Common-Mode Stability Test and Design Guidelines for a Transformer-Based Push-Pull Power Amplifier

We report design guidelines for a transformer-based push-pull power amplifier (PA) with a systematic stability test carried out in the complex frequency domain. By detecting the right half-plane zeros of the impedance matrix's determinant, the common-mode (CM) instability in a differential PA were accurately estimated with the established frequencies from the analysis. In the stability test, the parasitic series coupling capacitance of the input transformer and the parasitic inductance from the gate (base) biasing line were identified as the two primary mechanisms of the CM instability in a transformer-based PA contingent on stability. Based on the analysis of the instability mechanisms, useful stabilization methods were proposed and discussed for a robust push-pull PA design with a well-balanced performance. An onboard transformer-based push-pull PA was implemented for the verification purpose, and the effects of parasitic inductance on the stability had been investigated. The simulated and measured results corresponded well with the proposed analysis.