I. Introduction
Nonlinear RF and microwave circuits are prone to exhibit unstable behavior that can be revealed as autonomous oscillations [1]–[4] that are the function of the input power. This is the case of power amplifiers, which typically operate at substantial compression levels to reach acceptable efficiency values, or analog frequency dividers that need a large-signal input drive to generate a parametric oscillation. In some cases, input power may lead the nonlinear system to a bifurcation point, following which a qualitative change in its behavior is observed. Indeed, the presence of critical resonances that shift with input power is one of the main risks for the stable and robust operation of microwave power amplifiers. These resonances, although stable, are characterized by the presence of complex-conjugate poles with very low damping (small real part) and they affect circuit performances [5], [6]. Small stability margins make power amplifiers very sensitive against changes (bias, temperature, load impedance, aging, etc.). Consequently, a power amplifier can easily end up showing a spurious oscillation if its operating conditions are modified [7]. In addition, poles with low-damping factors can generate other problems in power amplifiers. When these resonances occur at low frequencies, they are responsible for the appearance of transient responses in pulses, affecting the pulse profile in radar applications [8]. These low-frequency resonances have also a direct impact on the amplifiers’ video bandwidth, limiting the effectiveness of digital predistortion systems [9], [10].