The circuit presented is from the Motorola Linear Databook of 1990. The integrated circuit is a transconductance operating and the power supply need not be symmetrical.
I found this circuit on a December 1976 Radio Elettronica. The Italian magazine no longer exists, but...
We find this circuit in a 1978 documentation. The circuit uses two inverters of a 4049 integrated...
This circuit is from 1995 documentation, but can be easily assembled today, as the components used are...