| 摘要 |
<P>PROBLEM TO BE SOLVED: To reduce variation due to variation of the external power supply voltage of the output signal of a D/A converter for AFC control for controlling the frequency of a reference frequency signal by an RF IC. <P>SOLUTION: A D/A converter 315 for AFC control for controlling a reference frequency oscillator 314 is configured of a voltage potentiometer type D/A converter including three voltage followers AMP1, 2 and 3. In at least the post-voltage follower AMP3, an NMOS differential input circuit and a CMOS output circuit and a bias circuit supply an external power supply voltage Vdd_ext, and a PMOS differential input circuit PMOS_DA supplies an internal stabilized power supply voltage Vdd_int generated by a reference voltage generator RVG. Thus, it is possible to suppress the increase of the currents of the MP3 of a PMOS current source due to the increase of the external power supply voltage Vdd_ext even when the pair property of PMOS differential MP1 and MP2 deviates. Consequently, it is possible to prevent the input offset voltage of an PMOS differential from being increased, and to reduce the variation of an AFC control analog output signal. <P>COPYRIGHT: (C)2008,JPO&INPIT |
