spikes (was:AW: FETs on the ASM-1 VCO.)

[Arthur Harrison]

If bipolars do the trick, then why not?  I suppose the key parameter is 
saturation resistance.

-Art

-----Original Message-----
From: Haible Juergen <Juergen.Haible at nbgm.siemens.de>
To: DIY <synth-diy at mailhost.bpa.nl>
Date: Wednesday, December 16, 1998 5:38 AM
Subject: spikes (was:AW: FETs on the ASM-1 VCO.)


>With all this talk about MOSFETs, JFETs and even diodes:
>
>Is there anything wrong doing it with BJTs ?
>I've used bipolar transistors for discharging in my VCOs all 
>the time. I haven't done much research myself, just copied what
>I saw in many commercial circuits. It works ok, though the spike
>in the (converted) triangle could be smaller ...
>Is this a general problem with BJTs, I mean do I get much shorter
>reset times with FETs ?
>
>IMO, the only reason for ultra short reset time is triangle
>conversion. And the problem is worst in LFO applications.
>Sure, I know, first rule in Building VCOs: Start with triangle,
>not with saw. But then I absolutely love the variable shape
>of Osc 3 of the VCS3 for modulation - it's a very unique
>effect - and you can only get that when your oscillator core 
>is producing a saw wave. So here is an application where
>short discharge times are important. (I don't speak of
>nanoseconds here, but not of 1nF capacitors either. It's
>a 1u electrolytic, if memory serves.) I could improove things
>a little when I used a darlington.
>
>JH.
>