Re: low capacitance zener
- From: Mike Monett <No@xxxxxxxxx>
- Date: Thu, 19 Apr 2007 23:18:36 +0000
Joerg <notthisjoergsch@xxxxxxxxxxxxxxxxxxxxx> wrote:
> John Larkin wrote:
>> Most 5-volt zeners have tons of capacitance, 100 pF maybe. The
>> b-e junction of a BFS17 is about 1.7 pF at zero bias and seems to
>> be a very nice 5.5 volt zener.
> Yeah but are you sure next month's shipment will be close enough
> to that 5.5V?
Doesn't matter these days with software calibration.
But does the junction capacitance stay at 1.7 pF when it goes into
Zener mode? I thought that's what caused the huge increase in
capacitance. I believe some typical ones may even reach 1nF.
There have been several discussions on using the base-emitter
reverse breakdown. Here's some info from 1999:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
some app notes say operating in avalanche mode, however briefly,
will degrade the noise figure in low-level RF amplifiers.
The 2N2222 base-emitter junction has a very sharp breakdown - about
7.2V from < 1uA to > 10mA. Very repeatable on a number of units from
the same purchase.
The 2N2369 (or 2N3227) is not so sharp but the knee is still under
10uA. About 6.4V.
The 2N5139 is sharp. About 7.8V.
The 2N4209 is sharp. About 6.0V.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Regards,
Mike Monett
.
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