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Re: Ultimate DIY debugger award/No Go

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5/26/1999 5:53 AM

Re: Ultimate DIY debugger award/No Go

Aron,

I gues when they say it's a replacement they mean that this opamp has:

a) same pinout as the original
b) equal or better performance parameters (like input impedance open loop gain, noise, etc.)

They probably don't mean that the guts of the IC is exactly like the original. In fact if they cloned the exact circuit it would be piracy.

This circuit is HEAVILY dependant on what's inside the chip - I could only give my explanation on how it worked after I read the datasheet and looked at the internal schematic of the opamp.

The pin 5 of the TL071 is internally connected to a ~1K resistor going to -V and a transistor that regulates the quiescent current of the JFET that is connected to pin 3.

From your voltages I guess that your replacement has an offset ballance that is not exactly like that of the TL071.

I'm afraid you need a REAL TL071/TL081, not a replacement.

Before you can get one, try one last thing: instead of connecting the collector to pin 5 connect it to pin 1 and leave pin 5 open. I don't expect this to work but it won't hurt trying.

GFR

5/26/1999 11:38 AM
aron	>Before you can get one, try one last thing: instead of connecting the collector to pin 5 connect it to pin 1 and leave pin 5 open. I don't expect this to work but it won't hurt trying. I did this. It didn't work. I have a real TL071acp. As for what acp means.... I don't know. Aron

5/27/1999 2:32 AM

Re: Ultimate DIY debugger award/IT IS OVER!

I can't believe it! The circuit really works.

I tried different zeners, new transistors and nothing worked.

I decided to do the one last thing - put in a TL081cp.

That did it. I couldn't believe it but the transistor biased. EVERY transistor biased after that.

It can't be a TL071acp - apparently the chip needs to be a TL071cp or TL081cp.

Amazing... In addition, I used 1N4730 zeners (3.9v) and they biased perfectly.

So I am happy that I can still follow a schematic, that I can still solder correctly, that I wasn't going crazy!

Thank you all for your help!

Aron

5/27/1999 4:44 AM
Michael Burgundy	Re: Ultimate DIY Congrats! Okay! Way to go! We can all stop scratching our heads now, have a shower and a shave and get back to regular life ;´) Any chance of a schem?

5/27/1999 6:52 AM
R.G.	Re: Ultimate DIY debugger award/IT IS OVER! It ain't over yet, guys. Now why must it be "cp" instead of "acp"? The data books I have don't even show an internal schematic difference.

5/27/1999 7:30 AM

The ACP has a 6mV max. input offset voltage. The CP has a 10mV max. input offset voltage. For normal aplications the ACP is a better chip than the CP, but since we're messing with the offset ballance, maybe for some reason the low-offset version will not work... I'm going to verify which version was the TL081 that I used.

My data books don't show any schematic difference too. I thought the only difference was a tighter matching of the internal transistor.

5/27/1999 8:23 AM

C!...uunh!... What is it good for??...

The "C" version was in the unit I saw as well. For the life of me, I can't figure out why better matched input devices would make that big a difference, as I suspect that they are all from the same wafer, and just separated on testing the chips. Especially considering that the transistor is stuffing current into one of the balance nodes under some conditions, which would cause massive momentary unbalances.

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