8 Nov
2025
8 Nov
'25
6:45 a.m.
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 <cctalk(a)classiccmp.org>
wrote:
On 10/30/25 03:30, Christian Corti via cctalk wrote:
On Wed, 29 Oct 2025, hupfadekroua wrote:
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
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.