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The figure-of-merit of an on-chip transformer is investigated by using a power gain approach in this paper. Adopting a microwave viewpoint, contrary to the lumped-circuit concept, to characterize the on-chip transformer, the transfer power gain between primary and secondary terminals is plotted in the Smith chart by changing the load impedances. Therefore, the peak value of power gain is found at a specific load impedance. To discuss the power loss of two transformers with different metal widths, the device layouts are carefully designed to maintain identical self- and mutual inductances in the transformers' coils. Results show that impedance matching plays a key role in improving the power gain of an on-chip transformer.