AW: diode ladders

[Haible Juergen]

	> So it is difficult to make a linear response !?

For a filter, you don't want the resistance to be controlled
in a linear manner, but rather the inverse of it. Which is,
as you have shown, easily accomplished by a current source
that feeds the ladder. 
If you want exponential control, you have the choice:
(1) Use an exponential current source: The moog ladder does
      this as well as the EMS VCS3 filter and the ARP Quadra
      Phase Shifter, or:
(2) Use a voltage source (but one which tracks well with temperature!)
      to drive the diaodes directly. The EMS HiFli Phaser does this.

JH.