With this circuit of a Popular Electronics from the 90's it is possible to test a diode with the oscilloscope visualizing its characteristic curve. The transformer can have currents starting from 100 mA.
The frequency of this circuit depends on C1 and, for the indicated values, it is 1 kHz. Equivalent...
This configuration with the LM383 appears elsewhere on this site. The circuit shown was obtained...
This circuit was obtained from HAM Radio magazine in an 80's issue. The FETs are types of the time...