Abstract
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.
Highlights
INTRODUCTIONModern wireless communications have led to the significant advancement of microwave / millimeter-wave circuits
Over the past decades, modern wireless communications have led to the significant advancement of microwave / millimeter-wave circuits
We show that a differential amplifier can suffer from CM instability due to the gate inductance from the biasing line as well as the parasitic series coupling capacitance between the two coils of the input transformer
Summary
Modern wireless communications have led to the significant advancement of microwave / millimeter-wave circuits. Park: CM Stability Test and Design Guidelines for a Transformer-Based Push-Pull PA a specific level of RF input [7], and a small-signal analysis cannot detect these oscillations. Since the multi-branch structure uses power dividers and combiners between PA stages, it faces potential odd-mode instability that has been investigated carefully both in the small-signal and large-signal domains [5], [6], [10]. We present analytical expressions for evaluating the stability of a transformer-based push-pull PA through a smallsignal analysis in the complex-frequency (s = σ +jω) domain to establish useful guidelines. (a) A pseudo-differential amplifier, (b) a push-pull amplifier, and (c) the simulated oscillation of the drain voltage of a transistor in a push-pull structure after turning on the supply voltage at 1ns when no input is introduced. An experimental example of the instability in the push-pull structure is given in Section V, followed by conclusions on the study
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