Couplers are four-port passive components that enable a defined amount of RF power in a transmission line to be coupled to another line. There are four common coupler types: hybrid, directional, dual directional, and impedance transforming. Directional couplers are unidirectional and are commonly used to sample high-power transmission lines. Hybrid couplers evenly split RF input signals with a 90-degree phase shift between output ports. In practice, hybrid couplers can serve as two-way splitters or two-way combiners. Dual directional couplers are specially designed with forward and reverse coupling paths. Finally, the impedance transforming coupler is a specialized hybrid coupler with input and isolation ports at 50 ohms and the –3dB ports at an impedance other than 50 ohms, such as 25 ohms. Such couplers can be especially useful combining wideband GaN HEMT transistors.