FETs on the ASM-1 VCO.

[Ian Fritz]

Rene Schmitz wrote:
> I use MOSFETs as discharge switches, and have compared them to jFETs.
> (though I'm still not sure if it acts more like a diode in my circuit, that
> would lead to another interesting topic, why not use diodes as discharge
> switches? A diode made from a jFET has pA leakage! This would work only for
> the VCOs that have the cap to the supply.)
> The MOSFETs have higher leakage, and there *are* effects from the capacitance.
> I get a tiny wiggle at the reset, the waveform that I see at the output is
> further
> distorted by the opamp buffer. When I use a FET here the wiggle disappears.
> I use small signal FET (BS108) these don't have the low Rds(on) of the
> power FETs
> (BUZ et al.)
> These small signal devices have channel resistances equal in magnitude to
> that of jFETs,
> so I would prefer the jFET (though I use MOSFETs... kind of schizophrenic :)

Thanks for the description of your experiments. One other point to watch
out for -- small signal JFETs have limited current carrying capability.
This can also limit the discharge time ( cap. charge = av. current X
disch. time). MOSFETS, of course, are much better in this aspect of the
problem.

> I personally prefer a true schmitt-trigger where there are two tresholds, a
> high one when the reset cycle begins, and when the cap is discharged to the
> lower threshold the pulse ends, that way you have the fastest possible
> reset spike, and if your trigger circuit is fast enough and of sufficiently
> low inpedance you won't have trouble with the MOSFETs higher gate source
> capacitance. Leakage is of course bigger than that of the jFET, but if you
> don't need the *centi* Hz ranges this is no problem. The performance in the
> ultrasonic range (20k+) is better with MOSFETS, but my ears are not that
> good to hear any THD at 100Khz ;-)

Interesting points. The old Electronotes triangle VCOs used a discrete
Schmitt trigger approach.  I got one of those to track reasonable well
at ultrasonic frequencies (it drives a 10-step waveform generator, which
is an example where good ultrasonic response is needed).

 Ian