This circuit was found in documentation from the 90s. The operational amplifier is NE5514, which is uncommon today. The configuration can be adapted to use modern components.
This circuit is used in transceivers, enabling input selection by diode switching through polarity...
This configuration was found in an American publication from the 90s. Operators admit equivalents just...
This circuit was found in an Italian magazine Elettronica Prática of 1977. The transistors can be...