips. The prices of those tend to start around 30 cents on AliExpress, the LM358 fakes go for 10 cents

author: wraper
author_rank: Unknown
timestamp: November 14, 2019, 05:24:59 pm
content: Quote from: magic on November 14, 2019, 05:23:27 pmYou can make 5 times cheaper than LM358 if you want. Look at fake NE5532 die shot, it's certainly not LM358. And it's simpler than LM358.

author: sheisthedarkness
author_rank: Newbie
timestamp: November 14, 2019, 05:34:16 pm
content: Quote from: imo on November 14, 2019, 05:21:50 pmJust tried, no luck, same thing. How could this help exactly ?

author: magic
author_rank: Super Contributor
timestamp: November 14, 2019, 05:36:12 pm
content: It's RC4558 topology, another cheap classic opamp and more suited for audio. But I admit that in the case of fake 5532 I'm not entirely sure if it's an off the shelf die because it's just so tiny. I don't know if it meets the power dissipation specs of a real 4558 and for how longThough on the second thought, some discrete transistors are even smaller, so who knows, perhaps it is a legit 4558 rather than a custom job.But you are right, going for custom tiny underpowered and oversimplified designs will probably be the final step required to break the magic 10 cent barrierQuote from: sheisthedarkness on November 14, 2019, 05:34:16 pmBy biasing the output in class A. But the current conducted by the resistor at all time needs to be higher than the output current of the entire circuit. What kind of load is this opamp driving and what are the supply voltages?

author: sheisthedarkness
author_rank: Newbie
timestamp: November 14, 2019, 05:42:01 pm
content: Quote from: magic on November 14, 2019, 05:36:12 pmInput is 1kHz about 2V pk-pk, supply voltages are + and -12V. I don't reallyneedto get it working, I'm going to buy some real ones from digikey, I was just curious about your post.

author: magic
author_rank: Super Contributor
timestamp: November 14, 2019, 05:45:08 pm
content: It wasn't my post. But a few kΩ resistor to either rail should eliminate this distortion if there is nothing more connected to the output.Okay, gain 2 so you have some gain resistors. Try unity gain or higher value gain resistors or lower value resistor to the rail, up to the point of cooking that resistor

author: sheisthedarkness
author_rank: Newbie
timestamp: November 14, 2019, 05:51:35 pm
content: Quote from: magic on November 14, 2019, 05:45:08 pmHey, that worked ! 1k resistor from out to vcc eliminated the distortion, although I still don't really get why. Thanks !

author: mikerj
author_rank: Super Contributor
timestamp: November 14, 2019, 06:02:44 pm
content: Quote from: sheisthedarkness on November 14, 2019, 05:51:35 pmIt's because you are forcing the output stage to work in class A operation, i.e. only one of the output transistors is ever conducting. This means you can only drive higher impedance loads however, once you load the output enough that the 1k resistor can't provide sufficient current the crossover distortion will be back.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 14, 2019, 07:08:22 pm
content: Quote from: sheisthedarkness on November 14, 2019, 04:43:50 pmRule 34 of electronics sourcing: if it exists, there's counterfeit of it!If it came from eBay, Ali or other unofficial source, assume it is counterfeit. Or salvaged. Doesn't matter what.And agree, looks like LM358 crossover distortion. Could be something even worse than that (a Chinese copy?), but not much worse without simply not working at all.Tim

author: RoGeorge
author_rank: Super Contributor
timestamp: November 14, 2019, 07:26:05 pm
content: See if this remedy works

author: iMo
author_rank: Super Contributor
timestamp: November 14, 2019, 09:08:42 pm
content: Quote from: sheisthedarkness on November 14, 2019, 05:34:16 pmThe designers of that opamp forgot to layout the resistor on the die. Next time you should order that opamp together with option #1 - "1k resistor for crossover distortion elimination"..

author: Kleinstein
author_rank: Super Contributor
timestamp: November 14, 2019, 09:13:50 pm
content: Quote from: T3sl4co1l on November 14, 2019, 07:08:22 pmIn an other thread about fake OPs they did die photos, showing something similar to LM358 - probably a Chinese version. The performance was more like slightly better than expected for a real 358. Given that the OPs are not that expensive - they could as well sold the chips as what they are. There definitely would be a market for a cheap improved 358.Someone around here even got fake 0 Ohms resistors. So don't expect anything to cheap to get fakes.The cross over distortion is kind of a compromise taken for lower supply current.

author: Audioguru again
author_rank: Regular Contributor
timestamp: November 14, 2019, 11:28:27 pm
content: Why bother adding an output resistor to eliminate crossover distortion when an LM324 or same opamps in an LM358 have trouble producing high audio frequencies due to the slow slew rate?They are also noisy (hisssss).

author: sheisthedarkness
author_rank: Newbie
timestamp: November 15, 2019, 12:51:05 am
content: Quote from: Audioguru again on November 14, 2019, 11:28:27 pmWe're not certain what it is but I tested it with the 1k resistor from 0 to 20kHz, I did not see the crossover distortion appear again at any of those frequencies. I can't comment on noise as I haven't done any real testing but just by looking at it, compares fairly well to the real TL081 I have, which is why I was so dumbfounded

author: wnorcott
author_rank: Regular Contributor
timestamp: November 15, 2019, 12:58:37 am
content: I don't get it because a TL081 only costs about 50 cents at Mouser electronics in quantity 1 and you know you are getting the real thing, from TI. Or if you wanty to go the discount route and don't mind to wait and wait, get them from Futurlec for 30 cents each. What is one minute of your time worth, that got wasted debugging bad chips? It's worth a lot more than 50c. I would not warm up my scope for fifty cents.

author: magic
author_rank: Super Contributor
timestamp: November 15, 2019, 08:02:46 am
content: Quote from: sheisthedarkness on November 15, 2019, 12:51:05 amTry 20kHz with enough gain for ±10V output swing and you will see other wonders of LM358.And noise is not something you can measure on a scope unless it's really bad.Quote from: T3sl4co1l on November 14, 2019, 07:08:22 pmSalvaged can at least be useful when it comes to obsolete components, sometimes. But you better test itQuote from: Kleinstein on November 14, 2019, 09:13:50 pmI'm pretty sure you can buy legit Chinese jellybeans, just not on AliBay unless you know what you are doing.If you are bored you could order one of each sort of LM358 from LCSC (there is a dozen) and see if they are any good.Heck, maybe you will even find this one. The topology looks more or less the same as TI version and die dimensions are similar so I presume it was originally designed as a legit LM358 rather than solely for the sake of faking TL072s and OP07s. For starters, you wouldn't bother with input swing to ground if you were doing the latter.

author: exe
author_rank: Unknown
timestamp: November 15, 2019, 09:58:12 am
content: Quote from: imo on November 14, 2019, 09:08:42 pmIt's a low-power opamp with 0.3mA supply current.

author: iMo
author_rank: Super Contributor
timestamp: November 15, 2019, 04:52:06 pm
content: Quote from: exe on November 15, 2019, 09:58:12 amQuote from: imo on November 14, 2019, 09:08:42 pm

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 15, 2019, 05:18:14 pm
content: That said, if you go for something like NE5532 you're basically using the same thing but with a class-AB biased output stage. Very power hungry, but at least it doesn't have crossover distortion.Tim

author: magic
author_rank: Super Contributor
timestamp: November 15, 2019, 09:06:11 pm
content: NE5532 is the secret LM358 option #1 with 1k pullup. The cat is out of the bag now.But seriously, what the hell is going on in this thread at this point?

author: MagicSmoker
author_rank: Super Contributor
timestamp: November 16, 2019, 12:26:28 pm
content: Quote from: wnorcott on November 15, 2019, 12:58:37 amThis is the beginner section, so one should expect that a lot of the people starting threads here will not realize that counterfeits of even <$0.01 components can be found on fleabay, Ali-whatever, etc.At least the OP learned this valuable lesson on a jellybean op-amp rather than, say, a $200 IGBT module...