In this study, we propose a broad-side coupled transformer with reduced capacitance for RF CMOS power amplifier applications. The width of the secondary winding is decreased to reduce parasitic coupling capacitance. Additionally, an auxiliary primary winding is added to improve the coupling between the primary and secondary windings. To prove feasibility of the proposed transformer, we design the transformer using 180-nm RF CMOS technology. From the simulated results of a typical transformer and the proposed broad-side coupled transformer, we successfully find that the parasitic coupling capacitance of the proposed structure is reduced compared to that of a typical structure. Additionally, the auxiliary primary winding increases the maximum available gain of the proposed transformer.
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