Found in a 1992 book, this circuit is intended for operation with audio signals. Equivalent operational amplifiers can be used. The optical coupler is an unusual type today.
This 6-transistor transistor radio was manufactured in Japan in the 1960s. It is a portable...
This signal follower, from a publication from the 1970s, uses old germanium transistors such as...
Found in Radio Electronics from the 90s, this circuit can vary the frequency in small values...