11 Apr
2012
11 Apr
'12
12:48 p.m.
On Apr 11, 2012, at 3:09 PM, Dave McGuire wrote:
"Too hard" being the key; you still want it to generally be in the same
ballpark or higher (e.g. a 2N2955 would fit the bill here except for its
hFE of ~15). Higher is generally better in most cases unless the design
tradeoffs required to get that higher gain impede something else
important (e.g. higher gain can be achieved if you don't care as much
about current handling, Vce(max), noise figure, etc, or if you go with
a Darlington pair which is really two transistors).
I think the 2N2907 part is basically aimed at the same application area
as the older A55, but it benefits from newer processes. You'll find
that in a lot of transistors; I'm still a little baffled as to what the
quantitative difference between a 3904 and a 4401 is aside from slightly
better current capabilities on the 4401. Typically I can swap one for
the other with no problem unless I'm blowing up 3904s.
- Dave
I have
identified some transistors with similar specs (e.g.
http://uk.rs-online.com/web/p/general-purpose-transistor/7390492/), but the
current gain hFE is higher at 100. Not sure if it is better for the hFE to
be higher or lower than the required spec.
It's pretty unusual for a circuit design to rely on a specific current
gain (Hfe), because it's not constant. It varies with the voltage
across the transistor, and varies a good amount from transistor to
transistor of the same model. Don't try too hard to match the Hfe.