Converter systems invariably require feedback. For example, in a typical dc-dc converter application, the output voltage v(t) must be kept constant, regardless of changes in the input voltage v g (t) or in the effective load resistance R. This is accomplished by building a circuit that varies the converter control input [i.e., the duty cycle d(t)] in such a way that the output voltage v(t) is regulated to be equal to a desired reference value v ref . In inverter systems, a feedback loop causes the output voltage to follow a sinusoidal reference voltage. In modern low-harmonic rectifier systems, a control system causes the converter input current to be proportional to the input voltage, such that the input port presents a resistive load to the ac source. So feedback is commonly employed.