In a modulated signal subjected to voltage conversion, in particular, a modulated signal subjected to multi-level modulation, such as QAM (Quadrature Amplitude Modulation), linear operation is required in a high-frequency power amplifier for transmitting power to an antenna. Hence, class A or class AB has been used as the operation class of the high-frequency power amplifier.
However, as broadband communication is promoted, a communication system that uses subcarriers, such as OFDM (Orthogonal Frequency Division Multiplex), has begun to be used; hence the conventional class-A or class-AB high-frequency power amplifier cannot be expected to attain high efficiency. In other words, in OFDM, subcarriers are overlapping, whereby high power is generated instantaneously at random. That is to say, the ratio PAPR (Peak to Average Power Ratio) of the instantaneous maximum power to average power is high. Hence, in order that a high-frequency signal having this kind of high power can also be amplified linearly, it is necessary to hold high DC power at all times. The power efficiency of the class-A operation is only 50% at the maximum; in particular, in the case of OFDM, since PAPR is high, the power efficiency is only about 10%.
For this reason, in the case of a portable wireless transmitter wherein batteries are used as a power supply, its operable time is short, thereby causing a problem in practical use.
In order to solve this kind of problem, the conventional EER (Envelope Elimination and Restoration) method has been proposed, which is known as Kahn's method (for example, see U.S. Pat. No. 6,256,482B1 (FIG. 6 on page 3 of the drawings)).
FIG. 6 is a block diagram showing the schematic configuration of the EER method. In FIG. 6, a high-frequency modulated signal 46, such as a QAM signal, input to a terminal 40, is divided into two branches; in one of the branches, the modulated wave 46 is subjected to envelope detection by a detector 41, whereby an amplitude component signal is generated. A power supply voltage Vdd is voltage-converted by a voltage converter (an amplifier for amplifying the amplitude component) 42. At this time, a class-S amplifier (a switching regulator or the like) capable of operating at high efficiency (up to 95%) is used as the voltage converter 42. In the other branch, the amplitude of the modulated wave 46 is controlled by an amplitude control amplifier (limiter 43), whereby a modulated wave having only phase information is obtained. The modulated wave having the phase information is input to the RF input terminal of a switch-type amplifier 44, whereby, for example, the gate voltage of a field-effect transistor, an element of the switch-type amplifier 44, is modulated.
The switch-type amplifier is herein a class-F amplifier wherein harmonics are controlled so that the waveform of the drain voltage becomes rectangular, or a class-E or class-D amplifier wherein load conditions are optimized so that the waveform of the drain voltage and the waveform of the drain current do not overlap each other.
The conventional class-A amplifier has a period in which the drain voltage and the drain current are generated simultaneously, thereby consuming power. On the other hand, in the switch-type amplifier 44, such a period in which the drain voltage and the drain current are generated simultaneously is made as short as possible, whereby power consumption can be reduced.
When a DC power of 200 mA and 3 V is supplied, for example, the DC power is 600 mW. When the switch-type amplifier 44 is OFF, no current flows and only the voltage Vdd is applied; hence DC power consumption is zero. On the other hand, when ON, a current of 200 mA flows; however, since the transistor conducts completely, the voltage VDS between the drain and the source can be assumed to be about 0.3 V at most. In this case, a DC power of 0.3×0.2=0.06 W, that is, 60 mW, is consumed inside the transistor. The power efficiency reaches a very high value of (600 −60)/600=90%. This effect is significant since the power efficiency of the class-A amplifier is only 50% at the maximum.
In other words, high power efficiency is attained by using the switch-type amplifier. However, since the switch-type amplifier is a nonlinear amplifier, the switch-type amplifier cannot be used because a modulated signal, such as the QAM signal, in which the amplitude level of the modulated wave changes, is required to be amplified linearly.
In order to solve this problem, the EER method separates a signal including amplitude information into an amplitude component and a phase component, and only the phase component is amplified by the switch-type amplifier. When the amplitude component is input to the power supply terminal of the switch-type amplifier, output power proportionate to the amplitude component is obtained; hence, a signal including the original amplitude information is reproduced eventually.
With this configuration, a highly efficient amplifier, such as the switch-type amplifier, although nonlinear, can be used, whereby high efficiency can be attained.
However, since the frequency band of the voltage converter 42 (for example, a switching regulator) for modulating the amplitude component is 5 MHz at most, the conventional EER method cannot be used in the modulated wave frequency band width of 20 MHz stipulated in the IEEE802.11a Standard, a wireless LAN standard.
In order that the frequency band is widened, it is necessary to decrease the inductance of the low-pass filter incorporated in the output of the voltage converter 42. However, since the Q value of the inductance is lowered, the amount of heat consumed by the inductance becomes unignorable, and the efficiency of the voltage converter 42 lowers. In addition, noise increases.
Furthermore, in the case when a series regulator is used as the voltage converter 42, the product of the amount of voltage conversion (the difference between the power supply voltage and the voltage of the amplitude component) of the series regulator and the drain current of the high-frequency power amplifier is power consumption. In OFDM, the average voltage value of the amplitude component is not more than a half of the power supply voltage; hence, high efficiency is not attained even in this case.