Transient Intermodulation Distortion-Part 2: Soft Nonlinearity
Transient intermodulation distortion (TIMD) in a feedback amplifier is associated with nonlinearity in the stages that precede the forward-path dominant pole. A method is given for calculating TIMD for three classes of input waveform: a single sinusoid, a combination of a small sinusoid with a large band-limited square wave. It is suggested that the intermodulation between two input sinusoids is the most appropriate means for quantifying amplifier nonlinearity at high frequencies. Specification of the in-band second-order and third-order intermodulation products between two pairs of sinusoids (such as 5 plus 7 1/2 kHz and 10 plus 15 kHz) gives the lay user a simple numerical basis for comparing amplifiers. Such a specification also enables the professional to locate the dominant nonlinearity as preceding or following the forward-path dominant pole, and enables the coefficients of the quadratic and cubic terms in a power-series expansion of the nonlinearity to be found.
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