[sdiy] Transistor questions

[René Schmitz]

On 03.03.2025 13:17, Matthew Skala via Synth-diy wrote:
> On Mon, 3 Mar 2025, Roman Sowa via Synth-diy wrote:
>
>> I don't think there's a way to achieve 3ns rise time in this circuit,
>> expecially when done in THT.

I also think this is off by about an order of magnitude here, maybe two.

The original circuit is not that fast.

> Having anything happen in 3ns implies bandwidth of at least about 300 MHz,
> i.e. the bottom of the UHF range, which is certainly possible on a
> carefully designed through-hole board.  I've seen ham radio projects do
> it, and note that ordinary FM broadcast recievers routinely handle(d) 108
> MHz signals on through-hole boards, let alone UHF analog television
> receivers.  Conventional wisdom today includes exaggerated ideas of the
> disadvantages of through-hole.  But the hams would usually use dead-bug
> construction instead of a through-hole board for this range.  Getting it
> to work is far from a gimmie.
>
>
A thing to watch out for is the maximum current. A faster discharge 
means more current.

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.


Best,

  René


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