A Dual-Band CMOS Tunable Duplexer Employing a Switchable Autotransformer for Highly Integrated RF Front Ends

A dual-band CMOS duplexer employing a switchable autotransformer is proposed for highly integrated RF front ends in mobile phones. Because on-chip switches are used to change the length of the autotransformer, the inductance is sized for two different frequency ranges. It is designed to support both low-band (LB) mode from 0.7 to 0.9 GHz and mid-band (MB) mode from 1.7 to 2.0 GHz for cellular applications. With different inductances for LB and MB modes, transmitter (TX)-to-receiver (RX) isolation is achieved by using two separate balance networks for LB and MB. The duplexer is implemented in a 65 nm CMOS process. In LB mode, TX and RX insertion losses (ILs) are less than 4.3 dB, while TX-to-RX isolation is more than 50 dB. In MB mode, TX IL and RX IL are less than 4.5 dB, while TX-to-RX isolation is more than 50 dB. The proposed duplexer proves feasible structure to cover both LB and MB frequency ranges in cellular 3G/4G applications.