Abstract
Potential instability of power amplifiers (PAs) under mismatch effects is analyzed, with emphasis on the impact of the PA termination at the fundamental frequency. A large-signal version of the μ factor is derived, considering the lower and upper sideband of the fundamental frequency as virtual observation ports. This requires the application of the conversion matrix approach to each steady-state solution obtained with harmonic balance when varying the fundamental termination. To evaluate the impact of the fundamental output load, a contour technique is used to assemble loads giving the same μ value. A stabilization procedure is provided, based on the mapping of the mismatched load onto the stable region of the fundamental termination Smith chart. The analysis is extended to consider the baseband frequency, using three virtual observation ports.
Highlights
Power amplifiers (PAs) that are stable under ideal 50 terminations may become unstable under antenna mismatch conditions in realistic wireless environments where the antenna input impedance is affected by objects in its vicinity [1]
The application imposes circuit stability for any possible load condition, including highly reflective loads [2]. These widely reported problems constitute a serious concern for PA manufacturers and several simulation procedures have been proposed for their prediction at the design stage
The impact of the fundamental frequency termination on power amplifier instability under mismatch effects has been analyzed with a contour technique that enables the identification of stable and potentially unstable load values
Summary
Power amplifiers (PAs) that are stable under ideal 50 terminations may become unstable under antenna mismatch conditions in realistic wireless environments where the antenna input impedance is affected by objects in its vicinity [1]. Being a large-signal extension of the factor [5], LS should have the same geometrical meaning: it should concord with the distance between the centre of the Smith chart at the upper sideband u (at fin+f) and the edge of the stability circle This stability circle will be boundary between u loads giving negative and positive resistance when looking into the circuit output at –fin+fp, corresponding to potential instability under mismatch effects. The fact that the stability circle in the u plane constitutes a boundary between loads giving rise to negative and positive resistance at the lower frequency sideband –fin+f is validated Results confirm that the stability circle constitutes a boundary between the u values giving negative and positive resistance at the lower sideband
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