[sdiy] Mind blown -- TL074 saturated outputs different

David G Dixon dixon at mail.ubc.ca
Wed Nov 28 08:06:53 CET 2018


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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