A device for providing wireless power comprising: a power supply; a power amplifier (120, 230) coupled to the power supply, the power amplifier (120, 230) comprising: a first switch and a second switch coupled to the power supply and to a common switch output, a first diode coupled between an output of the first switch and an input of the first switch, the first diode biased to enable current to flow from the output to the input of the first switch, a second diode coupled between an output of the second switch and an input of the second switch, the second diode biased to enable current to flow from the output to the input of the second switch, and a pulse-width modulator ("PWM") coupled to the power amplifier (120, 230), the PWM configured to substantially simultaneously toggle each of the first and second switches (M1, M2) between open and closed states, and to maintain the first and second switches (M1, M2) in opposite open and closed states; a sensor configured to detect an impedance of a load; and a controller (130, 900) coupled to the power supply and the PWM, the controller (130, 900) configured to: receive, from the sensor, a sensor signal that provides the detected load impedance to the controller; determine, by using the detected load impedance, a duty cycle of the PWM either (i) by access to a look-up table that determines the duty cycle of the PWM based on the load impedance, or (ii) by computation of a mapping function that accepts an impedance as an input and determines a duty cycle as an output, wherein the look-up table or mapping function are established according to a relationship that maps load impedance to duty cycle so that the power amplifier operates at or near optimal efficiency; and adjust, using the determined duty cycle of the PWM, an output voltage of the power supply according to a relationship that maps duty cycle to output voltage so that the RMS value of an output current is maintained at a substantially constant level. a power supply; a power amplifier (120, 230) coupled to the power supply, the power amplifier (120, 230) comprising: a first switch and a second switch coupled to the power supply and to a common switch output, a first diode coupled between an output of the first switch and an input of the first switch, the first diode biased to enable current to flow from the output to the input of the first switch, a second diode coupled between an output of the second switch and an input of the second switch, the second diode biased to enable current to flow from the output to the input of the second switch, and a pulse-width modulator ("PWM") coupled to the power amplifier (120, 230), the PWM configured to substantially simultaneously toggle each of the first and second switches (M1, M2) between open and closed states, and to maintain the first and second switches (M1, M2) in opposite open and closed states; a first switch and a second switch coupled to the power supply and to a common switch output, a first diode coupled between an output of the first switch and an input of the first switch, the first diode biased to enable current to flow from the output to the input of the first switch, a second diode coupled between an output of the second switch and an input of the second switch, the second diode biased to enable current to flow from the output to the input of the second switch, and a pulse-width modulator ("PWM") coupled to the power amplifier (120, 230), the PWM configured to substantially simultaneously toggle each of the first and second switches (M1, M2) between open and closed states, and to maintain the first and second switches (M1, M2) in opposite open and closed states; a sensor configured to detect an impedance of a load; and a controller (130, 900) coupled to the power supply and the PWM, the controller (130, 900) configured to: receive, from the sensor, a sensor signal that provides the detected load impedance to the controller; determine, by using the detected load impedance, a duty cycle of the PWM either (i) by access to a look-up table that determines the duty cycle of the PWM based on the load impedance, or (ii) by computation of a mapping function that accepts an impedance as an input and determines a duty cycle as an output, wherein the look-up table or mapping function are established according to a relationship that maps load impedance to duty cycle so that the power amplifier operates at or near optimal efficiency; and adjust, using the determined duty cycle of the PWM, an output voltage of the power supply according to a relationship that maps duty cycle to output voltage so that the RMS value of an output current is maintained at a substantially constant level. receive, from the sensor, a sensor signal that provides the detected load impedance to the controller; determine, by using the detected load impedance, a duty cycle of the PWM either (i) by access to a look-up table that determines the duty cycle of the PWM based on the load impedance, or (ii) by computation of a mapping function that accepts an impedance as an input and determines a duty cycle as an output, wherein the look-up table or mapping function are established according to a relationship that maps load impedance to duty cycle so that the power amplifier operates at or near optimal efficiency; and adjust, using the determined duty cycle of the PWM, an output voltage of the power supply according to a relationship that maps duty cycle to output voltage so that the RMS value of an output current is maintained at a substantially constant level.