Introducing positive envelope feedback – a new method for linearity improvement in radio frequency integrated circuit power amplifiers

Abstract

In the first part of this doctoral thesis, the author introduces Positive Envelope Feedback in Radio Frequency Integrated Circuit (RFIC) Power Amplifiers (PAs) - a new method for improving the PA’s linearity-efficiency trade-off and based on a signal flow directly from the PA output, through a single envelope detector, to its bias input. The proposed technique requires minimum additional circuit area, consumes negligible additional current, does not suffer from bandwidth limitations and does not degrade PA noise performances. The inherent simplicity of positive envelope feedback enables its incorporation into existing PA architectures with minimum re-design, as well as its use in conjunction with other circuit techniques aimed at PA performance enhancement. The theory, design and implementation of positive envelope feedback are described in detail while underscoring the various design conditions that must be taken into consideration to ensure PA performance improvement through the proposed technique. Its implementation is highlighted with the help of multiple PA designs, using results obtained through simulation as well as experiments on fabricated designs. Measurements on one experimental Complementary Metal-Oxide-Semiconductor (CMOS) Silicon-on-Insulator (SOI) PA design using positive envelope feedback show an increase of 1.7dB in the PA’s 1dB gain compression point (P1dB) and an output third-order intermodulation distortion (IMD3) improvement of up to ~3.44dB while requiring only an equivalent ~5% increase in chip area and 1.2% increase in quiescent current consumption. Simulations on a modified version of this PA design demonstrate an IMD3 improvement of up to 3.5dB and Adjacent Channel Power Ratio (ACPR) improvement of up to 6dB thanks to the use of positive envelope feedback with larger signal bandwidths, and at output power levels where the open-loop PA is under significant gain compression. In the second part of this thesis, the author develops the analytical foundations of a novel multi-port PA representation based on distinct sets of nonlinear complex polynomials that describe a combiner, a nonlinear baseband-to-RF converter and a nonlinear RF amplifying function, for the processing of the PA’s input modulated RF signal and any envelope-dependent dynamic biasing signal. The proposed representation is shown to allow an accurate prediction of the PA’s output distortion components as a function of an input RF multi-tone excitation and a multi-tone envelope-dependent biasing signal. This novel representation of a PA’s nonlinearities renders possible closed-form analytical formulations to describe a three-port PA system, and allows determining the adjustments necessary in the dynamic biasing signal and circuitry for PA linearity improvement under multi-tone as well as modulated excitation signals. It is intended for a context introduced for the first time in the author’s research work and envisioned as promising for current and future mobile communication equipment – the automatic optimization of linearity performance in RFIC PAs that employ envelope-dependent dynamic biasing, through embedded self-calibration functions implemented within the transmitter front-ends of mobile equipment. The proposed representation allows the optimization of the PA’s envelope-dependent dynamic biasing for linearity improvement from one mobile unit to another through embedded self-calibration, starting from quasi-static measurements alone of the PA’s input and output power, in contrast to more complex training sequences that are required for high-order Volterra-based and other PA representations. The applicability of the proposed representation is highlighted through simulation and benchmarking against experimental results, demonstrating accurate characterization of PA performances under different dynamic biasing techniques, for multiple RFIC PA platforms and in different semiconductor technologies. In one experimental implementation using an industry-designed Gallium Arsenide (GaAs) PA, it accurately predicts the necessary dynamic biasing adjustments to achieve more than 4dB reduction in the output IMD3. A similar reduction in Adjacent Channel Power (ACP) is demonstrated with a modulated signal. In a second experimental PA implementation, the proposed representation allows, for the first time, using an analytical approach for predicting the condition of system stability under closed-loop positive envelope feedback operation, as well as determining the optimum performance requirements for the feedback system components.

Date	13 Mar 2020
Original language	American English
Awarding Institution	École de technologie supérieure
Supervisor	Nicolas Constantin (Supervisor)