[sdiy] Mind blown -- TL074 saturated outputs different

David G Dixon dixon at mail.ubc.ca
Wed Nov 28 09:50:28 CET 2018

Thanks, René.  I think I will add an output resistor and a couple of zeners
to set the output voltages of the comparators to certain values, as in the
"precision schmitt trigger" of the link you provided.  I need to rely on a
fixed output voltage to generate the specific positive and negative
threshold voltages for the comparator logic, and I can achieve this with the

Tomorrow morning I'll get this damned thing finished.  Someone in Iceland is
waiting patiently, but I think I'll have to lay out and make a new board.
Oh well, that's why we build prototypes in the first place.

Oh, and in case anyone is wondering, I am actually losing a little money on
this one (plus, it's taking many hours of my time which I am providing for

> -----Original Message-----
> From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On 
> Behalf Of René Schmitz
> Sent: Tuesday, November 27, 2018 11:44 PM
> To: synth-diy at synth-diy.org
> Subject: Re: [sdiy] Mind blown -- TL074 saturated outputs different
> Hi David and all,
> On 28.11.2018 08:06, David G Dixon wrote:
> > 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?
> Even the diode drops varies with loading.
> > 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 would say it is generally avoided to rely on the saturation 
> voltage to be a specific value.
> That is why people will use schemes like this if you need any 
> precision: 
> https://en.wikipedia.org/wiki/Schmitt_trigger#/media/File:Opam
> pschmitt_realistic_xcircuit.svg
> Maybe use a rail-to-rail output opamp instead. (Even these 
> will a small bit away from the rail.)
> Best,
>   René
> --
> synth at schmitzbits.de
> http://schmitzbits.de
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