I got hold of some MPSH81. Unfortunately they are not a suitable substitute for a 2N3639 or 2N3640. I built a little test circuit with a 100 ns pulse generator replicating a real R-series flipchip circuit. This works well with a 2N3639 or 2N3640, but not at all with the MPSH81. The MPSH81 doesn't even start to turn on unless I extend the pulse duration to 400 ns. The datasheet for the MPSH81 doesn't even specify tON, tOFF and tS, but the fT of 600 MHz appeared promising. The search for a suitable substitute continues ...
Tom On Thu, Oct 30, 2025 at 11:48 PM Jon Elson via cctalk <[email protected]> wrote: > On 10/30/25 03:30, Christian Corti via cctalk wrote: > > On Wed, 29 Oct 2025, hupfadekroua wrote: > >> what do You think about: > >> > >> 2N3640 -> 2N3906 > >> 2N3693 -> 2N3904, 2N2222A, BC337-25 > > > > I've tested exactly those types. They are all too slow. > > For example, when you cascade the three independent stages > > on an R211 module, feed the first stage with a clock of > > around 1 MHz, and just replace one of the 2N3639 with a > > 2N3906, the turn-off slope after the last stage will be > > delayed from around 30ns to 200ns! > > But we will test the trick with the Baker clamp on the > > 2N3906. > > I think you should try the MPSH81. Depending on the bias > and base drive, you may also need the Baker clamp. (Use a > really fast, low capacitance diode for that.) > > The 2N3906 is an audio transistor, and not fast or low > capacitance. > > Jon > >
