[sdiy] Filters

[Magnus Danielson]

From: James Patchell <patchell at cox.net>
Subject: RE: [sdiy] Filters
Date: Sat, 03 Jan 2004 10:22:01 -0800
Message-ID: <5.2.1.1.2.20040103101539.00b450f8 at pop.west.cox.net>

> That is a really good question....thing is, the question should really be 
> implementing diodes as current controlled resistors...but, that is a quibble.

My recommendation would be following:

1) Read the Moog transitor ladder patent (except for the claims, which is only
   for the criminally insane, patent layers and perverts like myself).
   It is not a bad introduction really.

2) Read the SSM 2044 patent. Good continuation.

3) In the end, recall that the only thing you do is replace the frequency-
   setting resistors of some filter with diodes. You let the small-signal
   properties change by adjusting the large signal properties.

> I can only think of good examples.  The Neil Steiner mentioned below is a 
> very good example.  Also, if you can find a circuit for a diode ladder 
> filter (as opposed to a transistor ladder filter), that is another good 
> example.  Also, the Paia 2700 series lowpass and band pass filters are two 
> more examples of using diodes as current controlled resistors.
> 
> The thing you have to remember about a diode is that the dynamic resistance 
> is proportional to the current flowing though it.  You also need to keep in 
> mind that any changes in the diode voltage are going to make an exponential 
> change in the diode current....this is why in general you would not want to 
> use a diode as a current controlled resistor, because it is not very 
> linear, but, hey, for us, that is generally a good thing.  You do have to 
> be a lot more clever using diodes as control elements.

Indeed. Like building "large" diodes, i.e. things like 10-20 diodes in series,
then use those as variable resistors. The gain is that you can raise the
signal level and thus dynamic range. It's wicked and obscure. It's been done
before, but that shouln't stop anyone, now should it?

Cheers,
Magnus