[sdiy] Transistor questions

[rburnett at richieburnett.co.uk]

> but isn't the discharge loop between the transistor and whatever
> capacitor it is discharging from?

Correct.  I had thought you meant adding a smd bypass cap, like a supply 
decoupling capacitor across the switch.

-Richie,



On 2025-03-04 09:37, cheater cheater via Synth-diy wrote:
> but isn't the discharge loop between the transistor and whatever
> capacitor it is discharging from? can't get much better than moving
> the capacitance all the way up close to the bond wire, can it?
> 
> On Tue, Mar 4, 2025 at 10:28 AM <rburnett at richieburnett.co.uk> wrote:
>> 
>> I dont think so.  It's inductance in the entire discharge loop that
>> defines current rise time (assumming an infinitely fast switch.)
>> 
>> That same loop inductance is also likely to make the capacitor voltage
>> ring negative if the switch doesn't open at the precise instant that 
>> the
>> capacitor voltage reaches zero!
>> 
>> -Richie,
>> 
>> On 2025-03-04 08:19, cheater cheater wrote:
>> > 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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