[sdiy] Mind blown -- TL074 saturated outputs different

[René Schmitz]

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:Opampschmitt_realistic_xcircuit.svg

Maybe use a rail-to-rail output opamp instead. (Even these will a small 
bit away from the rail.)


Best,
  René

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