In-Circuit Characterization of Low-Frequency Stability Margins in Power Amplifiers

Low-frequency resonances with low stability margins affect video bandwidth characteristics of power amplifiers. In this paper, a nonconnectorized measurement technique is presented to obtain the low-frequency critical poles at internal nodes of a hybrid amplifier. The experimental setup uses a high-impedance probe connected to a vector network analyzer to obtain a fully calibrated closed-loop frequency response that is identified to get the poles of the device at low frequency. Compared to previous connectorized solutions, the approach avoids the ad hoc insertion of extra RF connectors to access the low-frequency dynamics of the amplifier. In addition, it simplifies the characterization at multiple internal nodes, which is worthwhile for an efficient detection and fixing of critical low-frequency dynamics in multistage power amplifiers. The technique is first applied to dc steady-state regimes and compared to the connectorized approach on a single-stage amplifier. Next, it is applied to a three-stage amplifier to show its potential to detect the origin of the undesired dynamics and the most effective way to increase stability margin. Finally, the technique has been extended to the large-signal case to increase its usefulness for the design and diagnosis of high-power amplifiers.