[sdiy] Article "Analyzing the Moog Filter"

[Bernard Arthur Hutchins, Jr]

In his discussion, the author (Sam) asks: “Why are there so few topologies? Why don't any of the existing topologies look much like filters?”

The first question depends on what you mean by “few” and what constitutes a DIFFERENT topology.   If he means that most VCFs are “Ladders” (by including the modern equivalent of four cascaded first-order low-pass with feedback) or state-variable (S-V), he is correct in his observation.

The second question depends on what a filter is SUPPOSED to look like. If he is thinking of a filter as a (fixed-parameter) “active filter”, perhaps VCVS, MFIG, etc., where you get TWO poles for EACH op-amp, then topologies where you expend an op-amp for each pole (4-Pole LP) or three op-amps for two pole (S-V) look strange perhaps.

BOTH questions have answers that are understood in terms of the same design restraints – a limited number of PRACTICAL ways of achieving voltage-control.

For example, if you started with a VCVS you have two floating resistors (not one end to ground or to virtual ground) and these would cost you two OTAs (and two op-amps) for each such float:
http://electronotes.netfirms.com/AN23.PDF

In such a case, getting 2nd order with two OTAs to virtual ground and two integrating op-amps and a summer is a good deal.  We speak of such as a S-V-“filter” but it came out of an analog computing topology.

But it is an explanation of engineers using methods/resources wisely.

Bernie

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