Power amplifiers (PAs) often exhibit instabilities giving rise to frequency divisions or spurious oscillations. The prediction of these instabilities requires a large-signal stability analysis of the circuit. In this paper, oscillations, hysteresis, and chaotic solutions, experimentally encountered in a high-efficiency class-E/F/sub odd/ PA with four transistors combined using a distributed active transformer, are studied through the use of stability and bifurcation analysis tools. The tools have enabled an in-depth comprehension of the different phenomena, which have been observed in simulation with good agreement with experimental results. The study of the mechanism generating the instability has led to a simplified equivalent circuit from which the optimum stabilization network has been determined. The network enables a global stabilization of the circuit for all the expected operating values of the amplifier bias voltage and input power. This has been achieved with negligible degradation of the amplifier performance in terms of drain efficiency and output power. The stable behavior obtained in simulation has been experimentally confirmed.

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Global stability analysis and stabilization of a class-E/F amplifier with a distributed active transformer

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.

/pdf/analysis-and-elimination-of-hysteresis-and-noisy-precursors-384ge705s5.pdf

Analysis and elimination of hysteresis and noisy precursors in power amplifiers

Multistage power amplifiers are prone to parametric oscillations of an odd-mode nature. In this article, a technique for detecting and avoiding odd-mode parametric oscillations in microwave power amplifiers under large-signal excitations is extended and improved. For that, two monolithic amplifiers, having different topology and different oscillation mode, are extensively analyzed. A previously analyzed X-band two-stage power amplifier is used as a first example. Then, the technique is applied to a novel L-band one-stage power amplifier exhibiting a different oscillation mode. The success in determining the oscillation nature of these two different examples and the strategies used for their stabilization clearly illustrate the potential of this technique to address the complex problem of parametric oscillations in microwave amplifiers. © 2005 Wiley Periodicals, Inc. Int J RF and Microwave CAE 15, 2005.

Detecting and avoiding odd‐mode parametric oscillations in microwave power amplifiers

Output power scaling based on class-J dynamic load modulation (DLM) theory is used to design an unprecedentedly high power microwave transmitter with varactor-based DLM functionality. Matching networks are realized on high dielectric constant substrates in order to reduce the form factor. The fully matched DLM PA incorporates a 24-mm GaN HEMT powerbar and a stack of four SiC varactors, all fit into a CuW package (40 mm × 20 mm). Peak output power is reconfigurable by changing the drain voltage, while retaining the DLMeffect. Under pulsed conditions at 40 V the PA delivers a peak power of 86 W at 2.14 GHz. Efficiency enhancement by DLM is 10-15 percentage-units at 6 dB output power back-off (OPBO). Employing digital predistortion (DPD) with a vector switched generalized memory polynomial (VS-GMP) the ACLR is -46 dBc at an average output power of 17 W and a drain efficiency of 34%. These results prove the potential for high output power levels in varactor-based DLM transmitters.

A packaged 86 W GaN transmitter with SiC varactor-based dynamic load modulation

In this paper, harmonic balance and envelope-transient tools are applied to the analysis and design of potentially unstable power amplifiers. The ranges of unstable behaviour are determined through bifurcation detection and the effects of odd-mode instability on the intermodulation distortion are investigated. The amplifier is stabilized using concepts from bifurcation control. Additionally, a feedback configuration for the amplifier stabilization is proposed and the results are compared with those obtained with previous methods, based on resistive loading. The techniques have been applied to a 4 GHz microwave amplifier.

Analysis and stabilization tools for microwave amplifiers

A novel nonlinear analysis of high power amplifier instability has been developed. This analysis method deals with a loop oscillation and presents the conditions for oscillation under large-signal operation by taking account of a mixing effect of FETs. Applying this analysis method to the high power amplifier instability that an output power decreases at some compression point where an f/sub 0//2-wave is observed, it has been found that this instability is caused by an f/sub 0//2 loop oscillation. In addition, it has been verified by analysis and measurement that the oscillation can be removed by employing an isolation resistor in a closed loop circuit. >

S. Tsuji

Papers

Nonlinear analysis of f/sub 0//2 loop oscillation of high power amplifiers