Found in the May 1988 issue of Radio Electronics magazine, this circuit is sensitive to small variations in input voltage. The power supply must be symmetrical.
This circuit is a from publication 1980, but still can be mounted with BC548 transistors for Q1...
This circuit for testing zener diodes by lifting their curve on an oscilloscope was presented in...
This high quality circuit with complementary symmetry was found in an Electronics World of 1966....