Abstract: A modulated signal from an orthogonal modulator is supplied to a mixer, and a second local oscillating signal from a second local oscillator is supplied through a variable gain amplifier to the mixer. A transmission signal from the mixer is supplied through a step-attenuator (ATT) to a power amplifier, and further supplied through a directional coupler and an isolator to an antenna. A signal from the directional coupler is supplied through a detector to a comparator, and a reference voltage from a reference voltage setting unit is supplied to the comparator. An output from the comparator is supplied to a control unit of the variable gain amplifier and a gain is thereby controlled such that a DC voltage from the detector and the reference voltage from the reference voltage setting unit become equal to each other. Further, an attenuation amount of the step-attenuator is set by an attenuation amount setting unit in accordance with a transmission output control signal from a base station.

Abstract: A high-frequency signal step attenuator including an amplifying circuit having a transistor element with its base or gate connected to an input terminal to which a high-frequency input signal is supplied, and supplying an amplified output signal of the input signal to an output terminal, a current detector for detecting a current flowing through a current path of collector-emitter or a current path of drain-source of the transistor element to generate an output voltage corresponding to a detected current, a voltage setter for selectively generating output voltages of different levels, and a comparator for comparing the output voltage of the current detector with the output voltage of the voltage setter, and supplying an output signal of a compared result as a feedback signal to the base or gate of the transistor element.