RF Amplifier Design Techniques for Linearity and Dynamic Range

Abstract

This thesis presents circuit- and device-level linearization techniques to reduce the intermodulation distortion as well as the DC power consumption in RF amplifier circuits. As a key method to accomplish the research goals presented in Chapter 1, a so-called orthogonal design approach is introduced that allows for an uncompromised optimization of the various design parameters, such as noise, impedance matching, gain, linearity, and efficiency. For low noise amplifiers (LNAs) this yields a reduction in the DC power consumption and a significant improvement in the so-called spurious-free dynamic range, which is a measure for the highest and lowest signal levels the amplifier can handle without seriously degrading the signal quality. For class-AB operated power amplifiers (PAs), higher linearity can be achieved up to the 1 dB compression point. To clarify the terminology used in this thesis, Chapter 2 introduces the fundamentals of nonlinear distortion and the dominant nonlinearities in bipolar and MOSFET devices. The main focus is on bipolar devices and the interaction with their surrounding circuitry, since these devices are the subject of most of the research presented in this thesis. Chapter 3 reviews some existing linearization techniques that are mostly used in traditional RF circuit design: current mode (translinear design), negative feedback, and transfer function shaping (multi-tanh design and Derivative Superposition). However, these techniques are not able to meet all desired specifications simultaneously in a particular application (LNA, Mixer, and PA). To alleviate the limitations of the above-mentioned techniques, Chapter 4 introduces input out-of-band tuning for bipolar-based amplifier designs. This technique provides an orthogonal way of implementing linearization into the design, since it relies on proper tuning of even-harmonic products at the difference and double frequencies instead of at the fundamental frequency. Consequently, more design freedom is created for a better trade-off between gain, noise, linearity, and most importantly, the DC power consumption (or efficiency). The out-of-band linearization technique becomes ‘truly’ orthogonal when implemented in a differential design, since we can distinguish between differential-mode (fundamental, and all odd-order components) and common-mode (all even-order components) signals. This allows for broadband linearization for low- and high-power amplifier designs and low DC power consumption, as theoretically and experimentally verified in Chapter 4. As mentioned before, out-of-band tuning in itself already gives more freedom in the design of RF amplifiers. This additional degree of freedom can be used to combine existing techniques, such as negative feedback, with out-of-band tuning. Chapter 5 introduces a bipolar-based Dual-Loop Feedback LNA (DLF LNA) with input out-ofband tuning and a unilateralization technique for high isolation and high gain at low DC current levels. This makes the DLF LNA a promising alternative for the cascode LNA configuration. Chapter 6 gives an example of an integrated differential Class-AB power amplifier in which the common-mode out-of-band products are properly terminated at the center tap of the input transformer balun. Moreover, it uses bridge neutralization of the collector-base capacitance to increase the gain, efficiency and isolation of the differential CE-stage, making it an interesting candidate for highly-linear balanced driver or PA designs. Chapter 7 introduces an LDMOS-based Class-AB power amplifier, which uses Derivative Superposition (DS) to tailor the FET characteristic in such a way that it only creates even and no odd-order frequency components. DS relies on the typical phase reversal in the third-order nonlinearity of the ID(VGS) characteristic, which is in principle not present in bipolar devices. Because distortion is directly combated in the device characteristic itself we can simultaneously optimize output power and efficiency by means of traditional harmonic load-matching techniques. Chapter 8 presents the conclusions and recommendations of this research. The most important conclusion is that the introduction of out-of-band tuning in the design allows optimization of linearity without compromising the in-band gain, noise, and efficiency performance. This orthogonal design approach provides more design freedom and has introduced a new method for the RF community to further reduce the DC power consumption in battery-operated handsets for wireless communication.