[sdiy] Mind blown -- TL074 saturated outputs different

Ben Bradley ben.pi.bradley at gmail.com
Wed Nov 28 20:04:38 CET 2018


Have I posted my "READ THE DATASHEET" rant before? I recall Bob Pease
saying (both in print and in person when National put him on tour
circa 2001) very emphatically that you never use "typical" values for
design, that you should always use Min and Max to calculate the WORST
CASE values, as these are the only guaranteed numbers. "Typical" means
it MIGHT be that value on a good, sunny day, but it's certainly not
guaranteed.

Here's the datasheet:
http://www.ti.com/lit/ds/symlink/tl072a.pdf
Looking on page 13, the "Maximum peak output voltage swing" shows
(with +/-15V supplies, temp at 25C and a 10k ohm load) minimum of
+/-12V and typical of +/-13.5V. There's no maximum given, but even
between the minimum and typical, there's a 1.5V variation.
Furthermore, there's no mention that these are or should be
symmetrical, so the output could well have a negative peak of 12V and
a positive peak of at least 13.5V, or vice versa.



On Wed, Nov 28, 2018 at 2:08 AM David G Dixon <dixon at mail.ubc.ca> wrote:
>
> Hey SDIY Team,
>
> 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.
>
> Tonight I have been testing a circuit which isn't working exactly right, and
> have discovered the reason why: I had presumed that the saturation voltages
> would be +/-13.5V for a +/-15V supply.  My supply voltages are actually
> about -15.07V and +15.14V, so off by about 70mV.  The saturation voltages
> were about -13.43V and +14.48V for a TL074CN (I tested two and they were
> about the same), and about -13.79V and +14.40V for a TL074BCN.  The fact
> 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.
>
> I thought that opamps of a certain flavour would always saturate a certain
> fixed voltage away from the rail voltage, because this was determined by
> internal diode drops, which are basically fixed.  Was (am) I smoking crack?
> Does everybody else know about this and I'm just finding out about after 10
> solid years of electronics?  Or am I just experiencing particularly bad luck
> with my ICs?
>
> 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
> 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
> I would get if the saturation voltages were the same.  Simulations of TL07X
> 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?
>
> Cheers,
> Dave Dixon
>
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