Steady trends in the personal portable communications market have demanded lower cost and better overall performance of the transceiver. Modern CMOS technologies, currently demonstrated to be feasible for implementing RF circuitry, offer the prospect of higher levels of integration, bringing low cost, smaller form factors and, with the elimination of many off-chip signal paths, the potential for reduced power consumption. At the system level, non-constant-envelope (non-CE) modulation schemes are attractive, as they offer better spectral efficiency than CE schemes, allowing higher data rates to be transmitted in a given bandwidth. The goal of this work is the design of an integrated CMOS radio transmitter for a non-CE modulation scheme.
Battery life is a major consideration in the design of portable radio units, and this is particularly so in the design of the RF power amplifier with its significant power consumption. At the same time, non-CE modulations require a linear transmit path for low distortion, and there exists a fundamental tradeoff between linearity and power efficiency, and CMOS device characteristics only make this tradeoff worse.
Cartesian feedback is a linearization architecture that can provide a low-distortion output from a nonlinear amplifier, offering the potential for integration without needing off-chip delay lines or couplers. This architecture requires a power amplifier whose output envelope can be modulated by varying the input envelope. Though Class-C power amplifiers allow this modulation, while offering good power efficiency, they also have bad AM/PM distortion, which can introduce instability with Cartesian feedback.
We are investigating a modification to the normal class-C amplifier architecture to reduce the severity of AM/PM distortion. Simulated peak drain efficiencies for a 0.18 Ám CMOS prototype amplifier are on the order of 55%. The larger goal of this work is to implement this modification together with the other elements of the Cartesian feedback architecture in an integrated transmitter targeting GSM EDGE specifications.