Amplifiers – Hum or noise or distortion bucking introduced into signal...
Reexamination Certificate
1998-12-29
2001-05-22
Lee, Benny (Department: 2817)
Amplifiers
Hum or noise or distortion bucking introduced into signal...
C332S159000
Reexamination Certificate
active
06236267
ABSTRACT:
BACKGROUND
1. Field of the Invention
This invention relates to communication systems. In particular, the invention relates to power amplifiers.
2. Description of Related Art
An important component in a communication or signal processing system is the power amplifier (PA). A PA usually consumes a large amount of power to provide power gain to a transmitted signal. A major problem in radio frequency (RF) communication systems is the non-linearities introduced by the operation of the PA in the signal processing chain.
The non-linearity of the PA results in distortion to the transmitted signal. Higher class PA's achieve theoretically better efficiency, expressed as the ratio of the generated output radio frequency (RF) power to the consumed direct current (DC) power, but introduce significant non-linearities to the transmitted signal. The distortion is normally in the form of severe compression which distorts the amplitude of the signal. In the extreme case, the compression behaves like a hard limiter which truncates the signal at a certain threshold.
Although constant envelope waveforms possess some inherent immunity to the compression effects caused by the PA's, they still suffer from spurious consequences. In addition, amplitude modulation (AM) to phase modulation (PM) conversion is another side-effect of these PA's in which the phase modulated signals are greatly distorted. For non-constant envelope modulation schemes, such as AM, the amplitude distortion due to the PA clearly produces undesirable results.
There exist a few techniques to reduce the non-linearity effect of the PA's. One such technique is to design the PA with selective components and carefully designed internal compensating circuits. For example, balanced amplifier circuits can be combined to achieve high powers. Internally matched structures can be included to reduce parasitic effects. However, this technique increases the circuit complexity, results in expensive or bulky PA's which may not be suitable for wireless applications, and provides marginal improvement at best. Another technique is to compensate the non-linearity by using a separate filtering system which has phase and amplitude characteristics complementary to the PA characteristics. A disadvantage of this technique is the difficulty in matching both the phase and amplitude of the filter characteristics to the PA characteristics. Another disadvantage is the inability to respond to the system dynamically as the frequency or phase of the signal changes, or the characteristics of the PA or filter change (independently of one another).
Therefore there is a need in the technology to provide a simple and efficient method to linearize the characteristics of the PA without compromising power efficiency.
SUMMARY
The present invention is a method and apparatus for linearizing an output signal which is generated by a power amplifier (PA). A phase compensation on the output signal is performed using a phase feed-forward path and a phase feedback path. An amplitude compensation on the output signal is performed using an amplitude feed-forward path and an amplitude feedback path. The amplitude compensation is synchronized with the phase compensation.
In a preferred embodiment, the phase feed-forward path includes a phase control parameter generator containing control parameters to control a direct digital synthesizer and a phase-locked loop to generate the input signal to the PA. The phase feedback path includes a phase detector to generate a feedback phase error signal between the input signal to the PA and the output signal generated by the PA. The feedback phase error signal is used to update the control parameters in the phase control parameter generator. The amplitude feed-forward path includes an amplitude control parameter generator to adjust the power level of the PA based on an amplitude error. The amplitude feedback path includes an amplitude detector to detect the amplitude of the output signal, and an amplitude comparator to compare the detected output amplitude and a predetermined amplitude based on the characteristic of the desired output signal. The amplitude comparator generates the amplitude error.
REFERENCES:
patent: 5493252 (1996-02-01), Takai
patent: 5532642 (1996-07-01), Takai
patent: 5565814 (1996-10-01), Fukuchi
patent: 5675286 (1997-10-01), Baker et al.
patent: 5675288 (1997-10-01), Peyrotte et al.
patent: 6011434 (2000-01-01), Sakai
patent: 6028477 (2000-02-01), Gentzler
Blakely , Sokoloff, Taylor & Zafman LLP
International Business Machines - Corporation
Lee Benny
Nguyen Khanh Van
LandOfFree
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