At 20.00 2012.01.11, you wrote:
>At 10:32 AM 11/01/2012, you wrote:
>
>>Hello,
>>what happens when a negative swing at the base of a NPN transistor=20
>>reachs the break down voltage?
>>
>>Does it turn on the transistor, and then as long as the current is=20
>>limited to a safe value, it won't damage it or do other weird things?
>>
>>I can see breakdown voltage specified in datasheets, but not what=20
>>will happen if it gets reached. As I understand it, PN junctions do=20
>>not suffer damage from breakdown like e.g. a Schottky or a MOSFET gate wo=
uld.
>>
>>Thanks!
>>Mario
>
>A few things that may be unexpected--
>
>- a permanent reduction of transistor beta. This is
>   essentially damage to the BJT that is (practically speaking)
>   irreversible. You should thus avoid that region of operation
>   if you want to use it again as a transistor (once a noise source..)
>   This is why you often see an inverse-parallel switching diode.
>
>- you can make a relaxation oscillator with just the
>   C & E pins if the transistor is flipped (so the base-emitter
>   junction breaks down). Again, permanent degradation of forward beta.
>
>- you can get a bit of current of unexpected polarity at the
>   collector if you break the E-B junction down- it's a result
>   of IR light being emitted by the E-B junction and received by
>   the C-E junction! Again, permanent degradation of forward beta.

Thank you. It's clear that I won't use it then. But I will remember it
in case I'll ever need a white noise source better than a zener, the beta
is irrelevant for this application and it can be useful in this case.

Cheers,
MarI/O


>
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>
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>
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