[sdiy] Mind blown -- TL074 saturated outputs different

mskala at ansuz.sooke.bc.ca mskala at ansuz.sooke.bc.ca
Wed Nov 28 12:34:39 CET 2018


On Tue, 27 Nov 2018, David G Dixon wrote:
> 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?

According to the schematic in the data sheet, the output stage has an NPN
transistor to the positive rail on the high side, with its base driven by
a PNP current mirror.  On the low side, it has a PNP transistor with its
base driven by an NPN transistor which has, notably, *an emitter
resistor*.  So I'd expect some difference because of the voltage dropped
by that resistor on only one side, even before we look at the likely
difference between the NPN and PNP transistors - which usually are not
perfectly complementary unless specifically designed to be so.

More generally, if a data sheet doesn't promise that any two numbers will
be equal, and there isn't a specific physical reason they necessarily will
be, then they probably will NOT be equal.

Rather than depending on the saturation voltages to be equal, if you need
predictable values for the high and low outputs from an op amp you might
try the non-saturating "fully clamped voltage comparator" from page 47 of
TI's "Handbook of Operational Amplifier Applications" (Carter and Brown):
   http://www.tij.co.jp/jp/lit/an/sboa092b/sboa092b.pdf

-- 
Matthew Skala
mskala at ansuz.sooke.bc.ca                 People before tribes.
http://ansuz.sooke.bc.ca/


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