[sdiy] Transistor questions

[Roman Sowa]

I don't think there's a way to achieve 3ns rise time in this circuit, 
expecially when done in THT.
Build it and measure, it's so simple, just a few parts. You're in the 
region of timing where actual PCB is more important than simulation results.
As Gordon said, I also think that 2Sxxx transistors models might not be 
fully accurate.
Well, even 30ns is ambitious plan.

Just think about reset current needed in this VCO to achieve 3ns reset 
time to see how ridiculous the simulation result is. It's way beyond 1A 
then.

Roman

W dniu 2025-03-03 o 06:16, David G Dixon via Synth-diy pisze:
> Hey Synth DIY Team,
> I have built sawcore VCOs with two-transistor cores based on the Korg 
> PS-3100.  I initially used 2N3904 and 2N3906, and they worked great, 
> but simulations suggested that the saw reset time would be around 
> 30ns, which was more than I wanted, because I want perfect tracking up 
> to about 6kHz without high-frequency compensation.  Hence, I tried 
> many other transistor models, and settled on 2SC945 and 2SA733.  My 
> simulations suggested that the reset time would only be 2 or 3ns, and 
> this is fast enough.
> My question is this: given that the 2N390X transistors have higher 
> transition frequencies and lower collector capacitances than the old 
> Japanese transistors (according to All Transistors), why should the 
> latter give significantly faster saw reset times?  What is the actual 
> figure of merit for this?
> Cheers,
> Dave Dixon
>
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