[sdiy] Two questions - VCO standards and Differential Op-AmpCircuit

[harrybissell]

Bob Weigel wrote:
> 
> While I'm on a roll...or..not :-)...

OK, rolling... :^)

Mostly correct. In most cases trying to command an opamp
output 'outside' the rails will be harmless. long term.

It CAN make the opamp latch up, and it will fail to recover
after the input goes to a normal state. The only solution
would be a power reset. 

Usually that's not real friendly...   :^P

Its not 'permanent damage'... just permanent for the rest of that
session...

H^) harry

> 
> I can't imagine there being a standard for such a thing. Each
> manufacturer seems to do radically different things for instance the
> discrepency in various mfgs CV scalings. And of course VCO chips have
> scaling inputs so they can be set up to work. And then each discrete
> circuit was set up from a complex set of 'convenience' parameters. As
> chips came into use and multiple manufacturer's used them at least it's
> more likely that some used them in similar ways as I seem to recall some
> did.
> 
> Anyway the op-amp... if the inputs are set up like this the op amp just
> goes to the available rail voltage (ground in this case)
> USUALLY...though there are some op amps which...(as I experienced a
> while back with a 4560 for the first time and I think Harry and some
> others noted the phenomena...) can actually flop the other way when
> taken beyond rail conditions.
> 
> However I don't think it actually ever hurts the op amp. The only time
> you hurt op amps usually is by
> 
> * exceeding the supply limits
> * exceeding the input limits
> * driving against the output too hard. On some like the 741 types which
> have only 25-35 ohms or like that for series resistance with the output
> transistors in there, this doesn't take much. For others they have like
> 600 ohms I seem to recall built in and it's obviously harder to damage
> those. -Bob
> 
> Chris Manders wrote:
> 
> >Hi folks
> >
> >Just two questions I have for you knowledgeable guys
> >to answer, please?
> >
> >Regarding VCOs. Is there some sort of standard
> >Frequency output setting? i.e. with the CV input at
> >Zero (or disconnected) the VCO tunes exactly to Middle
> >C? This would seem appropriate perhaps? Or is it best
> >to set them to a very low level, as the CV inputs will
> >generally be positive and cause a rising of pitch?
> >My VCOs work fine (built on breadboard) but I have not
> >set them up yet for anything specific. I know that I
> >could set them up for anything I wanted but I would
> >like to adopt an appropriate standard if there is one.
> >
> >Secondly, and on a different subject – Say I have an
> >Op-Amp stage set as a differential amplifier, and
> >running from a single 15 volt supply. If I set the
> >inputs up so that the output should be a negative
> >voltage, would it damage the Op-Amp as it couldn’t
> >output this negative voltage as it is powered from a
> >single supply rail?
> >For this situation, would the output of the Op-Amp be
> >an inverted positive difference, or would the Op-Amp
> >output be Zero for all negative results like this?
> >
> >Thank you in advance
> >Chris
> >
> >
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