This circuit was obtained in a 1975 documentation. The maximum frequency depends on the diodes and the FET. Type 1N34 diodes and a FET BF245 can be used for frequencies up to 100 MHz.
This circuit was obtained from a Radio Electronics from October 1963. The 4 transistors must be...
This shield circuit is from an American documentation from 1969. The components can be modified...
This circuit for very low frequencies was found in an American documentation from the 90's....