Theory of Hall effect isolators for tunnel diode amplifiers

Hall Isolators are four-terminal semiconductor devices similar to Hall generators that permit unidirectional signal transmission, like Ferrite Isolators. Compared to the latter they are inherently broad-banded, including d-c operation, but also lossy. The minimum theoretical loss depends on the product of the carrier mobility and the available magnetic field. A design is given that actually achieves this minimum loss, usually below 10 db, for a given product. The impedance of Hall isolators can easily be made to match that of tunnel diodes. By combining the two devices the diodes can be operated closely to critical matching, producing amplification large compared to the insertion loss of the isolator and resulting in substantial overall unidirectional gain. Because of the excellent isolation several stages can be cascaded. The amplifiers are frequency limited primarily by the tunnel diodes and by the skin effect in the isolator, but both limitations occur well inside the microwave region.