This circuit, from the 70's, can be assembled with a more modern FET like the BF245. The circuit is powered by a 9 V battery, as its consumption is very low.
In the figure we have a simple way to produce tone in a piezoelectric transducer. The frequency depends...
This absolute value amplifier was found in a 90's EDN. Equivalent amplifiers can be used. The...
The integrated circuit used in this configuration for noise reduction on FM radios is not common...