This is the circuit used by the MiniMoog to implement an amplifier with voltage controllable gain.
Schematic is here: https://5093fe08-a-62cb3a1a-s-sites.googlegroups.com/site/minimoogwiki/home/notes/911-filt-print-circ-schem.gif
The triangle waveform is the signal input and the sine wave input is voltage controlling the gain. The resulting triangle wave output amplitude is seen to be determined by the sine wave voltage.
Some parameters can be tweaked via the potentiometers present in the circuit. The 100k pot is a trimmer simulating "External loudness input" which determines the actual amount of gain that is controlled by the control voltage input.
