These circuits were found in Exar documentation from the 90s. The circuits use both a small speaker and a piezoelectric transducer. The integrated circuit is not common today.
This circuit can serve as a basis for a digital mixer or simply for a noiseless switch of audio...
This traditional mixer configuration with a high input impedance was found in a 90’s Popular...
This medium wave radio works with any general purpose NPN transistor. The diode must be made of...