1. Field of the Invention
The present invention relates to a data converter, a signal generator, a transmitter using the data converter and the signal generator, a communication apparatus such as a cell phone or wireless LAN using the data converter and the signal generator, and a data conversion method.
2. Related Art of the Invention
FIG. 27 shows an example of a conventional transmitting circuit (see B. Razavi, “RF Microelectronics” Prentice-Hall PTR, 1998 (p. 153, FIG. 5. 39), for example). Referring to FIG. 27, I (in-phase) data and Q (quadrature-phase) data are respectively output from two output terminals of a data generator 5001. These groups of data are input to and modulated by a modulator 5002. An output from the modulator 5002 is amplified by an amplifier 5003 and radiated from an antenna 5004.
In the transmitting circuit shown in FIG. 27, however, the envelope of a signal to be radiated from the antenna 5004 fluctuates and it is required that the amplifier 5003 have certain linearity to avoid degradation of such a signal. In order to ensure certain linearity of the amplifier 5003, it is necessary to make the amplifier 5003 operate in a linear region such that the output of the amplifier 5003 is substantially lower than a saturated level. In such a case, the power consumption is increased.
Description will be made of this more concretely with reference to FIG. 28. FIG. 28 is a graph showing the output power and the efficiency of the amplifier 5003 with respect to input power. From the viewpoint of improving the efficiency of the amplifier 5003, higher input power and higher output power are desirable. That is, when the input power is Pin2, the output power is Pout 2 and the efficiency is η2. At such an operating point, an input signal having such a characteristic that its envelope fluctuates is distorted. In practice, the amplifier 5003 can not be used at such an operating point. In order to reduce the distortion, it is necessary to reduce the input power and the output power. If the input power and the output power are reduced, the efficiency is reduced. That is, when the input power is Pin1, the output power is Pout1 and the efficiency is η1.