This circuit is from American documentation from the 70s. The circuit makes use of the operating system of the time, but it can be implemented with more modern equivalents.
This circuit was found in a 1992 publication, using an operational amplifier not very common...
This is a amplifier for XTAL high impedance earphones or piezoelectric types. Current drain is...
This circuit is from the LM111 manual of the 1980s. The component no longer exists, but...