Linearity Enhancements of Receiver Front-end Circuits for Wireless Communication

Sammanfattning: Technology scaling in advanced CMOS nodes has been very successful in reducing the cost and increasing the operating frequency, however, it has also resulted in reduced transistor intrinsic gain and increased thermal noise coefficient, and most importantly, deteriorated linearity performance. At the same time, advanced wireless communication standards offer ever increasing data rates and pose more stringent requirements on coexistence, leading to very stringent linearity requirements.The objective of this dissertation is therefore to investigate techniques for enhancing the linearity of the receiver front-end in CMOS technology. The bandwidth of two-stage operational transconductance amplifiers (OTAs) in closed loop configuration is addressed in paper I and a compensation technique is proposed using positive feedback RC links. Analysis shows that this introduces two left hand plane zeros and two high frequency parasitic poles. In paper II the compensated OTA is used in a fifth order active-RC channel select filter (CSF) for LTE-Rel 8. The filter ’s power consumption measures only 3.4mW, and the linearity at the band edge and out-of-band is not deteriorated.In paper III, the linearity of the well-known triode OTA with feedback amplifiers is investigated, and feed- forward linearization is proposed instead of using feedback amplifiers. Not only are the amplifiers removed along with their power consumption, but also state-of-the-art linearity is achieved.Paper IV proposes a novel linearization technique suitable for high frequency OTAs. The linearization draws no bias current and measurements show that it is robust to mismatch as well as temperature and voltage variations. A low noise amplifier is simulated and a fourth order OTA-C filter was measured, demonstrating the performance of the technique. Finally, in paper V a fully integrated receiver front-end with spectrum sensing is presented, including mea- surements with LTE signals. Different blocker scenarios were measured and it is concluded that spectrum sensing is very beneficial for blocker handling, resulting in significantly improved performance. Further- more, the effect of noise cancellation and improvements of the OTA linearity are demonstrated on the overall front-end performance.