Re: Where have all the video transistors gone? (needed for wideband amplifier)


Laurent Lamesch wrote:
The application: a discrete 50 Ohm wideband amplifier with


Still another option: In most datasheets, only Vceo is given. As the
transistors are not operated with open base in the application, how much
can Vceo be exceeded?


Good surmise! I recently measured some humdrum MPSA42's, which IIRC
are rated at 300 volts max. Well with 1K ohms between base and
emitter, to bleed off leakage current, they ran just FINE with 800
volts on the plates, er, collectors. Every one I measured out of a
batch of 100 worked fine (AT ROOM TEMPERATURE) at 800 volts, most
broke down around 830 to 860 volts. I wouldnt recommend doing this as
a regular thing, for mil-spec job, or for a nuke-bomb trigger, but you
might be just fine pushing the ratings to say 30% below the measured
breakdown voltage.

Also fainter in my memory was my measures of some high-speed
transistors to replace those whiz-bang ones Tektronix used as vertical
output transistors. The specs were around 25 to 30 volts, but with a
small drive resistor, none broke down below 75 volts.

Note that this isnt necessarily a poor practice: for example, A LOT
of transistors in Tek scopes are "selected" more humdrum types, being
pushed waay beyond their rated limits of voltage, leakage and speed.

For example, the CRT grid blanking driver transistors in the 4xx
series of scopes are asked to switch about 120 volts in a very few
nsec-- if you look up the ratings for the 2Nxxxx part, the rating is I
faintly recall, 65 volts and 15nsec. They must have culled a LOT of
these transistors to find a few golden ones, or maybe they found a
batch that were way better than spec-*** specs.

.

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