[sdiy] op amp substitutes

[Harry Bissell Jr]

Arrgh !!!

(as if this did not bite-me-in-the- at ss for the
one millionth time yesterday).

Any opamp might oscillate with at some capacitive
load...  It has to do with (basically) shifting the
feedback so that it becomes resonant.  Maybe MAGNUS
(hej!) will chime in with the math.

There are techniques to use (wish I had...) such as
decoupling the capacitance with a small series
resistance.  I believe (but am not positive) that
making a voltage follower with a resistor from the
output to the inverting input (instead of a wire) has
benefit as well.

Sometimes you can use a much BIGGER capacitor and
swamp out the effect.

TL07x and TL08x are the worst offenders I can think
of.
My case yesterday... oscillated with 3' of
UNTERMINATED
MIDI cable plugged in.  Not even connected to
anything...just 'there' (and of course its a
capacitive load to ground...)

H^) harry


--- anthony <aankrom at bluemarble.net> wrote:

> I was wondering just what parameter you look for
> when you want to see how 
> much an op amp output can take high capacitative
> loads.
> 
> More specifically I was wondering if I could
> substitute one TL034 or one 
> TL054 (or even '74's) for four LF356's.
> 
> The LF356's datasheet goes to the trouble to mention
> that it can handle high 
> cap loads. But the '34 and '54 are FET input amps
> too (yeah I really don't 
> care about the inputs - it's the OUTputs...)
> 
> This is in the Analog Shift Register circuit. 
> 
> 
>