Author Topic: LM741  (Read 1521 times)

Phend

LM741
« on: March 21, 2022, 02:38:49 PM »
Might find this interesting
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Fase or Phuzz that is the question.

Rob Strand

Re: LM741
« Reply #1 on: March 21, 2022, 07:01:42 PM »
Quote
Might find this interesting

Yes, very interesting that even the humble 741 isn't immune.  Also interesting is he mentions someone on EEVblog has deciphered the fake parts and the fakes pull from a library of a dozen or so "fake" opamp designs.

They say you get what you pay for,  with ebay parts what you pay for is uncertainty!
By buying ebay parts you are supporting the wrong side of the supply chain.
Send:     . .- .-. - .... / - --- / --. --- .-. -

soggybag

Re: LM741
« Reply #2 on: April 15, 2022, 10:24:12 AM »
Oh my god, thatís so crazy! I just bought a bunch of cheap op-amps! I need to test them!

imJonWain

Re: LM741
« Reply #3 on: April 15, 2022, 11:27:13 AM »
Use them in a pedal design and you could have the next pedal with a magic part :).
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TFRelectronics

soggybag

Re: LM741
« Reply #4 on: April 15, 2022, 12:56:07 PM »
I know, I was looking at that whacked up wave form thinking 🤔 I wonder if it sounds any good?


zbt

Re: LM741
« Reply #6 on: June 23, 2022, 12:08:23 PM »
For testing dual op amp






Rob Strand

Re: LM741
« Reply #7 on: June 23, 2022, 09:47:24 PM »
Quote
For testing dual op amp
A buffer isn't always a good choice to check output swing.

For a buffer 'input voltage' = 'output voltage'.   What can happen, depending on the opamp,
is the 'input voltage' can hit the common mode range limit before the output hits
the output swing limit.

To avoid that you need to give the opamp a gain of 2 (upto say 10).
You want to bias the opamp at mid supply to let it swing in both directions.

An inverting opamp configuration will make adding gain and biasing correctly a little simpler.

Another point about output swing is it depends on the load resistor.  Following the logic of the video
that means matching load conditions of set out in the datasheet.  Light loads means the opamp will
swing closer to the rails.  Another point is the datasheet output swings are often worst case and
it's not uncommon to see much less drop in practice.  I find the opamp internal circuit a good way to
guide the expected output swing, but it's not straight forward to estimate the swing with different
load conditions. 

The video has a DC load resistor to ground which means it does not provide a load on negative output swings.
Ideally you want the loads on both positive an negative swings.    That means the load resistor must connect
back to Vref -  it also means large Vref resistor values are out and you may need to use an opamp to create a 'stiff' Vref for the output load.   Another way is two load resistors, one to +V and one to ground.
(I'm assuming a single supply test set-up.)

« Last Edit: June 23, 2022, 09:50:31 PM by Rob Strand »
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PRR

Re: LM741
« Reply #8 on: June 23, 2022, 10:00:10 PM »
A buffer isn't always a good choice to check

The "tester" in that video has a major fool-you. With the high load impedance many kinds of defective chips will read "good". A particular test-case is a opamp with the supply pins cut off. Junction and substrate conductance may pull a 100k load directly from the signal generator. (Note that conductance across the inputs will also pull load through the "NFB" jumper.)


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Rob Strand

Re: LM741
« Reply #9 on: June 24, 2022, 04:27:17 AM »
Sometimes you will find the pics of opamp test jigs in the datasheets, especially the older datasheets from the 70's though 90's.

Here's a good slab of info,
https://www.eeweb.com/wp-content/uploads/articles-app-notes-files-opamp-test-procedures-1317081347.pdf
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pinkjimiphoton

Re: LM741
« Reply #10 on: June 24, 2022, 07:57:44 PM »
Quote
Might find this interesting

Yes, very interesting that even the humble 741 isn't immune.  Also interesting is he mentions someone on EEVblog has deciphered the fake parts and the fakes pull from a library of a dozen or so "fake" opamp designs.

They say you get what you pay for,  with ebay parts what you pay for is uncertainty!
By buying ebay parts you are supporting the wrong side of the supply chain.

or even worse, AMAZON. all the crud that don't sell anywhere ends up on there... and you get what you pay for
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"When the power of love overcomes the love of power the world will know peace."
Slava Ukraini!

Rob Strand

Re: LM741
« Reply #11 on: June 25, 2022, 12:32:34 AM »
Quote
or even worse, AMAZON. all the crud that don't sell anywhere ends up on there... and you get what you pay for
We hear a lot about green products these days but we see far more brown products.  :icon_mrgreen:
Send:     . .- .-. - .... / - --- / --. --- .-. -

anotherjim

Re: LM741
« Reply #12 on: June 25, 2022, 04:50:46 AM »
I've noticed many Chinese sellers describe things by a popular western jellybean IC part number when it really could be any other chip that can do the job. There are ICs types that have only ever been made in China and only have a Chinese language datasheet. I have some little PWM H-bridge motor driver modules that have a chip compatible with a popular BJT chip that is in fact MOS. In some respects, the Chinese chip is better as it includes flyback diodes and can handle higher currents. The part number on the IC is the Chinese one and I was able to find the datasheet but it was described by the seller as the western BJT part. Now, would you buy it if you only got the real part number and had to go on a hunt to find the datasheet? Could there be interesting and useful parts we are missing out on?

Croeso i Diystompboxes.

Comma a question.

iainpunk

Re: LM741
« Reply #13 on: June 27, 2022, 11:17:46 AM »
i have about 20 or so 741 opamps in my stock, all of em come from a collection from my uncle-ish family member, and he hadn't added to the collection since the early 90's. about half of em are metal cans, most are marked uA741 as god intended. some of the ones not marked uA have big HONG KONG stamps on the top and/or other letterings. some just say 741 on them, having no other markings. i even have a few DIP14 uA741's, with 7 unused pins on them.

cheers
friendly reminder: all holes are positive and have negative weight, despite not being there.

cheers