The name explains it all: this converts a triangle wave into a sinusoidal wave. It uses the familiar differential pair BJT structure that other designs use, but its feedback paths make it far more convincing than other tri/sine converters, at least to my eyes.
This circuit is one of the amazing designs by Don Tillman. Tillman is usually cited for his innovative through-zero FM VCO with a trapezoid core, but his other designs are also quite inspired. I've linked his website below. Give it a read when you have the time!
https://till.com/articles/sineshaper/index.html
