[sdiy] Transistor questions

[David G Dixon]

Well, all of the core components of the VCO are within a few tenths of an
inch of each other in the layout, but I see your point.  This is why I ask
this stuff here.  I'm a chemical engineer/extractive metallurgist who does
electronics for fun (and, occasionally, a small profit), so I'm not all that
well informed on these sorts of issues.



-----Original Message-----
From: rburnett at richieburnett.co.uk [mailto:rburnett at richieburnett.co.uk] 
Sent: Monday, March 03, 2025 3:49 PM
To: David G Dixon
Cc: synth-diy at synth-diy.org
Subject: Re: [sdiy] Transistor questions

[CAUTION: Non-UBC Email]

Assumming for a moment that the transistor is somehow capable of 
switching instantaneously, you would need less than 6nH of total 
inductance in the discharging loop for the current to even *reach* 1.36 
Amps within 2ns with an EMF of 4 Volts!!!  This just comes from the 
equation V = L di/dt.

Given the typical figure of 25nH per inch of wire inductance you can see 
that you would need an incredibly tight layout!!!  And that is just to 
get the current to ramp up in the available time, then the capacitor has 
to discharge once the current is actually flowing!

The figures just don't seem feasible to me.

As a power electronics engineer I used to smile when I saw MOSFET 
gate-drive chip datasheets boasting many amps of current source/sink 
capability. Whilst they are impressive figures for sure, it requires 
phenomenally low inductance in the drive loop to get anywhere near 
realising this potential in a real-life application!

-Richie,