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The load-modulated balanced amplifier (LMBA if you are too lazy to type it out) is a recent adaptation of the time-tested balanced amplifier. In this new application, a signal is injected into the isolated port on the output combiner to load-pull the amplifiers to a happy place, where they will have improved efficiency at various levels of back-off, over potentially very wide bandwidths. Dare we say, for "electronic warfare?"
In a balanced amplifier, a 90-degree coupler is used to combine two unit amplifiers in quadrature. This coupler is a four-port device, quite often a Lange coupler, where two ports are used as inputs, a third port is where the signals combine, and a fourth port is isolated.
One of the inventors of the LMBA is Dr. Steve Cripps of Cardiff University in Wales. In the video below he provides some history of hybrid micro-circuits using alumina thin-film circuits, describes how load modulation can be used in a balanced amplifier, and why it out-performs other schemes like the Doherty combiner. The video is over an hour long, you might have to watch it a little at a time.
Dr. Steve Cripps explains the Load Modulated Balanced Amplifier
Here's a bit of advice to newer microwave engineers. the LMBA is not some "dumb" combiner, it takes an elaborate control system to operate. If you want your career to take off, be sure to pay attention to digital and digital/analog systems designs. Microwave engineering is an old technology and it isn't going anywhere so you have plenty of time to master it.
References
D. J. Shepphard, J. Powell and S. C. Cripps, "An Efficient Broadband Reconfigurable Power Amplifier Using Active Load Modulation," in IEEE Microwave and Wireless Components Letters, vol. 26, no. 6, pp. 443-445, June 2016.