[time-nuts] ? phase comparison or other device

John Miles jmiles at pop.net
Sat Jun 30 20:17:15 EDT 2007

> John Miles wrote:
> >> The output of an opamp is always referenced to its power supply,
> >> regardless of the configuration.  If the power supply is noisy,
> >> the output is noisy.
> >>
> >
> > Beyond a few kHz this may be true, but at lower frequencies
> even a low-grade
> > opamp has excellent rejection of power supply noise.  A good opamp data
> > sheet will characterize the PSRR versus frequency.
> >
> > -- john, KE5FX
> >
> John
> Only when the opamp is operating within its linear region, not when the
> output is saturated.
> Opamp saturation has been proposed as a good method of reducing the
> output noise of an opamp when an amplifier is overdriven.
> This is clearly not true when power supply noise is significant, as is
> almost always the case.

Sure; I was mainly objecting to use of the term "always."  The statement
that "the output of an opamp is always referenced to its power supply" is
simply not a good model of the situation near DC, unless (as you note) the
output is saturated.

-- john, KE5FX

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