Opamps, real or fake

Have you been wondering about ICs being real or not? IMSAI Guy in this video takes a look at a few opamps and shows you an easy way to find out for some 741 opamps whether they are real or not:

5 Likes

I had a few LM358 dual opamps lying around and I used them in one of my projects. Often times I use the TL072 but because these were pin compatible I thought, why no?

Unfortunately they didn’t work properly. I found out that there is only one opamp in the ones I had in my circuit. Because I had some 14 of these devices I decided to try them all. I tried the LM358 pin configuration as per the datasheet, and also tried the pin configuration of a single opamp like the TL071.

The sad score:

  1. six of them contain one opamp which tested as an inverting amplifier did the job but the opamp pins are in an odd configuration (1 = out, 2 = neg in, 3 = pos in, 4 = V-, 5 = ?, 6 = ?, 7 = V=?, 8 = ?)
  2. two or them are broken, the opamp outputs a DC voltage when I feed it a square wave
  3. six remaining are also broken, they do not respond to anything.

Then I found some dodgy looking LM356 in my stash. I say ‘dodgy looking’ because the print on the chip is not straight, and I would assume someone who has mastered making silicon dies should be able to get the type printed the right way. Of course I didn’t check all the specs, but these turn out to work like one might expect from a single opamp.

Maybe 35 years ago it was more difficult to print the type of the chip on the casing than to make the chip.

Some weeks ago I bought some TL074 opamps. In fact I bought 3 batches of 10 pcs each from 3 suppliers. Given that I’d bought some opamps in the past that didn’t turn out to be working properly, I thought ‘better not get all from the same spot’. Several people asked me whether the new ones were working. My reply at that time was: “Time will tell. As soon as I use them in a project I’ll find out.”’
From the first batch of 10 I’ve now used a few and all appear to be working as expected. Note: I did not do any extensive testing, I only used them in some projects and there they appeared to work well.

This is a pic of an IC in batch 1. I found these to be unproblematic.

As soon as I have some experience with batch 2 and 3 I’ll report on those.

2 Likes

So far my bad experiences were all with chips I bought on eBay (especially the LF 398 sample and hold chips, there are lots of fakes out there). I prefer buying from well-known shops like reichelt.de, TME (https://www.tme.eu) and also RS (https://www.rs-online.com) when I need larger batches.
I don’t really see the benefit of saving a few bucks when I have to check them all afterwards :frowning:

TL074 costs 35cents at Reichelt and 33 cents from an eBay seller who also sells antireumatic cream. I let you decide where to buy :wink:

Of course, exotic chips are often expensive and one is tempted (or forced) to buy cheap ones from questionable sellers. I bought many of those from such sellers (e.g. LF 398, TP 2399, 3340, 3360 etc.) when I started building modular synths and there is a considerable amount of luck required to get what you want – lesson learned.

Luckily I have plenty of equipment at home to test these things but for beginners with a basic set of electronic gear I highly recommend sourcing components from reliable sellers. Fake or faulty components are frustrating anyways, let alone when you’re not able to test them properly.

4 Likes

In my stash I had some of these ICs. They are however not behaving to spec. Their outputs should be able to vary from rail to rail, but alas, they don’t. So are they MCP6002s ?

Do you have a signal generator and an oscilloscope? It’d be fairly easy to find that out :slight_smile: Or what do you mean by “not behaving to spec”? If they do not behave to spec, they are either not MCP6002 or broken :see_no_evil:

Btw. they are rated for a power supply range of 1.8V to 6.0V, keep that in mind.

1 Like

Their outputs do not go from rail to rail, but other MCP6002s (from another source) in the same circuit do.

I think you mean that they are not giving the upper/lower supply voltage on the output when the non-inverting input has a higher/lower potential compared to the inverting input. In that case, the chip is faulty (or fake).

Btw. a basic test (if you lack a signal generator and an oscilloscope) can also be done with a multimeter. Apply the supply voltages (within specs range), put e.g. the non-inverting input on ground and hook up a potentiometer with pin 1 on the low and pin 3 on the high voltage supply. The wiper goes to the inverting input of the opamp. Now check the output while you turn the potentiometer. You should see a flip of the voltage when you cross 0V (ground).

This is of course a very basic test and is not suited to check if the opamp works correctly for high frequencies.

If you have an oscilloscope but no signal generator, you can use your phone or the audio output of any device (e.g. use a 1000Hz tone from YouTube and hook up the 3.5mm jack to ground and the input). Then watch the output with an oscilloscope and try different frequencies. There are also signal generator apps for smartphones, which allow you to easily alter the frequency/waveform with some touch buttons.

1 Like

Thx for the suggestions.

This is a small part of the circuit I was trying to use them in:

As you can see, when rotating the POT the output should go from 0 to 5V (assuming no input voltage at the CV in) but in fact it only reached 3.9V. After using a different MCP from another source, it covered the full 0 to 5 V range. REF-5 stands for -5V reference source.

If you want to really characterize an op amp, a fairly simple circuit for doing so is discussed here:

1 Like

I’ve been using MCP6004 opamps in a project and noticed that their output when used as a non inverting amplifier does not reach the rail voltage. It stays 1 Volt below on all 4 outputs but should be able to approach it up to 25 mVolt according to the datasheet. I tried several of my lot and all the ones with the production info 1849AT2 can’t reach the rail, the other one shown in the picture with production info 1610TSM can.

The MCP6004 can be powered to 6V. Assuming the “1839AT2” version is not an MCP6004 I powered it at -12 and +12 Volts and it works like a run of the mill opamp. No excessive power usage, the non inverting opamp circuits work well albeit the max output voltage at 10 Volts stays well below the rail voltage of 12V. So this chip definately works quite a bit out of spec, to say the least. I’ve not tried this with the 1610TSM because, as fate will have it, I’ve got only one of the latter and loads of the former.