looking for fast zener diodes

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Hi,

standard 500 mW zener diodes in the 6 V range show junction capacitances
around 100 pF, which limits their switching speed.

Therefore, often the base-emitter diodes of a small-signal transistors
are substituted. But how much do their zener voltages scatter? In other
words, how many transistors does one have to check to find 5%-matching
voltages?

Or is there such a product as fast zeners with defined voltage? Perhaps
200 mW types?

Thanks,

Martin.

 

[Jim Adney]

If you mean something like a P6KE6.8V: the Lite-On datasheet shows a
capacitance of 2500pF! I suppose this does no harm for the intended
applications, it may even help suppress voltage spikes.

Yes, that's a Transorb, and, yes, they have a large junction
capacitance which is usually not a bad thing where they are usually
used.

I HAVE used them in RF applications where I needed to keep the C down,
so I added a 1N914/1N4148 in series. The internal C of a good signal
level diode is small until it is actually forward biased.

-

 

[K Wind]

Thanks for the suggestions.

My setup is a minimalist textbook one: I'm using a 'fast' Op-Amp
(Ha-2515) with back to back zener diodes (ITT ZPD5.6) between output and
inverting input to convert a 20kHz, 6Vpp sinusoidal signal into a square
wave. The signal is fed into the inverting input via a 2,2k resistor
(max. diode current 3mA).

The resulting signal is decisively affected by diode capacitances, (the
slew rate is 10V/us rather than the 60V/us expected for a HA-12515).
Putting a 150pF capacitor in parallel with the diode pair reduces the
slew rate to 5V/us; the pair should therfore have a capacitance of
150pF, implying 300pF for each diode. Using two BZX79C6V2 in place of
the ZPD5.6's, I find a similar (about 10% smaller) capacitance.

Many datasheets don't list zener diode capacitances, but my value is in
rough agreement with the data available: For a BZX79C5V6, Vishay list C
=
300pF (max.) whereas, for the same diode, Motorola list C = 95pF (typ.)
in their table, and show 150 pF (typ.) on their graph. For a MAZ4056,
Matsushita list C = 95pF (typ.) and 140pF (max.). As the manufacturer is
often unknown when you buy zener diodes, it is difficult to know in
advance what capacitances you get!




This will certainly do the trick, at the cost of an increased component
count. The zener diodes would here be used as level shifters at the
Op-Amp output.




If you mean something like a P6KE6.8V: the Lite-On datasheet shows a
capacitance of 2500pF! I suppose this does no harm for the intended
applications, it may even help suppress voltage spikes.

I've seen transistors used to clamp the output of an op-amp by connecting
them from output to inverting input. Is it good practice to use transistors
as low current zener diodes?

Ken