In a transmission apparatus, such as a base station, which transmits signals to mobile phone units, a Doherty amplifier is used as an amplifier to efficiently amplify the signals. When the Doherty amplifier receives a low-level signal, a main amplifier having characteristics, such as a class AB, operates, and when the Doherty amplifier receives a high-level signal, not only the main amplifier, but also a peak amplifier having characteristics, such as a class C, operates. Thus, highly-efficient signal amplification is achieved by controlling the amplifiers that operate depending on the level of the input signal.
A configuration example of a general Doherty amplifier will be described with reference to FIG. 10. The general Doherty amplifier distributes input signals and outputs the distributed signals to a main amplifier 110 and a peak amplifier 112. The signal input to the main amplifier 110 is amplified by the main amplifier 110, and is transmitted through an adjustment line 111. The signal input to the peak amplifier 112 is amplified by the peak amplifier 112, and is transmitted through an adjustment line 113. The signals transmitted through the adjustment line 111 and the adjustment line 113, respectively, are synthesized, and the synthesized signal is transmitted through an adjustment line 114. An adjustment line 115 is disposed at the preceding stage of the main amplifiers 110 and an adjustment line 116 is disposed at the preceding stage of the main amplifier 112 so as to match the phases of the signals in the synthesizing part.
The Doherty amplifier can amplify a signal having a predetermined frequency with high efficiency by adjusting the electrical length of each of the adjustment lines 111, 113, and 114. In this case, the electrical length is represented by a phase. The electrical length may be represented as, for example, 90 degrees. For example, an electrical length of 90 degrees indicates a 90-degree phase shift of a signal. The phase can also be represented by a wavelength λ. For example, a phase shift of λ/4 indicates a 90-degree phase shift.
For example, in the case of matching the output-side impedance of the adjustment line 114 to 50Ω and matching the input-side impedance of the adjustment line 114 to 25Ω, the adjustment line 114 having an electrical length of 90 degrees at 666 MHz and having a characteristic impedance of 35.5Ω is used. In this case, as shown in FIG. 11, assuming that a frequency band with a return loss characteristic of −30 dB or less is an effective band, the effective band is about 630 MHz to 700 MHz when the adjustment line 114 is used.
The electrical length of each of the adjustment line 111 and the adjustment line 113 is adjusted so that signals having a frequency ranging from 630 MHz to 700 MHz can be transmitted with high efficiency.
Thus, the Doherty amplifier which amplifies signals in a desired frequency band with high efficiency can be operated by controlling the electrical length of each of the adjustment lines 111, 113, and 114.