Bandwidth limits in PWM switching amplifiers

PWM buck switching power converters are good candidates for high efficiency power amplification of arbitrary band-limited signals. The correlation between the signal bandwidth and the switching frequency, in turn related to switching losses, imposes practical limits for high bandwidth applications. Stringent specifications appear in such applications as audio amplifiers (kHz signal bandwidths), and lately, in fast envelope tracking power amplifiers (MHz signal bandwidths) for the Envelope Elimination and Restoration technique in polar RF power amplification. Bandwidth limitations in PWM amplifiers are explored in this work by proposing design criteria for obtaining the switching frequency to signal bandwidth ratio (f_s/f_x) so as to guarantee a given aliasing error. To achieve that purpose, PWM spectra are reviewed for single tone, two-tone and multitone signals. Subsequently, by taking into account the analogy between PWM and FM spectra, bandwidths around the switching frequency are estimated by extending Carson´s rule to an arbitrary error. This allows obtaining an extension of the Nyquist criterion for pulse-width modulation. System-level simulation results are reported to validate the analysis, showing that the conventional f_s/f_x factor used in the power converter design field might be too conservative