To reduce the influence due to the leakage of an oscillation output signal of a reference frequency oscillator DCXO, generated when the level of the oscillation output signal of DCXO is converted by a level converter Lev_Conv to the vicinity of an oscillation output signal of a voltage control oscillator TXVCO for RF transmission by harmonic components.

The level converter Lev_Conv converts the level of the oscillation output signal of DCXO and supplies a level conversion signal to a phase comparator PDC of a PLL fractional synthesizer Frct_Synth, which controls oscillation frequencies of the voltage control oscillator TXVCO for RF transmission. The Lev_Conv includes a self-bias type voltage amplifier Self_Bias_Volt_Amp, which amplifies the reference frequency signal of the reference frequency oscillator DCXO. The self-bias type voltage amplifier includes a coupling capacity C1, an amplifier transistor Qn5, a load Qp5 and a bias element R1 and suppresses the level fluctuations of the harmonic components, even if external supply voltage Vdd_ext fluctuates.

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