We found this circuit in an old documentation. The operational can be of any type and the frequency is given by the formula next to the diagram.
We found this circuit in a 1980s documentation. The circuit uses general purpose transistors,...
This amplifier with gain control was obtained in a documentation from Signetics of the 80's....
This circuit works with only 1.5 V and can be powered by a solar panel. The circuit is from 1991....