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Power amplifiers are the major source of nonlinearity in communications systems. Such nonlinearity causes adjacent channel interference and increases bit error rate. Polynomials are often used to model the nonlinear power amplifier or its predistortion linearizer. In this paper, we present a novel set of orthogonal polynomials for baseband Gaussian inputs to replace the conventional polynomials, and show how they alleviate the numerical instability problem associated with the conventional polynomials. The orthogonal polynomials also provide an intuitive means of spectral re-growth analysis.