Patent ID: 8532591

Claim:
A transmission circuit that polar-modulates an input signal by switching an operation mode between a high-output mode in which the transmission circuit operates in a high-output region and a low-output mode in which the transmission circuit operates in a low-output region, the transmission circuit comprising: a signal generation section that generates an amplitude signal and a phase signal on the basis of the input signal; an amplitude signal driving section that generates a first control voltage based on the amplitude signal in the high-output mode, and generates a second control voltage at a predetermined level and a first bias current based on the amplitude signal in the low-output mode; a variable gain adjustment section that adjusts a power level of the phase signal; and a power amplifier: to a power supply terminal of which the first control voltage is supplied and which amplitude-modulates the phase signal having the adjusted power level on the basis of the first control voltage, in the high-output mode; and to the power supply terminal of which the second control voltage is supplied and to an input terminal of which the first bias current is supplied and which amplitude-modulates the phase signal having the adjusted power level on the basis of the first bias current and the second control voltage, in the low-output mode, wherein in a specific region in the low-output region and close to the high-output region, the variable gain adjustment section adjusts the power level of the phase signal to a predetermined upper-limit level, and the amplitude signal driving section operates in a special low-output mode in which a gain of the power amplifier is adjusted by changing the second control voltage and the first bias current to obtain a desired output power level.