27 Oct
2025
27 Oct
'25
2:53 p.m.
On 10/27/25 14:17, Brent Hilpert via cctalk wrote:
On 2025Oct 27,, at 9:37 AM, Christian Corti via cctalk
<cctalk(a)classiccmp.org> wrote:
The MPSH81 shows Ccb = 0.85 pF and Cce = 0.65 pF and fT 600
MHz. They don't give a Cob spec.
Jon
On Mon, 27 Oct 2025, Paul Koning wrote:
Cob, fT not quite as good, but close:
2N5400: Cob=6p0, fT=400MHz
2N4916: Cob=4p5, fT=400MHz
vs 2N3639: Cob=3p5, fT=500MHz
Have you tried with an ordinary switching
transistor that's readily available? I'm surprised at that storage time, why
would it need to be that low for a machine that runs at just a couple of MHz?
The
storage time is very crucial. We have tested many different switching transistors. The
point is, the PDP-8 uses needle impulses (around 30ns) to switch a flip-flop. They just
don't work with today's transistors. And on modules like the R211/S211, they
introduce a too long delay.