[sdiy] Mind blown -- TL074 saturated outputs different

Roman Sowa modular at go2.pl
Wed Nov 28 10:21:34 CET 2018


Hi,

> My mind is now blown.  I had always thought that the positive and negative
> saturation voltages from an opamp would be the same if the supply voltages
> were equal.

only if an opamp is designed for such behavior, that's Rail-to-Rail opamp.

> that they aren't +/-13.5V doesn't worry me too much, but the fact that they
> are so different (by as much as a volt) does.

TL07x is not symetrical, look at internal diagram

> I thought I could essentially bank on the saturation voltages being a
> certain value, and more importantly, be equal and opposite, and use that for

TL07x was not made for that, you cannot rely on features, or should I 
say side effects that were not important even for designers of the chip.

> design.  I'm trying to divide down these voltages to use as references for
> logic comparators, and I need the same + and - voltages at the inputs which

add a resistor with back-to-back zeners or even TL431 inside a bridge to 
get symetrical precise voltage. Or RR opamp, although it may be not so 
many of them out there working up to 30V and in THT.
And maybe you're loading that poor opamp too much when it's at negative 
side? If there is probably a voltage divider, add another resistor to 
-15V to adjust symetry. Not sure how precise the voltage you want.

> always assume a saturation voltage of +/-13.5V, which the datasheet says is
> typical.  I guess that assumption is false.  Was I stupid to assume that?

It would be very impolite to answer that question

Roman



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