[sdiy] Outputs and Impedances

mskala at ansuz.sooke.bc.ca mskala at ansuz.sooke.bc.ca
Sun Oct 28 13:40:40 CET 2018

On Sun, 28 Oct 2018, Tony Kalomiris wrote:
> The OP said he was driving a reactive load (transformer) and "2 voltage
> dividers".

He said the transformer was on the far side of another voltage divider.
If despite that it's still presenting significant reactance to the op amp,
then we have another can of worms opened because (as you describe later in
your message) reactive loads on an op amp output are asking for stability
problems.  But such problems if they occurred would have nothing to do
with evenly distributing the signal.

I don't think there's much value in trying to dream up exotic scenarios of
what *could* happen in this circuit unless the OP comes back and provides
more information about component values, circuit topology, what the
purpose of this circuit is at all, and so on.  Saying "If you had this
weird special-case situation, then you would need to do that" does NOT
contribute to better understanding when someone is probably dealing with a
much simpler typical case and already having trouble with the basic
concepts for the simple case.  With that in mind, I probably shouldn't
even have mentioned possible exceptions in my first reply myself - but
I'm conscious of the presence of the well-actually kids on this list, who
don't miss a chance to try to catch me in a supposed "error" any time I
leave out an irrelevant detail for pedagogical reasons.

The original question seemed to be based on an incorrect understanding
that there would be a need to evenly distribute the signal from an op amp
to multiple loads in the general case, let alone that series resistrs
would help with that somehow, and I think there's value in clearing that
point up.  It isn't a problem that needs solving, because the op amp
output looks like a pure voltage source and the currents into the loads
are independent of each other.  That remains true even if it's not
possible without more information to give a complete account of everything
else that could possibly occur in an op amp circuit, and even if there are
hypothetical situations in which series resistors might be useful in
solving other, genuine, problems.

Can we at least agree that a series resistor right before a voltage
divider is pointless?  You could just make the top resistor of the voltage
divider that much bigger.

Matthew Skala
mskala at ansuz.sooke.bc.ca                 People before tribes.

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