Use of Diodes in Filters??

Martin Czech martin.czech at
Thu Nov 20 10:00:50 CET 1997

> Also, you usually don't get more than 0.75 V over most diodes, it will
> just be a lot of current going thougth it... this put a upper limit on
> the voltage applied and this results in a need to keep signals down
> before you clip.
> One trick one may play with the diode is to stack multiple of thems
> such that you migth get better signal to noise ratio (less noise that
> is). An extreme case of this is found in the EMS Hi-Fli where the
> phaser section contains a stack of 20 diodes per AP filter, there is a
> total of 12 in the Hi-Fli which results in just 240 diodes for the
> core phase block itself.

Yes, in this case a large number of diodes is used for each filter block.
But I thought the question was about the three diodes in the EMS filter etc.
In this case there is only one  "transconductance" diode per stage,
and I think the stack of three diodes is used to symmetrise (?) the ladder.
This way, the manufacturer would not necessarily have to select the diodes.
This is very expensive in mass production.

So , using multiple diodes makes matching easier, just by the first law
of statistics : "law of the large number" (in case of a large number of 
elements the distribution will "sharpen" arround some mean value).

What do you think?

But the S/N ratio in the example obove is also a good reason to
stack diodes, for shure.


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