Is there a quick'n'dirty way to estimate the useful frequency limit of old Ge RF transistors? - I was thinking of maybe an inherently unstable untuned RF amp that would tend to oscillate at the highest frequency that the transistor gave useable gain!
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?
Unusually that spec is given and its a short form data book not a proper data sheet! IIRC the values are identical for both devices - but measured at the respective frequencies.
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