Using the 741 this basic circuit was found in the November 1973 Radio Electronics magazine. The power supply must be symmetrical. The components can be recalculated for other frequencies.
Found in old Philips documentation, this circuit uses components from the 60's. | Clique na imagem...
The circuit shown uses the LM324, but can be implemented with other operational ones. The source...
Found in Linear Technology documentation, this circuit drains only 70 uA of the monitored circuit....