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An analysis is presented for the transfer functions of a particular pulsewidth modulator and power-stage subsystem that has been widely used in practical switching-mode dc regulator systems. The switch and filter are in a Â¿buckÂ¿ configuration, and the switch is driven by a constant-frequency, variable duty-ratio, push-pull magnetic modulator employing square-loop cores. The two transfer functions considered are that with modulator control signal as input and that with line voltage as input. For ac signal,the corresponding describing functions (DF) are derived. It is shown thatcurrent-source drive to the modulator extends the control DF frequency response over that with voltage drive, and that complete cancellation of the effects of line variations can be obtained at dc, but not for ac. Experimental confirmation of the analytical results for the control DF are presented.