Pedro M. Cabral

The Linearity-Efficiency Compromise

This article attempts to improve the average efficiency and linearize the work-horse of RF power amplification- the class-B (or class AB) PA-relying, for example, on predistortion, dynamic load modulation or ET. Nevertheless, whenever higher efficiencies are desired, the current-mode class-B PA must be replaced by switchedmode PAs (for example, class-E or class-F), completely modifying the paradigm of RF amplification. In fact, as these are saturated circuits, they cannot respond to excitations of variable amplitude; and thus the original signal envelope must be restored by operating the PA as an AM modulator via dynamic supply adaptation. A good example of this is the EER arrangement.

A Metric for the Quantification of Memory Effects in Power Amplifiers

This paper presents a quantitative metric for memory effects in power amplifiers (PAs) and applies it to various active device technologies and wireless system contexts. The proposed metric is mathematically founded in the dynamic two-tone distortion response, has a clear physical meaning in the important field of PA linearization and can be easily evaluated from either harmonic balance simulations or measurement data gathered in a microwave laboratory. In addition, a memoryless PA linearizer, optimum for reducing the integrated intermodulation distortion (IMD) power in the operation bandwidth for a two-tone excitation, is derived, providing a rigorous figure-of-merit of PA linearizability under static IMD compensation. The application of this figure-of-merit is then illustrated for three different PA prototypes based on Si LDMOS, InGaP/GaAs heterojunction bipolar transistors, and GaN high electron-mobility transistors, designed for 900-MHz (global system for mobile communications), 2.1-GHz (wideband code division multiple access), and 3.5-GHz (WiMax) wireless systems, respectively

A Multiple Time-Scale Power Amplifier Behavioral Model for Linearity and Efficiency Calculations

This work describes a new power amplifier, PA, behavioral model intended to predict both the PA amplitude and phase input-output signal relationship and the average or instantaneous dc power consumption and thus average or instantaneous signal-dependent efficiency. Hence, contrary to other previously published PA behavioral models, such a model is not only capable of describing the complex low-pass equivalent fundamental zone output, but also the real low-pass current consumption output. In addition, a special attention is paid to accurately account for the multiple time-scale dynamics shown by modern GaN HEMT based high power amplifiers.

New method for phase characterization of nonlinear distortion products

This paper presents a new method for phase characterization of nonlinear mixing products based on statistical averages of signal windows captured by a synchronous two-channel sampler. Passing some of the signal processing operations, previously performed by complex laboratorial arrangements, to the digital domain, the present method simplifies the measurement bench and allows absolute phase measurements, i.e., not depending on any arbitrarily selected phase reference nonlinearity.

Design of a Power-Amplifier Feed-Forward RF Model With Physical Knowledge Considerations

In this paper, the design of a general feed-forward topology for modeling RF power amplifiers (PAs) is presented. It consists of producing the best nonlinear nonrecursive approximator of a feedback model which traduces the physical characteristics of general PAs. Following the procedure presented in this paper, the feed-forward structure, which has a polynomial form with memory (Volterra series), can be exactly determined up to the desired order of approximation. Evidently, increasing the order of the feed-forward topology results in a more complex structure, with more laborious model extraction procedures. This paper presents in detail the structures of the general nonrecursive model up to the fifth-order kernel. A systematic extraction procedure, based on standard RF laboratory continuous wave and two-tone tests, is proposed under the assumption that the PA linear output does not vary within the considered fundamental frequency band (flat behavior). Model validation is demonstrated by means of ADS simulation data and from real data measurements.

A Simple Method to Estimate the Output Power and Efficiency Load–Pull Contours of Class-B Power Amplifiers

This paper describes a simple method to estimate the output power and efficiency load-pull contours of a class-B power amplifier (PA). Using successively improved-but still simple and intuitive-field-effect transistor nonlinear iDS (vGS,vDS) current models, this work starts by explaining the poor efficiency predictions provided by the Cripps load-line method, to then derive refined expressions to predict the output power load-pull contours and a novel analysis that produces useful estimates of the efficiency load-pull contours. These estimates are believed to constitute an invaluable tool for the analysis and design of single-ended class-B or Doherty PAs.

AM/PM distortion in GaN Doherty power amplifiers

The carrier power amplifier (PA) of a Doherty architecture is designed with a load that maximizes efficiency for a particular load-pull ratio. With the increase of the input drive, the peaking PA provides the necessary current to reduce the carrier PA load impedance. From this point on, the carrier PA operates at a constant output voltage, suffering a severe voltage gain compression at its terminals. Since the input Miller reflected Cgd depends on this gain, the devices total input capacitance changes substantially with drive level, inducing a significant input phase shift. Hence, in this paper we address the impact of this capacitance variation on the AM/PM distortion characteristic of a GaN HEMT based Doherty PA.

A physical model of power amplifiers AM/AM and AM/PM distortions and their internal relationship

Signal-dependent phase variation, AM/PM, along with amplitude variation, AM/AM, are known to determine nonlinear distortion characteristics of current-mode PAs. However, these distortion effects have been treated separately, putting more weight on the amplitude distortion, while the AM/PM generation mechanisms are yet to be fully understood. Hence, the aim of this work is to present a large-signal physical model that can describe both the AM/AM and AM/PM PA nonlinear distortion characteristics and their internal relationship.

Speed requirements of supply modulators used in polar architectures

This paper investigates the minimum supply modulator linear processing bandwidth and slew rate (SR) requirements necessary to meet the standard specs of total signal distortion and adjacent signal power ratio in modern polar wireless transmitters. System level simulations, performed under CDMA2000FW and CDMA2000RV excitations, showed that, depending on the filter behavior in the amplitude envelope spectrum main lobe, relaxed bandwidths between 2 and 5 times the original signal are sufficient to guarantee desired specs of 45dB of ACPR and −35dB of NMSE. On what the SR is concerned, we concluded that 1/(2.5) times the maximum AM(t) signal slope is enough to achieve the same desired specs.

A linearized polar transmitter for wireless applications

This paper reports on a new design method to overcome the nonlinear distortion associated to polar transmitters. The proposed methodology complements the modulation of drain bias, vDD(t), of conventional envelope elimination and restoration power amplifiers with a proper variation of the input drive level, vg(t). Allowing vg(t) to vanish when vDD(t) goes to zero, it obviates the inherent nonlinear distortion caused by the amplifier carrier feed-through. In addition, and beyond guaranteeing linear operation, such a design methodology is also beneficial for power added efficiency as it prevents the dissipation of instantaneous input power for a desired null instantaneous output power.