Today I tested the input common mode voltage range for LM358 and TL072.

The test setup:

View attachment 174367

Input signal comes form function generator, F = 1kHz and the amplitude is 10V peak to peak. All Opamp are supplied from symmetrical supply +/-5V.
So we can easily exceed the allowed input common mode voltage.

First the TL072 form ST:

View attachment 174369

As you can see in the TL072 phase inversion phenomenon exist.

In LM358 from ST we have this situation:

View attachment 174370

As you can see no phase inversion only clipping for Vin > Vcc - 1.4V. And no crossover distortion due to lack of load current.

For the NE5532 from Philips

View attachment 174372


As for the crossover distortion. For the LM358 the crossover distortion is clearly visible

View attachment 174373

But no sign of visible crossover distortions in the TL072.

It's official. The TI branded TL072's I was previously using for my Wien Bridge Oscillator were FAKES!

Here is the sine wave produced by the new, genuine TL072 I ordered from Arrow:



Perfect sine wave. No crossover distortion!

Here is a picture of the fake TI TL072:



The real TI TL072:



I have to admit that the fake looks pretty good. It IS a dual op amp of some sort. It has the same pin-out as the TI TL072. It is just not a genuine TI TL072.

Question: Why in the world would you make a fake of a part that costs only US$0.54 when it is genuine and ordered from a reputable vendor? I can understand making a fake of a chip that costs US$54 but NOT a fake of a chip that costs 54 CENTS.

Jeez. If they would just use their powers for good instead of for evil.

 

Question: Why in the world would you make a fake of a part that costs only US$0.54 when it is genuine and ordered from a reputable vendor?

Which begs the question: Why would any hobbyist buy a cheap, possible dubious part from China, when he can get the real McCoy for US$0.54?

 

Which begs the question: Why would any hobbyist buy a cheap, possible dubious part from China, when he can get the real McCoy for US$0.54?

I didn't know any better! I did not realize that many of the parts on ebay were fakes. I just figured they were good deals because they were "direct from the factory" or something like that. I also liked the free shipping from most vendors on ebay.

Live and learn.

 

Live and learn.

But always pay.

 

But always pay.

So what do I do with these fake TL072’s? I asked for a refund of my US$5.00. We shall see. I guess I could use them when I need a garden variety op amp and I don’t care about crossover distortion. Is there a way to tell if they really have JFET inputs?

 

Is there a way to tell if they really have JFET inputs?

The input of the real device is two p-channel J-FETs:

Since the input bias current is very small for a FET (fraction of a nanoamp), much smaller than most bipolar input amps, you could connect the op amp as a non-inverting follower with gain (say 100), using a plus and minus supply, and place a large resistor (say 10-100 megohm) from the (+) input to ground.
A bipolar amp will generally show a significant offset output voltage change with the resistor and without (input connected to ground), but the FET should be essentially the same for both conditions.

 

So what do I do with these fake TL072’s?

Throw them away. Forget and move on.

You deserve to spend your time (an asset that you will NEVER recover) in something useful.

Nice sinewaves yours, man!

 

Throw them away. Forget and move on.

You deserve to spend your time (an asset that you will NEVER recover) in something useful.

Nice sinewaves yours, man!

Now I have to go back and re-run some experiments that did not work, but this time I will use the REAL TL072’s. Sigh. The fake parts may have caused me to give up on projects where I thought I was the problem. Maybe it was the fake parts!

 

Maybe it was the fake parts!

It's unfortunate how many things we learn the hard way.

 

Is there a way to tell if they really have JFET inputs?

Build this circuit

And check the respond

 

Texas Instruments make many little power amplifier ICs and you can buy genuine ones locally.