17-05-2017, 06:16 AM
(16-05-2017, 03:40 PM)ppppenguin Wrote: I think alpha mattered a lot in the very early days of transistors where beta was very low and you couldn't usefully approximate alpha as unity. Leakage current mattered too in the days of germanium devices and made it more difficult to design circuits that were sensibly immune to temperature variations etc.
That is absolutely the case. Germanium transistor beta was typically in the range 40 to 100, and leakage current from 50uA to 300uA, and highly temperature dependent. Finger heat will easily triple leakage current. And when trying to measure beta, you have to wait for the transistor temperature to stabilise, measure leakage and then subtract that from the collector current when measuring beta.
Got heavily into that when building a fuzz pedal for my guitarist son. Why Ge rather than Si? Well using the "correct" Ge transistors such as AC128 gives a soft edge to the distortion because of the truly dreadful frequency response. Si transistors have much higher frequency response, and hence produce faster rise times to hard fuzz, and hence produce a harder and edgier tone.
First time I have ever built a circuit whose purpose is to distort!
