PURPOSE: To apply a necessary minimum operating supply voltage corresponding to the amplitude value of a signal to be amplified across an analog power amplifier so as to improve the efficiency of the amplifier, by increasing and decreasing little by little the output voltage of a digital power amplifier in a voltage step corresponding to the smallest bit in accordance with the amplifying value of an input signal voltage.

CONSTITUTION: The output of an AD converter circuit 7 increases and decreases correspondingly to the positive side peak value of an input signal Vi and the output voltage of a variable voltage source 1a is switched in stages. Namely, when the positive side peak value is high, the collector voltage +Vc of a transistor (TR) Q1 becomes higher and, when the peak value is low, the collector voltage +Vc becomes lower. Similarly, outputs of the AD converter circuit 7 and another variable voltage source 1b increases and decreases correspondingly to the negative side peak value of the input signal Vi and, when the peak value is high, the collector voltage -Vc of another TR Q2 becomes lower. When the peak value is low, the collector voltage -Vc of the TR Q2 becomes higher. Since the number of switching stages is 16 stages (4 bits) to the positive side and another 16 stages to the negative side in this case, switching of the supply voltage is performed little by little with a less number of power sources.