EMRFD Message Archive 3777

Message Date From Subject
3777 2009-11-16 06:56:00 Alan Melia Re Re: Great Learning Site
Hi Russ Ah yes I had forgotten about the low saturation voltage in inverse
mode. This is probably de to the fact that there is a big area "emiiter"
junction and a very low resistance "collector". Anyone for a "inverse mode"
active mixer ??

To Cecil, I am not sure about the Ft but this does depend on the collector
capacitance Cob The normal transistor collector N region is kept high to
reduce this an a epi layer is used to reduce the series resistance.....(in
traditional planars of 40 years ago anyway :-)) ) The emitter is heavily
doped and have a relatively high capacitance because the depletion region
does not move very fay so i guess the Ft would be much lower. Ft is not much
dependent on DC gain....cf the old gold doped switching transistors, which
had the gain reduced as a result of introducing recomination centres in the
base region to shorten the decay of the stored charge, and speed the
switch-off.. (I think this is all getting a bit hazy now at this distance in

On the "avalanche" transistors mode, I have a feeling that Ft doesnt come
into their speed performance because the effect is not a minority carrier
effect so you can get nanosec pulses out of a standard "audio" transistor
like a BC107. (I could be wrong here as well ?)

Fascinating stuff this "old" electronics.

Alan G3NYK