In radio transmitters used for high-speed communication, high linearity (low distortion characteristic) is required in amplifiers to transmit data correctly. An explanation follows below regarding the configuration and operation of an amplifier of the related art.
FIG. 1 is a block diagram showing a feed-forward amplifier for realizing a low-distortion characteristic. The feed-forward amplifier includes: amplifier 1, delay lines 10 and 11, subtractor 2 for synthesizing the signal from amplifier 1 and the signal from delay line 10 and extracting third-order intermodulation distortion 6, error amplifier 3 for amplifying third-order intermodulation distortion 6 to generate third-order intermodulation distortion 7, and synthesizer 4 for offsetting third-order intermodulation distortion 6 and third-order intermodulation distortion 7.
As a simple explanation of the operation of the feed-forward amplifier shown in FIG. 1, when input signal 12 is applied as input from input terminal 13, input signal 12 is applied to amplifier 1 and delay line 10. The output signal from amplifier 1 includes third-order intermodulation distortion 6 in addition to main signal 5 that has been amplified. Upon the input of the output signal from amplifier 1 and input signal 12 via delay line 10, subtractor 2 extracts third-order intermodulation distortion 6 generated in amplifier 1 by combining these signals. Error amplifier 3 amplifies third-order intermodulation distortion 6 that is received as input and supplies the result as third-order intermodulation distortion 7. Synthesizer 4 adds together main signal 5 that is received as input via delay line 11 and third-order intermodulation distortion 6 with third-order intermodulation distortion 7 from error amplifier 3, whereby third-order intermodulation distortion 9 resulting from the addition of third-order intermodulation distortion 6 and third-order intermodulation distortion 7 is offset in the signal supplied from output terminal 14. In this type of feed-forward amplifier, distortion compensation is achieved by using the distortion generated from the amplifier itself and can therefore achieve a high distortion compensation amount, as described in “MWE 2004 Microwave Workshop Digest,” pp. 575-584, November 2004 (hereinbelow referred to as “Document 1”).
In the feed-forward amplifier shown in FIG. 1, however, the adoption of a two-loop structure leads to the problems of large circuit scale and increases in cost, size, and power loss. A method of circumventing these problems is proposed in U.S. Pat. No. 2,973,684 (hereinbelow referred to as “Document 2”). FIG. 2 is a block diagram showing an amplification device of the distortion compensation method proposed in Document 2.
FIG. 2 is a block diagram showing an example of the configuration of an amplification device of the related art. As shown in FIG. 2, the amplification device is of a configuration that includes low-distortion amplifiers 21 and 22, high-distortion amplifier 23, phase equalizer 24 provided in the section following high-distortion amplifier 23, hybrid transformer 25, and synthesizer 26.
As a simple explanation of the operation of the amplification device shown in FIG. 2, upon input of input signal 37 from input terminal 35, hybrid transformer 25 splits input signal 37 into main signal 28 that is in-phase with input signal 37 and main signal 27 in which the phase of input signal 37 is inverted 180°. Main signal 28 is then applied as input to low-distortion amplifier 21 and main signal 27 is applied as input to high-distortion amplifier 23. By the amplification of main signal 28 by low-distortion amplifier 21 and low-distortion amplifier 22, main signal 29 in which main signal 28 has been amplified and third-order intermodulation distortion 30 are supplied from low-distortion amplifier 22.
In contrast, upon receiving main signal 27 from hybrid transformer 25, high-distortion amplifier 23 generates third-order intermodulation distortion 32 for distortion compensation from main signal 27. High-distortion amplifier 23 next supplies main signal 27 and third-order intermodulation distortion 32 as output. Phase equalizer 24 provided in the section following high-distortion amplifier 23, upon receiving main signal 27 and third-order intermodulation distortion 32 from high-distortion amplifier 23, corrects for the phase shift in low-distortion amplifier 21 regarding third-order intermodulation distortion 32 and corrects phase such that the phase is anti-phase with third-order intermodulation distortion 30 supplied from low-distortion amplifier 22. Phase equalizer 24 then supplies third-order intermodulation distortion 32 in which phase has been adjusted and main signal 31 that replaces main signal 27.
Synthesizer 26 combines main signal 29 and third-order intermodulation distortion 30 supplied from low-distortion amplifier 22 and main signal 31 and third-order intermodulation distortion 32 supplied from phase equalizer 24. In this way, third-order intermodulation distortion 34 supplied to output terminal 36 can be offset and the relative strength of third-order intermodulation distortion 34 with respect to main signal 33 at output terminal 36 can be decreased. By means of this approach, distortion compensation is possible by a single-loop construction and a reduction in circuit scale can be achieved.
On the other hand, a Doherty amplifier is described in the above-mentioned Document 1 as a method of improving efficiency at times of low power in a parallel construction. FIG. 3 is a block diagram showing an example of the configuration of a Doherty amplifier of the related art.
As shown in FIG. 3, a Doherty amplifier is of a configuration that includes main amplifier 41, auxiliary amplifier 42, λ/4 lines 43 and 44, synthesizer 45, input terminal 46, output terminal 47, and distributor 48. In addition, main amplifier 41 is set to a Class-A to Class-AB bias state, and auxiliary amplifier 42 is set to a Class-C bias state.
As a simple explanation of the operation of the Doherty amplifier shown in FIG. 3, upon the input of input signal 51 to input terminal 46, distributor 48 distributes input signal 51 to the path to main amplifier 41 and the path to λ/4 line 43 and auxiliary amplifier 42. Each of main amplifier 41 and auxiliary amplifier 42 amplify input signal 51 that is received. Main amplifier 41 amplifies input signal 51 and transmits the result to synthesizer 45 by way of λ/4 line 44. Auxiliary amplifier 42 amplifies input signal 51 that is received by way of λ/4 line 43 and transmits the result to synthesizer 45. Synthesizer 45 combines the signal that is received from main amplifier 41 via λ/4 line 44 and the signal received from auxiliary amplifier 42 to supply output terminal 47.
The configuration explained in FIG. 3 can be designed to use change of the output impedance of auxiliary amplifier 42 that accompanies increase of the input power to optimize efficiency of the load impedance of main amplifier 41 over a broad range of input power, whereby efficiency can be improved over a broad range of input power compared to a single main amplifier 41.