The Moog VCF (1966)

Structure: four identical one-poles in series with a feedback loop:

This implements a voltage-controllable four-pole filter:

In the Moog analog ladder (the ``Moog 904A''), the one-pole filters consist of the capacitors and the AC resistance of the transistors, which is determined by the current source, which is varied to control tuning. See US Patent 3,475,623