This circuit has a sensitivity of 2uW. We found it in documentation from the 1980s. The integrated circuit is from that time. Equivalents can be tried.
This circuit is from an Electronics World of 1964. The circuit can be made with modern...
This circuit is from an old American documentation from the 70s. Transistor 2N3904 can be replaced by...
We found this circuit in the March 1980 Radio Electronics magazine. The circuit used is the HA2400...