Abstract:
The invention relates to an amplifier arrangement for amplifying an input using a distorting main power amplifier such as one operating as a class AB amplifier. The method and apparatus add to the input signal to the main amplifier a predistortion signal intended to compensate for the distortion added by the main power amplifier. In accordance with the invention, the input to the predistortion circuit is modified by measuring peak-to-peak signal values of the distortion in the output of the main power amplifier. The peak-to-peak values are measured in a signal derived by comparing an output derived from the main amplifier with an output derived from the input signal. A digitally controlled processor iteratively modifies various phase and gain controls to adjust the output of the amplifier.

Description:
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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects, features, and advantages of the invention will be apparent from the following description; taken together with the drawings in which: 
     FIG. 1 is a schematic block diagram of a preferred embodiment of the predistortor amplifier and control circuitry in accordance with the invention; and 
     FIG. 2 is a flow chart illustrating operation of the digitally controlled processor in accordance with a preferred embodiment of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, an amplifier arrangement  10  has a predistortion circuitry  12  ,a main power amplifier  14 , and a control circuitry  16 . Amplifier  14  is typically a high power class AB amplifier whose output over a line  18  can be either used directly, or can be the input to a feed forward distortion cancellation circuitry such as that described in my U.S. Pat. No. 5,796,304, entitled BROADBAND AMPLIFIER WITH QUADRATURE PILOT SIGNAL, the contents of which are incorporated herein by reference. 
     The input to the amplifier arrangement, over a line  20 , is split (or sampled) for a number of purposes. First, a line sampling coupler  22  directs part of the input signal to a delay element  24 . The output of the delay element is directed to a comparison device  30 . 
     The remaining input signal over line  36  is received by a controlled gain circuitry  38  and then a controlled phase circuitry  40 , and the output of circuitry  40  is sampled by another coupler  42 . The output of coupler  42  is received by a signal splitter  40  which divides the signal equally for output over lines  43  and  44 . The remaining input signal from line  45  is directed to a delay element  46  of predistortor  12 . 
     The output of splitter  40  over line  43  is directed to a delay element  48 , the output of which is received by comparison circuitry  50 . The other output of splitter  40  over line  44  is received by a gain controlled amplifier  52 , the output of which is directed to auxiliary amplifiers  53 ,  54 . The output of amplifier  54  is received by a controlled gain circuitry  60 . The output of controlled gain circuitry  60  is delivered to a controlled phase circuitry  62  the output of which is compared with the signal from delay element  48  in comparison circuitry  50 . The difference between the two signals, representing distortion components introduced primarily by amplifier  54 , is passed through a linear amplifier  64  to a controlled gain circuit  66 . The output of the control gain circuit  66  is received by a controlled phase circuit  68 . The output of the controlled phase circuit  68  is delivered to a coupler  70  which combines the output of the phase circuit  68  with the output of the delay element  46 , to generate an input, over a line  72 , to the main power amplifier  14 . 
     The output of the main power amplifier  14 , over a line  74 , is sampled by a coupler  76  and the sampled output signal is compared (differenced) to the output of the delay  24  available over a line  78 , to generate a distortion error signal on a line  80 . The distortion error signal over line  80  is sampled by coupler  81 , and is amplified using amplifiers  82  and  84 , connected in series with each other, and the output of amplifier  84  is detected, here using a Schottky diode  86  to measure the energy in the signal, for input to a digital controller  90 . The digital controller  90  outputs digital signals over lines  92   a  and  92   b  to control digital to analog (D/A) converters  100   a  and  100   b  respectively. The analog outputs of the digital to analog converters are directed to control various gain and phase elements of the predistortor circuitry  12  and a Cartesian loop circuit  110 . 
     In order to operate the adaptive predistortion circuitry in a transient environment, the invention provides for speeding up the effective response time of the control process of the control loop so that tuning of the predistortion circuit can be quickly evaluated. This allows the adaptive predistortion circuit to be effective in the transitory environment. 
     There is known in this field to use a Cartesian loop for controlling gain and phase elements, of the various circuitries. The Cartesian loop senses both amplitude and phase, and can operate quite quickly. The Cartesian loop couples over a coupler  120  to the distortion error signal over line  80 , and over a coupler  122  to the delayed input signal provided by delay  24 . Both signals are amplified using amplifiers  124  and  126  respectively, and are respectively split using a splitter  128  and a quadrature splitter  130 , the output being applied to mixing elements  132  and  134 . The output of the mixing elements are respectively directed to control amplitude and phase adjustments of controlled circuitries  38  and  40  and are also passed to peak detectors  150 ,  152 . 
     This system operates quite fast, so that for signals spaced within about 100 kilohertz, the loop performs cancellation of the distortion products coupled from line  120 . The limitation in the circuit is the time delay of the system, since the circuit uses a feed back technology. The signals which are left at the cancellation point, over line  80 , are the high frequency distortion components. Ultimately, these are fed to the error amplifier when a feed forward system is used as described in U.S. Pat. No. 5,796,304. 
     In a Cartesian loop such as that illustrated in FIG. 1, the absolute carrier null is a function of the balance of the phase detector elements. This is true whether the phase detector elements are diode mixers, or active mixers. A conventional detector can be used to allow the microprocessor or controller  90  to tune for the lowest possible power level at the cancellation point, line  80 . This process can be accomplished at a very low speed since any drift will be due to temperature and aging as described in co-pending patent application Ser. No. 09/057,332. Power can be detected, as described in that previous application, using a Schottky diode arrangement such as elements  82 ,  84 , and  86 , illustrated in FIG.  1 . In practical circuits, nulls of 25dB can be obtained without offset correction and it is expected that this value can be increased to 30-40dB depending upon the spacing between tones (amplitude and phase linearity) of the main amplifier  14 . 
     In accordance with the present invention, however, it is the high frequency components which are left on line  80  which are further removed, using the microprocessor/controller  90  to adjust the predistortion circuitry to minimize the level of these distortion components. The distortion and remaining main signals over line  80  are in the form of an RF envelope which represents the instantaneous error signal from the main amplifier. The apparatus of the invention detects this error envelope and adjusts the predistortion circuitry for a minimum level, which is the optimum tuning point for the predistortion circuitry. 
     To implement this aspect of the system peak detectors  150 ,  152 , operating at high speed, measure the value of the distortion signal peaks and provide that information to the controller  90 . The lower the peak-to-peak voltage, the lower is the distortion in the main amplifier. The lower frequency components, which could confuse the distortion detector, are eliminated by the very fast response time of the loop. That portion of the detected envelope which can not be detected, would be in the RF form and, in a feed-forward amplifier system, would be sent to the error amplifier for RF cancellation. Thus, in accordance with the invention, a peak-to-peak detector is used to detect distortion power in addition to (or instead of in some systems) the Schottky diode  86  which is more of an RMS device. This system is, of course, applicable to both feed forward and conventional predistortion only single channel amplifiers. 
     In operation, the microprocessor  90  can first detect the energy (corresponding to RMS power) in the output of comparison circuitry  30  over line  80  as sampled by coupler  81  and can act to minimize that value by controlling, through digital to analog converters  100   a,    100   b,  the amplitude and phase of elements  38 ,  40  and  66 ,  68 . This is done in an iterative process as illustrated in the flow diagram of FIG. 2, first adjusting elements  66 ,  68  and thereafter adjusting elements  38 ,  40 . Note that the other gain and phase controlled elements  52 ,  60 , and  62  can also be controlled by the microprocessor if desired or set manually. 
     In the preferred embodiment of the invention, referring to FIG. 2, the controller first checks to determine whether amplitude and phase elements  66  and  68  should be adjusted. If they are to be adjusted, as indicated at decision point  170 , the gain and phase are adjusted at  172  and  174 . The elements controlled by the Cartesian loop, that is gain element  38  and phase element  40 , are considered next for adjustment at  176 . If elements  38 ,  40  are to be adjusted, then the gain is adjusted at  190  and the phase is adjusted at  192 . For each iteration of the control loop in FIG. 2, new values for the distortion signals are read at  178 . Once the predistortor has been adjusted, then further adjustments within the predistortor are not made and the signals over line  80  contain primarily the high frequency distortions. These are then peak-to-peak detected using the peak detectors  150 ,  152  and in response to the signal values provided by those detectors, the controller  90  makes a decision whether to further adjust gain element  38  and phase element  40  at decision  176 . In this manner, the peak to peak value of the high frequency components is reduced to a minimum. 
     Thus the controller  90 , operating substantially in a feed back loop environment, iteratively adjust the varying control elements to which it is connected and determines whether adjustment improves, has no effect, or renders worse, the error products such as the output of the first loop over line  80 . The object is to minimize the distortion at the output of the linear amplifier  14  upon reaching a null. A typical control processor is the Motorola model MC68HC11E9 which allows for a correction sequence approximately every 50 milliseconds. 
     In this manner, the Cartesian loop operates at a speed great enough that it does not cause the predistortor settings to be changed in order to determine whether, in a transient environment, the settings still represent a proper null. 
     Additions, subtractions, and other modifications of the described and preferred embodiment of the invention will be apparent to those practiced in this field and are within the scope of the following claims.