[sdiy] Passive filters and impedances

Donald Tillman don at till.com
Sat Oct 31 19:02:36 CET 2015

> On Oct 31, 2015, at 9:24 AM, Tom Wiltshire <tom at electricdruid.net> wrote:
> 	http://electricdruid.net/images/PassiveFilters.png

Yikes!  It would be a lot nicer to just draw the schematic on paper and scan that.  :-)

> For the first order case, I've been able to derive the transfer function by thinking of it as a potential divider with impedances.

Yes, that works fine.

> However, I can't make the second order (or by extension third order) case work the same way.

Right.  It gets messy, error prone, and unsolvable.

To analyse a non-trivial circuit you need to label the N nodes, know that the current summed into each node must be zero (because "what comes in must go out"), write out an equation for the current into each node in terms of its neighboring voltages, and solve the N simultaneous equations.

Like many techniques, there's a barrier to entry, but after that it becomes really efficient.

  -- Don

Don Tillman
Palo Alto, California
don at till.com

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