High Linearity Transmit Power Mixers Using Baseband Current Feedback

Transmit mixers frequently use Gilbert cells driven by common-source baseband amplifiers. Class-AB biasing is used in the latter for high efficiency, but this causes poor linearity. In this article, linearity is enhanced by using negative feedback to directly sense and linearize the baseband current flowing into the switching pair of the mixer. By suppressing both gate- and drain-induced nonlinearities, it achieves higher linearity and permits a higher power output compared to using replica devices of the common-source amplifiers in negative feedback. The technique is demonstrated using two prototypes: a 130 nm prototype designed for 1.9 GHz long term evolution (LTE) signals and a 65 nm LP prototype designed for Bluetooth basic data rate (BDR) and enhanced data rate (EDR) modes. The 130 nm prototype has an adjacent channel leakage ratio (ACLR) of -41.3/-40.4/-37.6 dBc for LTE5/10/20 at 11.7 dBm output, -148.6 dBc/Hz noise at 80 MHz offset for 10 dBm output, and -53.8 dBc CIM3 for 9 dBm output power. It consumes 335 mW from 3 V and 1.3 V supplies, including 79 mW in the LO buffers. The 4.4% efficiency represents 70% improvement over the state-of-the-art. The 65 nm prototype provides 4 dBm output power (EDR) while consuming 50.5 mW from 1.8 V.