The present disclosure relates to a circuit that includes an input port for applying a sinusoidal input signal, and a first buffering means for converting the sinusoidal input signal into a square wave signal. A DC level of the square wave signal may be defined by an adjustable threshold voltage level. The circuit also includes an output port for outputting the square wave signal to a power amplifier. Further, the circuit includes a feedback loop having a low pass filtering means arranged for filtering the square wave signal and comparing means arranged for comparing a DC level of a filtered signal received from the low pass filtering means with a pre-set reference level. The reference level may be selected for cancelling a given harmonic component. The comparing means is further arranged for outputting to the first buffering means a correction signal for adjusting the threshold voltage level of the first buffering means.