SV: Re: SV: Re: SV: [sdiy] More MOSFET saw VCOs

[harrybissell]

Yes... its the Rol at nd style core. Its used in many places, for instance the
ramp generator from the GR-300.  How would I know that ??? :^P

Still... I have no explanation (yet).  It should run to the rails. I'll play
around with some differences in the base drive (add diodes) and see
if I can whup this

H^) harry

karl dalen wrote:

> --- harrybissell <harrybissell at prodigy.net> skrev:
>
> > since we are plating with reset circuits...
> >
> > I'm using an NPN to reset a sawtooth core... the opamp with
> > capacitor feedback type.
>
> Actually i have seen somtime in the very past a Roland
> synth schematic that used a NPN as core switch, i just cant
> remeber in what schemo it was!
>
> > In my case... I allow it to ramp up to an unknown voltage. It
> > clips at 7V.  I'd expect that to go higher...
> >
> > Am I seeing base-emitter breakdown, in your humble opinions ?
>
> Most likely  no, since  you are at virtual GND and
> you are not using "THAT" high base currents.
>
> Ohh, yea , i remember, check out JX3p or JX8! :-)
>
> KD
>
> > That would be about the right voltage for that to happen. OTOH
> > the emitter is at the summing junction (so that is ground) and the
> > collector at the output (which is positive) so where is this happening ?
> >
> > H^) harry  (you did not expect work to stop for holidays ???)
> >
> > karl dalen wrote:
> >
> > > --- Ian Fritz <ijfritz at earthlink.net> skrev:
> > >
> > > > At 06:22 PM 12/22/05, karl dalen wrote:
> > > > >There is a parameter that says "gate capacitance changes
> > > > >with frequency"!
> > > > >
> > > > >Do you se any differences regarding this parameter?
> > > >
> > > > Hmmm ... interesting question.  Anyone know the answer?  :-)
> > > >
> > > > There are other odd effects, too, like change in on resistance with
> > > > voltage.
> > >
> > > Yes, but thats a well known one!
> > >
> > >  I suppose the gate capacitance mostly comes into play during the
> > > > discharge phase, which shouldn't depend on the VCO frequency.  But if
> > there
> > > > is some feedthrough during the ramp ...
> > >
> > > Most likely if it was a power mos,
> > > the decay curve is very exponential.
> > >
> > > I think the variation of Vgs is a problem, wheter this
> > > is diffrenet between devices or the same for one entire batch
> > > im not to shure but temp and voltage will do change it!
> > >
> > > > The waveform isn't perfect and changes a bit with frequency.  So maybe
> > > > these effects come into play.  To first order, though, tracking is very
> > > > good and that's the main thing.
> > >
> > > I had also quite good tracking but the overshot/ringing
> > > at start of retrace caused tri wave be a bit messy at higher
> > > frequencies! Could be filtered by small pf cap ofcourse!
> > >
> > > Well, Ian was it 1,5 years ago or not??
> > >
> > > REg
> > > KD
> >
> >