[sdiy] Transistor questions

[rburnett at richieburnett.co.uk]

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,


On 2025-03-03 19:56, David G Dixon via Synth-diy wrote:
> The cap I'm using is 680pF, and it is discharging about 4V.
> 
> According to my rough calculation, it would require a collector current 
> of
> 1.36A to discharge that in 2ns, but only 0.136A to discharge it in 
> 20ns.
> Hence, the 3904 could do it (with its maximum I_c of 0.2A) but the 945 
> could
> not (with its maximum I_c of only 0.1A).
> 
> Well, I have both versions of the VCO built.  I'll calibrate them and 
> see
> which one gives better tracking.
> 
> 
> 
> -----Original Message-----
> From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of 
> René
> Schmitz
> Sent: Monday, March 03, 2025 4:43 AM
> To: synth-diy at synth-diy.org
> Subject: Re: [sdiy] Transistor questions
> 
>> On Mon, 3 Mar 2025, Roman Sowa via Synth-diy wrote:
> 
> To discharge a 10nF cap in 2ns you need currents in the amps range. You
> don't get that with these transistors.
> 
> And even if you managed, you spew fast transients all over the place.
> Faster is not always better.
> 
> 
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