Geoffroy Soubercaze-Pun

Systematic Approach to the Stabilization of Multitransistor Circuits

This paper proposes a systematic approach for the elimination of spurious oscillations in circuits with multiple active elements. The procedure is based on detecting the sensitive parts (nodes or branches) of the complex circuit at which we can have a strong control of the dynamics responsible for the instability. To do so, stability analyses based on pole-zero identification are performed at multiple observation ports. Once sensitive locations are detected, stabilization is achieved by adding series or shunt stabilization networks at the suitable node or branch. Standard techniques from linear control theory (pole placement strategies) are used to automatically calculate the values of the stabilization elements ensuring circuit stability. Here, the methodology is applied to the stabilization of a two-stage Ku-band high-power amplifier for active antenna space applications.

Stability analysis of microwave circuits

It is common knowledge that stability analysis is a critical step of RF design flow. Nowadays powerful CAD tools allow accurate simulation of the performances of microwave and RF circuits. But the prediction of instabilities giving rise to frequency divisions or spurious oscillations is still a challenge, especially for MMIC designs whose adjustment after fabrication is impossible. This paper presents a new tool for stability analysis of microwave circuits, valid for small-signal and large-signal regimes. The technique used in this tool allows to detect and determine the nature of oscillations, such as parametric oscillations in power amplifiers that can be for example function of the input drive signal. Knowledge of the type of oscillation mode facilitates the insertion of stabilization networks, with a better balance between the required oscillation avoidance and maintaining the original circuit performances. The integration of this tool in the design flow with commercial CAD tools will be presented.

Sensitivity enhancement in pole-zero identification based stability analysis of microwave circuits

In this work the use of a dual Current-Voltage (I-V) probe is proposed in order to improve the sensitivity of pole-zero identification based stability analyses. The benefits of this approach are shown here by its application to a parametric oscillation in an X-band multi-stage MMIC power amplifier and to a bias oscillation in a medium power FET amplifier built in hybrid microstrip technology.

Monte-Carlo stability analysis of microwave amplifiers

Pole-zero identification is being increasingly used as a method to analyze the stability of microwave circuits. However, in its current form, the stability analysis through pole-zero identification relies on a quality assessment that is based on visual inspection. This implies a manual approach that limits in practice the handling of a large amount of data as in the case of a yield or sensitivity analysis. Here, an automated methodology based on pole-zero identification is applied to the stability evaluation in the context of a Monte-Carlo analysis. The methodology is based on an algorithm that prevents the adverse effects due to undermodeling and overmodeling on the stability criteria. The approach is illustrated through its application to two in-home built amplifier prototypes: a medium power band L FET amplifier exhibiting a low frequency oscillation and a SiGe HBT reconfigurable amplifier for Wifi/Wimax applications that shows an undesired frequency division for some particular input drive conditions.

Experimental Characterization of Stability Margins in Microwave Amplifiers

This paper proposes a method for the experimental estimation of the stability margins in microwave amplifiers. The approach is based on measuring a closed-loop frequency response representing the linearization of the circuit about a steady-state solution. Critical poles of the amplifier are then obtained by applying conventional pole-zero identification techniques to the measured frequency response. As circuit parameters are modified, the evolution of these critical poles on the complex plane provides a practical way to assess the robustness of the design regarding its stability. Two types of common instabilities in microwave amplifiers are studied: low-frequency bias oscillations and parametric oscillations. For the low-frequency oscillations, the approach proposes the inclusion of an observation RF port into the amplifier bias path to experimentally obtain the critical poles of the circuit from a reflection coefficient measurement. Pole-placement techniques are then applied to increase the stability margin of detected critical resonances. For the parametric oscillations, pole-zero identification is applied to a frequency response obtained from a mixer-like characterization equivalent to the measurement of a “hot” reflection coefficient. The methodology is applied to two amplifier prototypes: an L-band field-effect transistor amplifier and a dual-mode WiFi-WIMAX amplifier that exhibit different kinds of unstable behavior.

Increasing low-frequency stability margins in microwave amplifiers from experimental data

A key aspect in the robust design of microwave amplifiers is to ensure circuit stability beyond the nominal operating conditions. In this work, a method for measuring and controlling the stability margin of low-frequency resonances in microwave amplifiers is proposed. The approach adds an extra RF port in series with an R-C stabilization network that is connected to the amplifier bias path. The extra RF port is used to experimentally obtain the critical poles of the circuit. Then, from the obtained pole-zero map, pole-placement techniques are applied to get the R-C values that increase the stability margin of the critical resonances. In this way, the risk of running into a low frequency oscillation when amplifier conditions are varied can be significantly reduced. The complete approach is experimentally validated in a demonstrator prototype built in printed circuit board technology.

Joint RF and large-signal stability optimization of MMIC power combining amplifiers

In this paper, authors report on an enhanced approach for the design of monolithic microwave integrated circuit (MMIC) power combining amplifiers. Commonly used techniques for the stabilization of such circuits are empirical and too conservative. This leads very often to a non-desired degradation of the radio frequency (RF) performances that are inherent to the physical properties of such stabilization networks at the fundamental frequency of operation. The methodology proposed here is based on the use of large-signal optimization processes that combine RF and stability analyses from the early stages of the design. This approach results in an improvement of the RF performances while sufficient stability margins are preserved. The optimization procedure is explained on a Ku-band MMIC power amplifier for space-borne communications.

Stability analysis of multistage power amplifiers using Multiple-Input Multiple-Output identification

Determining the origin and nature of the possible instabilities is key for an effective elimination of the unstable dynamics in multistage power amplifier design. In this work, a novel technique is proposed to provide a quantitative metrics that serves to locate and categorize the sensitive sections of the amplifier at which the unstable dynamics can be controlled and eliminated. The technique is based on an automatic Multiple- Input Multiple-Output (MIMO) frequency identification performed at different observation ports, followed by a residue analysis of critical poles. A three-stage amplifier exhibiting two common types of instabilities has been used to illustrate the complete approach.

Continuous-time modeling of a Traveling-Wave Tube amplifier

This paper deals with the modeling of a Traveling-Wave Tube (TWT) amplifier. The continuous-time modeling approach is presented and the model is used to capture the carrier memory effect of the TWT amplifier. Some processing on measurements are described and an assessment of the model under 2-tone signals is achieved for validation.

Discontinuity at origin in Volterra and band-pass limited models

Discontinuities at origin have been used to better approximate measured curves in recent papers but generally not explicitly and their physical validity has not always been demonstrated. In this communication, we show that these discontinuities can be explained by physically acceptable discontinuities in the real physical device. We propose simple criteria to accept or reject these discontinuities, in either passive or active devices, depending on the order of the discontinuity. In addition, we show that models having such discontinuities behave differently from classical models. In particular, these discontinuities explain non-integer dB/dB slopes of harmonic power and intermodulation power as a function of input power. Recent and older measurements of intermodulation products in passive devices, telephony base-station and RF transistors show such a behavior so that supposed lack of measurement cannot be used as a reason to reject discontinuities as non-physical.