Based on the LF256 operational amplifier, this circuit was found in the September 1989 issue of Radio Electronics magazine. The indicator is analog, and the power supply must be symmetrical.
This circuit was found in a 1987 edition of the United States' Hands-On Electronics magazine. The...
This amplifier circuit is from a 1952 documentation. The frequency of the amplifier signal depends on...
Found in Popular Electronics from the 1980s, this circuit can use more modern transistors such as...