Detector circuit and semiconductor device using same

A semiconductor device comprising:

• a detector circuit for detecting degradation in distortion characteristics of a power amplifier based on signals derived from a coupled line of a directional coupler, wherein the directional coupler includes a main line connected to the power amplifier and to an antenna, and a coupled line having a coupled terminal at a first end of the coupled line and an isolated terminal at a second end of the coupled line, said detector circuit comprising:
• a phase shifter/attenuators connected to said coupled terminal of said coupled line of the directional coupler and shifting phase of the signal from said coupled terminal of said coupled line, and attenuating the signal from said coupled terminal of said coupled line and outputting an output signal;
• difference output means connected to and receiving the output signal from said phase shifter/attenuator and to said isolated terminal of said coupled line and outputting a difference signal representing difference between the output signal from said phase shifter/attenuator and the signal from said isolated terminal of said coupled line;
• a wave detector circuit for converting the difference signal into a DC signal;
• and
• a comparing circuit for determining whether voltage level of the DC signal exceeds a predetermined level, wherein, when load conditions at the antenna cause degradation in the distortion characteristics of the power amplifier, said phase shifter/attenuator phase shifts the signal from said coupled terminal of said coupled line so that the output signal is 180° out of phase with the signal from said isolated terminal of said coupled line;
• and
• means for varying the load impedance of the power amplifier in accordance with the output from said comparing circuit to prevent degradation of the distortion characteristics.