On the "low noise transistor" thread, we got into how BJT transition
frequency (f sub tau) and current gain (beta) vary with collector
current. I posted graphs of beta vs. Ic for two very common
transistors, the 2N3904 and 2N4401 (and their cognates in other
packages with other prefixes, MMBT, etc.). Here are two graphs
showing beta vs. Ic and Ft vs. Ic for a range of popular transistors
(a few of which are obsolete by now). The graphs are from the second
edition of The Art of Electronics (not the new, 3rd edition).
Once again, note that the current gain curves represent the static
(DC) values. As Rick pointed out, the DC value is an upper bound on
the AC current gain, which is approximately equal to the DC value up
to the transition frequency (Ft) divided by the DC current gain
(usually at least 1MHz for small-signal transistors). Above that, it
declines at 20dB per decade.
Best regards,
Charles
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