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

[karl dalen]

--- 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
> 
>