in a communications system , a power amplifier is dynamically biased in response to a control signal indicative of the required transmission power . in delivering relatively high transmission power , the dc operating point of an amplifying element in the power amplifier is placed far from the cutoff and saturation regions of the output characteristics of the amplifying element . as a result , the corresponding input signal at a high drive level to the power amplifier is virtually undistorted thereby and , at the same time , a high power efficiency is achieved . in delivering relatively low transmission power , although the dc operating point is placed relatively close to the cutoff region of the output characteristics of the amplifying element , because of the corresponding input signal which is at a low drive level , the power amplifier minimally distorts the input signal . however , a high power efficiency is maintained because the input dc power to the power amplifier is reduced as well as the transmission power .