Some questions

[Joachim Verghese]

On Sun, 7 Jul 1996, Mikko Helin wrote:

>   I built a synth with Electronotes sawtooth VCO. It goes down to
> 0.01 Hz (but what do I do with that low frequency?). For some
> reason it doesn't start oscillating every time I turn the power
> on. I used BS170 MOS-FET instead of 2N4856 (or 2N4859), maybe
> that's it. I measured voltages with DMM, and integrator
> output goes up to +15 V and comparator fires (goes up to
> 0.0 V). LM311 comparator open collector output is connected
> to ground through 3.3k resistor, maybe BS170 needs a slightly
> positive voltage at gate to switch? (like 0.5 V or so, have to
> try). 

The BS170 is an enhancement type MOSFET which means that it is
normally off, and turns on when you apply a positive gate-to-
source voltage (VGS).

If memory serves, the EN sawtooth VCO uses a depletion type JFET.
The JFET is normally on but shuts off when you apply a negative
VGS. In other words, MOSFETs and JFETs are substantially different,
so I recommend that you try a JFET instead, a 2N4392, for example.
A BF244/245 might also work, although it's not optimised for
switching applications, and requires a larger VGS to shut off.

-joachim

PS I've got some spare 2N4392s, if you can't find them elsewhere.