Periodic Loop Structure for Combining Power FETs
Power dividers and combiners are important components in combined power amplifiers. For ease of integration with transistors (or amplifier modules) on a planar circuit, the output ports of the divider and the input ports of the combiner should be aligned in a straight line. When fork or radial power combiners are used, irregular meandering of interconnecting transmission lines is required . Microstrip corporate power combiners  and suitably designed tapered microstrip combiners , on the other hand, have input ports in a straight line. Most power dividers and combiners are meant to be terminated at all ports with matched loads. When integrating with transistors, matching is required, and this adds to the circuit complexity and size. In the interests of improving integration of transistors and power combiners and dividers, a number of approaches have been developed such as the dual-fed distributed amplifier  and the distributed active transformer . The power combiner needs to transform an external load (typically 50 Ω) to a suitable load impedance for the transistors which is low for power FETs. The load-impedance presented to each FET of the dual-fed distributed amplifier is proportional to the number of FETs, N, , and inversely proportional to N for the distributed active transformer . The distributed active transformer therefore permits combining of microwave power FETs without needing additional impedance transformers. The distributed active transformer requires tightly coupled lines and implementation requires a multi-metal-layer technology. In this work we propose an N-way circuit level power combiner for FETs that achieves a 1/N impedance transformation ratio and can be implemented in a single layer microstrip technology.