This circuit uses three inverters to generate a rectangular signal whose frequency depends on R1 and C1. The circuit is from a March 1996 Electronics Now.
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....