Sorry Lads, really dumb opamp question

[Happy Harry]

Ok... like I said here's my "theory" of what might be
happening... maybe the mechanism is different.

I was driving cables (slightly capacitive load...) and
of course, my first take at fixing the instability was
to put a resistor in series with the output as you suggested.

Even a value as low as 10 ohms was effective. There was some
loss.

But I also noticed that a "feedback" resistor was equally
effective. Here the loss in nearly non-existant because the
opamp is approaching unity gain... (feedback resistor is
small and input resistor is almost infinite.

Why... I don't know.  I'll entertain any better suggestion.
Possibly I just changed the resonant point of the circuit to where
it no longer hurt me.... and it is waiting to bite me in the @ss
somewhere else ???

H^)  harry


>From: Martin Czech <czech at Micronas.Com>
>Reply-To: Martin Czech <czech at Micronas.Com>
>To: harrybissell at prodigy.net
>CC: synth-diy at node12b53.a2000.nl
>Subject: Re: Sorry Lads, really dumb opamp question
>Date: Mon, 15 Jan 2001 09:50:45 +0100 (MET)
>
>
>:::Sometimes the resistor-less unit does not work so well
>:::with all op-amps and loads, especially capacitive loads.
>:::Putting a resistor between the output and the inverting
>:::input can help these cases. I suspect the mechanism is that
>:::the input is decoupled a little from the output by the RC
>:::of the resistor and the input capacitance. Usually I just'
>:::use the resistor-less method, but its good to have the trick
>:::if you need it.
>
>?
>
>I do not understand how a resistor including some more parasitic
>capacitance and making the additional feedpack pole worse
>can help when you allready have a lag problem due to large capacitive
>load.
>
>Maybe a resistor with parallel capacitor would make more sense because
>of some lead effect, but that would be dominant only in configurations
>with another resistor from "-" to reference.
>
>Resistor do have some parallel capacitance, but very little.
>
>I guess decoupling the output with resistor or inductor
>would be more applicable.
>
>m.c.
>

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