[sdiy] SSM2220 dual PNP clones?

[David G. Dixon]

Thanks Paul and Renee for your excellent responses.  I hadn't thought of
using the diode tester to test Vbe, but it's a bit of a forehead slapper,
isn't it?

Incidentally, I just tested 10 1N4148 diodes which I desoldered from a
circuit to see if they were OK for reuse, and they all read exactly 0.586 V!
I guess if my 2N3906's are similar, then I won't need to drop $7 on SSM2220
(because there doesn't seem to be a suitable alternative).

(I also tested a bunch of 1M and 499k resistors which I had hand-sorted to
be within 0.1% for use in an R/2R ladder, and they hadn't budged either...
It would appear that normal soldering and desoldering isn't enough to change
the values of metal film resistors or diodes.  Good to know, I think.)



> > And another, closely related question:  If I were to try to match PNP
> > transistors by hand, what would I actually be matching?  Beta?  If it's
> > beta, then that's easy, because my multimeter has a beta-reading
> doohickey.
> 
> You would need to match Vbe for a given collector current. Beta itself
> is relatively unimportant, which you can see from the fact that it
> doesn't show up in the equations describing the exponential convertor.
> 
> If you want to short cut it using a DVM, then use the diode tester.
> (Effectively matching Vbe at Ic = 0)