PWM encodes a reference voltage as the duty cycle of a fixed-frequency carrier. What determines the average output voltage after filtering?

• A. The carrier frequency alone
• B. The duty cycle of the PWM signal
• C. The instantaneous carrier amplitude
• D. The power supply voltage only

Answer: (C)

The key idea is that a low-pass filter turns a PWM waveform into a DC level that corresponds to how long the output stays high in each switching period. The average voltage after filtering is set by the high-level amplitude (typically the supply voltage) multiplied by the duty cycle—the fraction of time the output is high. So if the high level is Vsupply and the duty cycle is D, the average is approximately D × Vsupply. The instantaneous carrier amplitude changes within each period, but the filter smooths those rapid fluctuations, leaving only the average value. Hence the duty cycle, not the instantaneous carrier amplitude, determines the DC output level (with the supply setting the maximum scale).