Transmission circuit by polar modulation system and communication apparatus using the same

A transmission circuit by a polar modulation system, comprising:

• a transmission power control section for generating a transmission power control signal for controlling transmission power of the transmission circuit;
• an attenuator for attenuating a magnitude of an amplitude signal by an attenuation amount in accordance with the transmission power control signal;
• a voltage control circuit for outputting a voltage controlled signal which is controlled in accordance with the amplitude signal input via the attenuator;
• a phase modulator for performing phase modulation on a phase signal and outputting the resultant signal as a phase modulated signal;
• and
• a power amplifier for amplifying the phase modulated signal in accordance with the voltage controlled signal which is output from the voltage control circuit to perform amplitude modulation on the phase modulated signal, and outputting the resultant signal as a transmission signal;
• wherein the attenuator includes a plurality of resistor elements, a plurality of switching elements for switching the connection between the plurality of resistor elements and an input terminal in accordance with a value of the transmission power control signal, a variable capacitor element connected to each of the plurality of resistor elements in parallel, and a capacitance value control section for controlling capacitance values of the variable capacitor elements based on the transmission power control signal.