This high impedance amplifier is from 1985 documentation by General Electronic. The amplifier with FET at the entrance is from the period and the source is 9 V.
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...