ladder filter theory?

[Lawrence Archard]

On 24 Jul 97, Rob Hukin on the subject of ladder filter theory? 
wrote:

> Can someone point me in the direction of a theoretical explanation of how
> diode/transistor ladder filters work? Anything in Electronotes?

Don't know about a proper reference, but on a hand-waving level....

The pair of transistors at the bottom of the ladder form a 
differential amplifier or long-tailed pair (so match them). Signal 
levels on the bases of this pair can be increased to achive the 
desired level of 'overdrive'. 

The current drawn through the 'tail' and so also through the legs of
the ladder can be varied. The effective dynamic resistance of the
diodes/transistors in the chain varies as a function of the current
through them so you effectively have a ladder in which the 'rungs'
are fixed capacitors and the uprights are variable resistors - hence
a tunable filter. 

A differential amplifier (often an op-amp) takes the ac part of the
signal off the top of the ladder, part of which may be fed back to
increase the resonance of the filter.

Is that any use? I'd also be interested in seeing a full analysis.
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