We found this circuit in a Hands-On Electronics from the 90s. The circuit must be connected to an external amplifier. Frequency control is capacitive, using an antenna for this purpose
The figure below shows an AC transconductance amplifier with the National Semiconductor LM3900. The...
This common circuit using operational amplifiers was found in the 1993 issue of Radio Electronics Special...
This circuit provides a voltage at the output only when the input voltage exceeds a value that...