This circuit takes advantage of the noise generated at the reverse biased junction of a germanium diode. Other types of diodes can be used. The signal is weak.
This circuit provides a symmetrical voltage of 15 V output from the 6.3 V secondary of a small...
We found this circuit in an American 1980s documentation. The operational ones used are not common...
This transistor source can supply currents up to 2 A and the voltage determined by the zener...