Novel Miniature Dual-Band Rat-Race Coupler with Arbitrary Power Division Ratios Using Differential Bridged-T Coils

In this article, a novel miniature dual-band rat-race coupler design is proposed. The proposed dual-band rat-race coupler employs differential bridged-T coils (DBTCs) as building blocks such that a very compact circuit size and a dual-band response can be achieved at the same time. Especially, through the proposed dual-band design of DBTC, arbitrary power division ratios over the two operating bands can also be realized. Explicit design equations of the proposed dual-band rat-race coupler are presented, and practical limitations on the achievable frequency ratio, power division ratio, and bandwidth are thoroughly investigated. As design examples, two 2.45-/5.5-GHz dual-band rat-race couplers with 0-/0- and 3-/0-dB power division ratios for the two bands are implemented in a silicon-based integrated passive device process. Very compact circuit sizes of less than 4.4 mm × \,\, 3.1 mm as well as good measured performance are achieved. To the best of our knowledge, these are the smallest dual-band rat-race couplers ever reported.