A Power amplifier circuit based on a cascode structure and to be powered by a power source voltage, e.g. a battery, said circuit comprising -a first transistor having a grid, source and drain terminal; said first transistor being connected in a common source mode; -a second grid source transistor having grid, source and drain terminal, said second transistor being connected in common grid mode; -a biasing circuit for biasing said first transistor and said second transistor. The PA is characterized in that it includes a circuit for sensing the value of the power source voltage and for generating at least a first and a second biasing voltage for the grid of said second transistor in accordance with the power source voltage sensed, said first biasing voltage providing substantially equal protection to said first and second transistors when said power source voltage is sensed to be at a high voltage and said second biasing voltage providing more voltage to said first transistor when said power source voltage is sensed to be at a low voltage.