Robert Baer said:
Ft is a function of collector current; nominally incerases linearly with
collector current.
When internal resistances become dominate due to "high" current (Rbb',
Re' mainly) then that breaks down.
At a given current, measure the gain at a given frequency F and the gain
at 2*F and extrapolate; verify that the gain is one near that extrapolated
frequency.
In particular the Ft is known for the AF11x family of RF transistors, as I
have these listed in a Mullard data book. A curious point is both the AF116
& AF117 have Ft listed as 75MHz yet the 116 was sold for use in 10.7MHz
FM/IF strips and the 117 is only deemed suitable for AM L/O & 455kHz IF
strips - almost all the specifications are identical so what makes the 116 &
117 different?!
There are also non Pro-electron numbered Ge transistors in the bits box with
no easy way of distinguishing between RF & AF types especially Newmarket
"NKT" types It would be handy to be able to classify these.
One contributor to this thread suggested a ring oscillator, at first glance
this didn't look very helpful since I suspect that the oscillator might be
limited by the slowest transistor in the ring. On the other hand, if there
is no problem mixing Si & Ge transistors in the same ring, I'm sure I can
find a few PNP RF Si transistors that far outperform the Ft of the Ge ones I
want to test. Hopefully then the Ge transistor in the ring would be the
slowest - and limit the maximum frequency the ring can oscillate at. Am I
anywhere near on this?
TIA.