Yesterday I found a few LM13700s sitting around, which lead me to this interesting two part mini-series about OTAs, from gain control and voltage-controlled filters to voltage-controlled oscillators. It's an amazing subject:

UNDERSTANDING AND USING ‘OTA’ OP-AMP ICs

by Ray Marston

Part 1

https://www.nutsvolts.com/uploads/magazine_downloads/11/April%202003%20Ray%20Marston%20-%20Understanding%20and%20Using%20OTA%20Op-Amps.pdf

Part 2

https://www.nutsvolts.com/uploads/magazine_downloads/11/May%202003%20Ray%20Marston%20-%20Understanding%20And%20Using%20OTA%20OP-Amps.pdf