Bifurcation analysis of stabilization circuits in an L-band LDMOS 60-W power amplifier

n this letter, the global stability analysis of an L-band push-pull power amplifier is presented. The analysis is carried out for the amplifier operating in different modes, such as Class AB, Class B, and Class E/F, considering variations in the bias voltages, the input power and the input frequency. After determination of the oscillation mode, three different stabilization techniques are applied and compared: feedback resistors, neutralization capacitors, and odd-mode stabilization resistor. The element values of each stabilization network, ensuring a stable behavior for all the operating conditions, are calculated with a bifurcation-analysis technique. Good agreement is found between measured and simulated results

Nonlinear Design Technique for High-Power Switching-Mode Oscillators

A simple nonlinear technique for the design of high-efficiency and high-power switching-mode oscillators is presented. It combines existing quasi-nonlinear methods and the use of an auxiliary generator (AG) in harmonic balance. The AG enables the oscillator optimization to achieve high output power and dc-to-RF conversion efficiency without affecting the oscillation frequency. It also imposes a sufficient drive on the transistor to enable the switching-mode operation with high efficiency. Using this AG, constant-power and constant-efficiency contour plots are traced in order to determine the optimum element values. The oscillation startup condition and the steady-state stability are analyzed with the pole-zero identification technique. The influence of the gate bias on the output power, efficiency, and stability is also investigated. A class-E oscillator is demonstrated using the proposed technique. The oscillator exhibits 75 W with 67% efficiency at 410 MHz

Analysis and elimination of hysteresis and noisy precursors in power amplifiers

Power amplifiers (PAs) often exhibit instabilities leading to frequency division by two or oscillations at incommensurate frequencies. This undesired behavior can be detected through a large-signal stability analysis of the solution. However, other commonly observed phenomena are still difficult to predict and eliminate. In this paper, the anomalous behavior observed in a Class-E PA is analyzed in detail. It involves hysteresis in the power-transfer curve, oscillation, and noisy precursors. The precursors are pronounced bumps in the power spectrum due to noise amplification under a small stability margin. The correction of the amplifier performance has required the development of a new technique for the elimination of the hysteresis. Instead of a trial-and-error procedure, this technique, of general application to circuit design, makes use of bifurcation concepts to suppress the hysteresis phenomenon through a single simulation on harmonic-balance software. Another objective has been the investigation of the circuit characteristics that make the noisy precursors observable in practical circuits and a technique has been derived for their elimination from the amplifier output spectrum. All the different techniques have been experimentally validated

Efficient analysis methodologies for emerging oscillator configurations

Oscillators enable a low-cost and compact implementation of radio-frequency identification (RFID) readers and radar systems, among other. The function integration comes at the cost of a more complex performance and a demanding analysis since the circuit must respond to several specifications, while maintaining the oscillation condition. Here a review of recently introduced semi-analytical formulations, intended for these oscillator-based circuits, is presented. They are based on the use of a numerical model of the standalone oscillator, extracted from harmonic-balance or envelope-transient simulations, which is introduced in an analytical description of the oscillator interaction with the external elements. Various types of formulations, applied to self-injection locked radar, RFID readers and super-regenerative oscillators, will be described and validated.Author is grateful to S. Sancho, F. Ramírez and M. Pontón, from U. of Cantabria, R. Melville from Emecom, and S. Hernandez from AMCAD Engineering. Work supported by project TEC2017-88242-C3-1-R (AEI,FEDER/UE)

Stability and phase-noise analysis

This contribution emphasizes the relevance of Prof.Vittorio Rizzoli’s works on stability and phase-noise analysis and describes how they have impacted more recent investigations. Regarding the stability analysis, in 1985 he developed a frequencydomain formulation that provided unvaluable insight into the way how the perturbed system should be described and analyzed. This formulation enabled the application, for the first time, of the Nyquist criterion to circuits simulated with harmonic balance (HB). In 1994, he derived a HB formulation for phase-noise analysis, which considered both the frequency modulation, associated with the timing noise, and the frequency conversion effects; it provided a complete prediction of the noisy oscillator spectrum at small and large offset frequencies from the carrier. Departing from these relevant contributions, more recent advances in the two topics will be described.Work supported by the Spanish Ministry of Science and Innovation (MCIN/ AEI / 10.13039/501100011033) under grant PID2020-116569RB-C31

Nonlinear microwave simulation techniques

The design of high performance circuits with short manufacturing cycles and low cost demands reliable analysis tools, capable to accurately predict the circuit behaviour prior to manufacturing. In the case of nonlinear circuits, the user must be aware of the possible coexistence of different steady-state solutions for the same element values and the fact that steady-state methods, such as harmonic balance, may converge to unstable solutions that will not be observed experimentally. In this contribution, the main numerical iterative methods for nonlinear analysis, including time-domain integrations, shooting, harmonic balance and envelope transient, are briefly presented and compared. The steady-state methods must be complemented with a stability steady-state analysis to verify the physical existence of the solution. This stability analysis can also be combined with the use of auxiliary generators to simulate the circuit self-oscillation and predict qualitative changes in the solution under the continuous variation of a parameter. The methods will be applied to timely circuit examples that are demanding from the nonlinear analysis point of view.This work has been supported by the Spanish Government under contract TEC2014-60283-C3-1-R and the Parliament of Cantabria (12.JP02.64069)

Prediction of odd-mode instabilities under output mismatch effects

A methodology is presented to predict odd-mode instability in power amplifiers under output mismatch effects, as in the case of amplifiers connected to an antenna. This kind of instability is often observed in power-combining configurations, due to their symmetry properties. Unlike the single-ended situation, there is a cancellation of odd multiples of the oscillation frequency at the circuit output, so there is no impact of the load impedance values at the sideband frequencies. The odd-mode instability depends on the terminations at the fundamental frequency and its harmonic terms, and can only be detected within the circuit unstable loop, instead of the output plane. Here a methodology for the prediction and suppression of odd-mode instabilities is presented. Low-pass filtering effects and the use of a shorted stub allow the stability analysis to be limited to the fundamental-frequency termination. Then, the stability boundaries are efficiently determined through bifurcation detection inside the unstable loop, using the magnitude and phase of the reflection coefficient as the analysis parameters. Results have been validated through pole-zero identification and experimental measurements.This work has been funded by the Spanish Government under contract TEC2014-60283-C3-1-R, the European Regional Development Fund (ERDF/FEDER) and the Parliament of Cantabria (12.JP02.64069)

Global stability analysis and stabilization of power amplifier

Power amplifiers often exhibit undesired behavior from certain input power value which cannot be predicted with a small-signal stability analysis. Among the commonly observed undesired phenomena are spurious oscillations, frequency divisions, hysteresis or chaos. In this contribution, simulation tools are presented, enabling an in-depth study of the origin and characteristics of these phenomena. The developed global stability analysis and stabilization tools have allowed an efficient suppression of the undesired behavior in a switching-mode power amplifier, with minimum degradation of the amplifier performance, in terms of drain efficiency and output power

Systematic methodology for the global stability analysis of nonlinear circuits

A new methodology for the detection of Hopf, flip, and turning-point bifurcations in nonlinear circuits analyzed with harmonic balance (HB) is presented. It enables a systematic determination of bifurcation loci in terms of relevant parameters, such as input power, input frequency, and bias voltages, for instance. It does not rely on the use of continuation techniques and is able to globally provide the entire loci, often containing multivalued sections and/or disconnected curves, in a single simulation. The calculation of Hopf and flip bifurcations is based on the extraction of a small-signal admittance/impedance function from HB and the calculation of its zeros through a geometrical procedure. The method is ideally suited for the investigation of the global stability properties of power amplifiers and other nonlinear circuits. The turning-point locus, associated with either jump phenomena or synchronization, is obtained by taking into account the annihilation/generation of steady-state solutions that is inherent to this type of bifurcation. A technique is also presented for the exhaustive calculation of oscillation modes in multidevice oscillators and oscillators loaded with multiresonance networks. The new methodologies are illustrated through their application to a power amplifier and a multimode oscillator.This work was supported by the Spanish Ministry of Economy and Competitiveness and the European Regional Development Fund (ERDF/FEDER) under research projects TEC2014-60283-C3-1-R and TEC2017-88242-C3-1-R