This very simple amplifier was found in the Philips Databook of 1990. The circuit is suitable for headphones and other battery powered applications.
This TTL circuit for digital signals up to a few dozen megahertz was found in old documentation, but...
This circuit with an operational amplifier was found in the magazine Radio Electronics of...
This detector is from the 1990 Motorola Dataar Databook, but can be adapted to work with more...