Abstract
This paper presents a stability analysis of microwave power amplifiers (PAs) driven by binary pulse trains, as in the case of class-S PAs. First, using a simplified digital PA test bench in class-D configuration, different qualitative behaviors are obtained when varying the pulsewidth, including subharmonic and incommensurable oscillations. The mechanisms affecting the stability properties are studied with a harmonic balance-based formulation, by means of pole-zero identification and bifurcation detection. A sufficiently high number of harmonic components must be considered, together with the Krylov decomposition for an efficient computation of the inverse of the Jacobian matrix. It is demonstrated that, when varying the pulsewidth, the distinct pairs of complex-conjugate poles may shift to the right-hand side of the complex plane and, therefore, lead to different kinds of unstable behavior. This phenomenon is related to the dependence of the critical poles on the average value of the input signal. Boundaries of the various types of unstable behavior are traced in the plane defined by the pulse repetition rate and pulsewidth, using bifurcation detection techniques. All the predicted phenomena have been confirmed experimentally. In a second step, the algorithms derived from the simple class-D circuit are transferred to study the stability of a more complex tri-band class-S amplifier. It has been analyzed versus the input bitrate, obtaining a fully stable behavior that has been validated experimentally.
Highlights
The growing demands towards wideband, multi-band and multi-standard features go along with the need for highly flexible, highly efficient and cost-effective nextgeneration wireless communication architectures [1]
We present for the first time a complete stability analysis of a digital microwave power amplifier, which is important for further research on digital transmitter architectures in future wireless communication networks
Frequency-domain techniques have been applied to investigate the impact of the input signal on the circuit natural frequencies
Summary
The growing demands towards wideband, multi-band and multi-standard features go along with the need for highly flexible, highly efficient and cost-effective nextgeneration wireless communication architectures [1]. A very simplified proof-of-concept digital PA circuit of class-D type is studied and realized, in order to analyze the influence of varying input pulses on the stability This very basic circuit for a single frequency band helps understand the typical instability phenomena in digital PA circuits. A sufficiently high number of harmonic terms must be considered, using Krylov decomposition [18]-[19] for an efficient computation of the inverse of the HB Jacobian matrix The advantage of this frequency-domain analysis is its efficient combination with continuation methods to follow the evolution of the stability properties versus relevant parameters, such as the pulse amplitude, width and repetition rate.
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