This circuit, found in a 1958 magazine, is for reference only, since the components are not easy to find today. The transistors are germanium types and power supply is made by common cells.
This circuit with the 741 operational amplifier was found in the March 1995 issue of Electronics Now...
Found on an 80's Elektor, this circuit uses common components, being the diode a general purpose...
Found in a 1967 documentation this circuit can be applied to any operational amplifier. Components...