[sdiy] Transistor questions

[cheater cheater]

Hi Richie,
wouldn't an smd bypass cap soldered directly to the pins of the
transistor where they come out of the body help satisfy the tight
layout requirement during rise time?


On Tue, Mar 4, 2025 at 12:53 AM <rburnett at richieburnett.co.uk> wrote:
>
> 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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