CMOS active transformers and their applications in voltage-controlled quadrature oscillators

This paper introduces CMOS active transformers synthesized using MOS transistors only. The proposed active transformers are evolved from their active inductor counterparts and offer a number of attractive characteristics as compared with spiral transformers including a tunable coupling ratio, large and tunable self and mutual inductances, high and tunable quality factors, and a small silicon area consumption. The characteristics of the proposed active transformers including the tunability of the self and mutual inductances, power sensitivity, noise, and quality factor are investigated both analytically and numerically using simulation. The application of the proposed active transformers is exemplified using a 1.6 GHz quadrature VCO implemented in TSMC-0.18 μm 1.8 V CMOS technology and analyzed using SpectreRF from Cadence Design Systems with BSIM3v3 device models. The total transistor area and power consumption of the VCO are 107 μm² and 10.8 mW, respectively. The phase noise of the VCO is -114.6 dBc/Hz at 1 MHz frequency offset.