Amplifiers – Hum or noise or distortion bucking introduced into signal...
Reexamination Certificate
1999-10-22
2001-04-03
Pascal, Robert (Department: 2817)
Amplifiers
Hum or noise or distortion bucking introduced into signal...
C330S151000, C375S297000
Reexamination Certificate
active
06211733
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates generally to power amplifiers, and in particular to linearizing the input/output transfer function for amplifiers, particularly high power class AB power amplifiers.
High power, broad band power amplifiers are well known. These amplifiers may operate in a feed forward configuration, or may have other forms of linearization which are required when the main power amplifier operates, for example, as a class AB amplifier. Although class A amplifiers usually produce less distortion than class AB amplifiers, class A amplifiers are also less efficient than class AB amplifiers. Thus, in order to retain the advantages of efficiency while minimizing distortion, class AB amplifier configurations have been developed which implement various forms of error or distortion correction.
One form of error correction attempts to distort the input signal in a manner which compensates for the distortions of a class AB amplifier. Thus, a predistortion circuit can be provided with various manual adjustments to produce a distortion signal from the original signal, so that when the distortion signal is combined with the input signal, and the combination is input to the power amplifier, operating for example, as a class AB amplifier, the output is substantially a linear amplification of the original input signal to the amplifier arrangement.
Such predistortion circuities typically employ a low power amplifier, preferably having the same general distortion characteristics as the main amplifier, so that its output, properly processed, can be used to obtain the necessary distortion components required to be combined with the input signal to the predistortion circuitry to generate a predistorted input to the main amplifier. Such configurations operate to substantially reduce the intermodulation frequency distortions produced by a class AB amplifier when the variable elements of the predistortion circuitry are properly adjusted.
However, even in properly adjusted amplifier arrangements using predistortion, a certain amount of instability, that is, drift in the operating point gain and/or phase, can be observed. Thus, a microprocessor can adjust the critical parameters of a predistortion circuit to minimize the distortion components, but has the limitation that as the predistortion circuit is adjusted, it unbalances the cancellation loop in the feed-forward configuration. This requires the microprocessor to re-null the signal cancellation loop before a decision can be made as to whether an improvement has actually been achieved. This is a very time consuming process, and if the input signals are of a type which are in a constant state of change, for example, turned on and off, the loops will become “confused” and not yield an adaptive improvement. Thus, the prior art adaptive predistortion techniques such as that disclosed in U.S. application Ser. No. 09/057,332, filed Apr. 8, 1998, and entitled, DYNAMIC PREDISTORTION COMPENSATION FOR A POWER AMPLIFIER, the contents of which are incorporated herein, in their entirety, by reference, works well for steady state signals but fails to give the results sought if operated in a transient environment. That transient environment exists for certain types of multi-channel operation, with the analog cellular system being a good example. In that system, carriers are continually being switched on and off.
SUMMARY OF THE INVENTION
The invention relates to an amplifier arrangement for amplifying an input signal with which a distortion cancelling input is combined. The amplifier arrangement features a main amplifier, a predistortion circuitry having an output connected to the main amplifier, and a feedback loop. The feedback loop features a comparator which differences a delayed derivative of the input signal to the amplifier arrangement with a signal representative of the output of the main amplifier to provide an error signal, an energy detector for receiving the error signal and generating a detected energy error signal output, a peak-to-peak detector for receiving the error signal and generating a peak-to-peak error signal output, a controller responsive to the detected energy signal output and the peak-to-peak error signal output for generating correction signals, the predistortion circuit being responsive to at least a portion of the correction signals for modifying its output to the main amplifier, and a control circuitry for receiving the input signal and delivering it to the predistortion circuitry, the control circuitry being responsive to at least a peak-to-peak value of the error signal for reducing the peak-to-peak signal value of the error signal.
In specific embodiments, the main amplifier is a broad band RF amplifier operating in a Class AB mode, and the feedback loop controller is responsive to the energy measurements for iteratively adjusting the signal correction circuitry amplitude and phase.
In another aspect, the invention relates to a method for correcting distortion in an amplified signal output from a main amplifier of an amplifier arrangement, the main amplifier being part of an amplifier arrangement and the arrangement having an input and an output. The method combines predistortion signals with the input to the amplifier arrangement for delivery to the main amplifier. The method features generating the predistortion signals in a predistortion circuit derived from an input to the amplifier arrangement, generating an error signal from an error signal derived from the output of the main amplifier representing distortion energy, generating a peak-to-peak error signal from the error signal representing peak-to-peak signal values, and iteratively and successively correcting, using a digitally controlled processor responsive to at least the energy error signal and a peak-to-peak error signal, gain and phase adjustments in and prior to the predistortion circuitry.
The method in a particular embodiment features comparing a signal derived from the output of the main amplifier and a signal derived from the input to the amplifier arrangement for generating a difference signal, and measuring peak-to-peak levels of a signal derived from the different signal for generating the peak-to-peak error signal.
The invention thus advantageously quickly predistorts, in a dynamic manner, the input signal to a class AB or other distorting main amplifier and thereby causes a substantial linearization of the input-output characteristics of the entire amplifier arrangement in a short period of time.
REFERENCES:
patent: 4639938 (1987-01-01), Kennett
patent: 5598127 (1997-01-01), Abbiati et al.
patent: 5742201 (1998-04-01), Eisenberg et al.
patent: 5892397 (1999-04-01), Belcher et al.
patent: 6008698 (1999-12-01), Dacus et al.
Fish & Richardson P.C.
Nguyen Patricia
Pascal Robert
Powerwave Technologies Inc.
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