This circuit was taken from Maxim documentation from the 1990s. It can be implemented on the basis of modern operational amplifiers. Power supply doesn't have to be symmetrical.
Found in American documentation from the 1970s, this circuit employs an integrated circuit from...
This circuit was obtained in the publication 101 Electronics Projects of 1979. The circuit can be...
This assembly circuit using a small motor as a generator was found in Radio Electroics magazine from...