A directional coupler is provided on the output side of a power amplifier. The cable retainer implementations can be useful for supporting, ordering, and otherwise managing cables coupled to multiport equipment found installed in communication racks or in stand-alone applications. Preparation of a mono-substituted 3-bromomethyl-3-methyloxetane premonomer via a simple, high yield process amenable to commercial scaleup is also disclosed. A common-mode feedback circuit generates a feedback signal to the first Class AB amplifier circuit based on the output signal.