Distortion cancellation for RF amplifiers using...

Details Distortion cancellation for RF amplifiers using... Distortion cancellation for RF amplifiers using...

2003-02-05

2004-12-14

Nguyen, Khanh Van (Department: 2817)

Amplifiers

With plural amplifier channels

Redundant amplifier circuits

C330S289000, C330S295000

Reexamination Certificate

active

06831511

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is in the field of nonlinear devices, and more specifically, is in the field of reducing distortion produced by nonlinear devices.
2. Discussion of the Prior Art
In the prior art, digital modulation schemes are widely employed in various multi-carrier communication systems (for instance, in wireless communication systems, in satellite communication systems, etc.) to achieve multiple objectives, like capacity improvement, better transmitted quality of data, and higher data rate transmission. In non-constant envelope digital modulation schemes, the information is included in both amplitude and phase of the modulated signals.
In the prior art, a linear amplifier is a preferable device to amplify such signals, because, in theory, an ideal linear amplifier does not cause signal distortions. On the other hand, a non-linear amplifier causes degradation of signal quality due to amplitude and phase distortions caused by nonlinear devices. In addition, linear amplifiers are also beneficial in amplifying multi-carrier signals simultaneously, in applications such as cellular base stations, without creating significant distortions.
One more advantage of employing linear amplifiers is that it reduces the number of amplifiers used, as well as eliminates high power combiner chains. Thus, usage of linear amplifiers directly results in reducing size, complexity and cost of the overall amplification systems which is highly important in applications such as satellite systems and cellular base stations.
The DC power consumption for linear amplifiers should also be minimized in order to attain high efficiency, higher performance reliability and reduction of operating costs. Such features of linear amplifiers are highly desirable in all communication systems.
In order to meet the linearity amplification requirement for non-constant envelope modulated signals in wireless communication systems, conventional linear amplifiers usually operate at a certain output power level (back off power level) that is different from a saturated power level. However, operating a linear amplifier at a back off power level has its own drawbacks. Indeed, the tradeoff of operating a linear amplifier at a back off power level is a low DC-to-RF efficiency on the amplifiers since peak DC-to-RF efficiency is usually achieved near a saturated output power level.
The prior art amplifier linearization techniques are widely used to improve the efficiency of non-linear amplifiers. However, conventional linearization techniques require the use of external circuitry to reduce distortion levels at the output of non-linear amplifiers.
The prior art common linearization techniques, such as feedforward, predistortion, and feedback techniques, have been disclosed in “Feedfoward—An alternative approach to amplifier linearization,” by T. J. Bennett et al., The Radio and Electronic Engineer, vol. 44, no. 5, pp. 257-262, May 1974; “Feedforward linearization of 950 MHz amplifiers,” by R. D. Stewart et al., IEEE Proceedings-H, vol. 135, no. 5, pp. 347-350, October 1988; “An automatically controlled predistorter for multilevel quadrature amplitude modulation, by J. Namiki, IEEE Trans. Commun., vol. COM-31, no. 5, pp. 707-712, May 1983; U.S. Pat. No. 4,465,980 by Huang et al.; U.S. Pat. No. 5,523,716 by Grebliunas et al.; U.S. Pat. No. 5,886,572 by Myers et al.; U.S. Pat. No. 5,821,814 by Katayama et al; U.S. Pat. No. 5,781,069 by Baskin. These techniques, however, usually involve very complex circuit configurations and require extensive alignment in production.
Recently, predistorters with simpler configuration have been disclosed in “A normal amplitude and phase linearizing technique for microwave power amplifiers,” M. Nakayama et al., 1995 IEER MTT-S Dig., pp. 1451-1454; “A novel series diode linearizer for mobile radio power amplifiers,” by K. Yamauchi et al., 1996 IEEE MTT-S Dig., pp. 831-834; “Passive FETMMIC linearizers for C, X and Ku-band satellite applications,” A. Katz et al., 1993 IEEE MTT-S Dig., pp. 353-356; U.S. Pat. No. 5,191,338 by Katz et al; U.S. Pat No. 6,396,327 by Lam; U.S. Pat. No. 6,307,436 by Hau; U.S. Pat. No. 6.346,853 by Kangaslahti et al.
As shown in
FIG. 1A
, the prior art recently developed circuitry
10
for linearized power amplifier (PA)
12
utilizes the conventional miniaturized predistorter design
24
. Similarly, as shown in
FIG. 1B
, the recently developed prior art circuitry
30
for linearized power amplifier (PA)
32
utilizes the conventional miniaturized predistorter design
40
. Though these recently developed prior art predistortion schemes achieve the circuitry size reduction over conventional size of circuitry design, they still require extra matching circuits, as shown in predistortion circuitry
24
of
FIG. 1A
, as well as shown in predistortion circuitry
40
of FIG.
1
B.
In addition, these prior art predistortion schemes (
10
of
FIG. 1A and 30
of
FIG. 1B
) are difficult to use because they have poor isolation. Indeed, since conventional predistorters usually experience poor reverse isolation, power amplifiers incorporating the predistorters require additional isolators to improve circuit isolation to avoid interaction between the predistorters and amplifier stages which would degrade overall circuit performance.
The predistortion schemes (
10
of
FIG. 1A and 30
of
FIG. 1B
) also experience high loss. Indeed, predistorters
24
(of
FIG. 1A
) and
40
(of
FIG. 1B
) are all passive in nature with insertion loss level ranges from 4 dB to 20 dB depending on the design. Extra buffer amplifiers (
14
of
FIG. 1A
;
34
of
FIG. 1B
) are usually added to compensate the high insertion loss. The use of buffer amplifiers is of particular concern as that would increase overall DC power consumption. Even though the overall efficiency of the linearized amplifier is improved, the increased DC power requirements increase the size and cost of the power supply or battery needed.
What is needed is to develop a novel linearized circuitry for a power amplifier that is free from the above-identified problems.
SUMMARY OF THE INVENTION
To address the shortcomings of the available art, the present invention provides novel linearized circuitry for a power amplifier that is free from the above-identified problems related to the prior art predistortion schemes (
10
of
FIG. 1A and 30
of FIG.
1
B).
One aspect of the present invention is directed to an apparatus and method for improving linearity of an RF signal. In one embodiment, the apparatus of the present invention comprises: (a) a splitter; (b) an over-biased non-linear RF power amplifier; (c) an under-biased non-linear RF power amplifier; (d) a combiner; (e) a bias controller; and (f) a circulator.
In one embodiment of the present invention, the splitter is configured to receive an input RF signal, and configured to split the input RF signal into two RF signals comprising a first input RF signal, and a second input RF signal.
In one embodiment of the present invention, the over-biased non-linear RF power amplifier is configured to receive the first input RF signal and configured to generate an over-biased non-linear output signal having an over-biased non-linear distortion component. The over-biased non-linear RF power amplifier is configured to receive an over-biased DC signal being greater than an optimum bias DC voltage.
In one embodiment of the present invention, the under-biased non-linear RF power amplifier is configured to receive the second input RF signal and configured to generate an under-biased non-linear output signal having an under-biased non-linear distortion component. The under-biased non-linear RF power amplifier is configured to receive an under-biased DC signal being lower than the optimum bias DC voltage. In one embodiment of the present invention, the combiner is configured to combine the over-biased non-linear output signal and the under-biased non-linear output signal.
In one embodiment of the present invention, the bias controller is configured to ke

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