Moog 904A lowpass filter Question

[jh.]

>After thinking about this, it occured to me that
> the signal at the top of the ladder is read out across the base emitter
> junctions of the transistor pair, these junctions have a logarithmic
> characteristic which cancels out the exponential characteristic of the
pair
> at the bottom, hence a linear overall response.
>
> I then installed the ladder capacitors.  The ladder was still linear for
> signals at all amplitudes well below its cutoff point, but when I ran a
> sine wave through and swept the filter cutoff around the sine frequency, I
> could clearly see distortion of the sine wave near the cutoff point.  The
> distortion is very dependent on input signal level.   I'm guessing this is
> because the caps are within the non linear part of the ladder, before the
> log response at the top of the ladder.

Exactly how I understand it. As strange as it looks, it's the linear
capacitors that cause distortion, because they "shunt" the high freq part
of the current, and the distortion won't cancel from diff pair to
top stage anymore. Same goes for each intermediate stage, btw.:
Without caps, it's just a cascode (highly linear with current source
type input from the collector below), but with the caps you get
a current divider from nonlinear BE junction and linear capacitor.

JH.