Hi guys,

At the risk of starting another holy war or Mass Thread of Off-Topicness=20
as I seem apt to do...

I was recently chatting with a friend, and we got onto the subject of=20
'jellybean' parts, via a discussion on the construction of read-write=20
amplifiers for disc drives (seriously!)

Anyway. He proposed a current-mirror with two switches, good to 1MHz or=20
so, based on a bunch of 2N3904s. I commented "That's great, but I don't=20
have any 3904s, just a box full of BC547s and BC847s". To which he=20
replied "Are you serious? 3904s are jellybean NPNs and you don't even=20
have one of them?"... He proposed that I get a box of 2N3904s and=20
2N3906s as a matter of some urgency...

So I'm skimming the datasheets for the 3904 and the 547, and they don't=20
seem all that different....
   * Vceo is slightly better on the 547 (45V vs 40V)
   * Vcbo is slightly worse  on the 547 (50V vs 60V)
   * Vebo is still the same old 6V on both
   * Max collector current is worse on the 547 (100mA vs 200mA)
   * Hfe is roughly the same, assuming we're talking about the B-spec=20
BC547 (aka BC547B)
   * Slightly less input and output capacitance on the BC547
   * Better typical noise figure on the BC547 (2dB vs 5dB)
   * They both have the same Ft (gain-bandwidth product) of 300MHz,=20
suggesting they'll both go to 100MHz if the current gain is kept below 3x
   * Fairchild don't tell you as much about the 547 (no detailed on-off=20
characteristics, just a few vague graphs). No details on rise/fall time,=20
breakdown voltages or cutoff current.
   * The pinouts are mirrored -- the 547 is C/B/E, the 3904 is E/B/C. So=20
you could put one in a socket intended for the other by rotating the=20
transistor 180 degrees...

But other than that they seem to be basically the same thing by a=20
different name -- "a rose by any other name". Which brings me to my=20
questions:

   - Is there any point to getting some 3904s and 3906es? So far the=20
best reason I've got is "so I've got a few to play with" (which isn't=20
that good a reason). All my R. A. Penfold books call for the BC547 (or=20
one of its brethren), Horowitz & Hill demands the 3904.
   - Are there any other jellybean parts I should look into keeping=20
around? I already have some BD139 and BD141 medium-power parts and a few=20
MJE3055 and MJE2955 TO220 high-power parts, and some 2N7000 MOSFETs, but=20
that's mostly it (aside from a few ST and IRF high-power FETs, and some=20
BF998 dual-gate MOSFETs and 2N3819 JFETs)...

Thanks,
--=20
Phil.
piclist@philpem.me.uk
http://www.philpem.me.uk/
--=20
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