Combined control of drain video bandwidth and stability margins in power amplifiers for envelope tracking applications

Envelope tracking amplifiers require significant video bandwidths to accommodate the dynamic variations of the drain bias. However, the increase of drain video bandwidth in a power amplifier can raise the risk of low frequency oscillations when some circuit parameters are changed. In this work, an experimental methodology is presented to monitor the drain video bandwidth and the low frequency stability margin in combination. The methodology is based on adding observation ports to both drain and gate bias paths in order to measure and control the critical and dominant poles that govern the low frequency dynamics of the power amplifier. The complete approach is illustrated in a demonstrator prototype specifically built for that purpose.