implementing as it does a complete simple op-amp with five transistors. Looking at the circuit diagram it follows the classic op-amp with a long-tailed pair of NPN transistors driving a PNP gain ...

Inside was a simple servo, with a differential amplifier ... and the germanium transistor AC amplifier was able to do a good job of working with the resulting 50 Hz square wave.

In order to develop the necessary technology, the project Magellan—High Efficiency mm-Wave GaN Transistor High Power ...