Abstract:
An apparatus and method reduce out-of-band frequency components of an amplified, multichannel RF signal able to have at least two frequency bandwidth limited signal channels, each channel having a respective carrier frequency which is not known in advance. The channels can, for example, be dedicated to CDMA signals. Each channel of the amplified RF signal has both in-band frequency components and out-of-band frequency components. The apparatus and method feature a network for amplifying an input signal for producing the amplified RF signal, the network having adjustable electrical characteristics, and a control system connected to the network for locating a frequency within the bandwidth of one of the channels of the amplified RF signal and for detecting energy in the out-of-band frequency components for the one located channel for producing control signals relating to the energy in that one channel. As a result of adjusting the electrical characteristics of the network, the energy in the out-of-band frequency components for that channel, and all other channels of interest, can be reduced.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to amplification systems and more particularly to methods and apparatus for reducing distortion in amplifiers used in such systems. 
     As is known in the art, amplifiers have a wide variety of applications. Amplifiers can be biased to operate in one of a number of so-called Classes. When biased to operate in Class A, the amplifier provides a linear relationship between input voltage and output voltage. While operation in Class A has a wide range of applications, when higher power output and efficiency are required or desired, the amplifier is sometimes biased to operate in Class A/B. When biased to operate in Class A/B, however, the Class A/B amplifier power transfer curve  10  is less linear than for Class A amplifiers, illustrated in FIG. 1 by trace  14 . To increase efficiency, communication systems often operate amplifiers in the non-linear region  12 . This practice, however, does introduce amplitude and phase distortion components into the output signal produced by the amplifier. 
     As is also known in the art, most communication systems have FCC allocated frequency bandwidths  18  (that is, in-band frequencies) centered about a carrier frequency  20  as shown in FIG.  2 A. For example, a CDMA (Code Division Multiple Access) communication system signal has a predefined bandwidth of 1.25 MHz. Different CDMA communication channels are allocated different bands of the frequency spectrum. Amplifiers are used in such systems, and are frequently biased to operate in Class A/B. Referring to FIG. 2B, signal processing such as amplification by an amplifier operating in the non-linear region  12  (FIG. 1) can produce distortion frequency “shoulders”  22   a - 22   b  outside a signal&#39;s allocated bandwidth  18 . (These are called out-of-band frequencies.) These distortion frequency components  22   a - 22   b  can interfere with bandwidths allocated to other communication signals. Thus, the FCC imposes strict limitations on out-of-band frequency components. 
     Many techniques exist to reduce out-of-band distortion. One such technique is shown in FIG. 3 where a predistortion unit  24  is fed by a signal  25  to be amplified. The predistortion unit  24  has a power transfer characteristic  24   a  (FIG. 1) and compensates for distortion introduced by subsequent amplification in Class A/B amplifier  26 . More particularly, the predistortion unit  24  transforms electrical characteristics (for example, gain and phase) of the input signal such that subsequent amplification provides linear amplification to the phase and frequency characteristics of the input signal. The predistortion unit  24  is configured with a priori measurements of the non-linear characteristics of the Class A/B amplifier. Unfortunately, the amplifier characteristics (amplification curve  10  with region  12  of FIG. 1) change over time and temperature making effective predistortion more difficult. For example, as the temperature of the amplifier increases, its non-linear region  12  may become more or less linear, requiring a compensating change in the transform performed by a predistortion unit  24 . Some adaptive predistortion systems use look-up tables to alter predistorter characteristics based on environmental factors such as temperature. These look-up tables include predetermined predistorter control settings for use in predetermined situations. However, environmental factors alone do not determine the alterations in an amplifier&#39;s characteristics. Thus, over time, amplifier characteristics vary unpredictably due to aging of amplifier components. 
     Another approach to reduce amplifier distortion is to use feedforward compensation, as shown in FIG.  4 . Here, a feedforward network  31  is included for reducing out-of-band distortion. The feedforward network  31  includes a differencing network or combiner  30 , a main amplifier  33  operating as a Class A/B amplifier, an error amplifier  32 , delay circuits  28  and  28   a,  and a combiner  29 . The differencing network  30  produces an output signal representative of the difference between a portion of the signal fed to the amplifier  33  operated Class A/B and the signal fed to the amplifier  33  prior to such amplification. The frequency components in the differencing network  30  output signal are, therefore, the out-of-band frequency components  22   a - 22   b  introduced by amplifier  33 . Amplifying and inverting the output produced by the differencing network  30 , by error amplifier  32 , produces an out-of-band correcting signal. More particularly, the combiner  29  combines the correcting signal produced by differencing network  30  and amplifier  32 , with the delayed signal output of amplifier  31  thus reducing the energy in the out-of-band frequencies  22   a - 22   b  (FIG. 2B) of the signal output by amplifier  33 . Feedforward network  31  includes delay line  28  to compensate for the delay in error amplifier  32 . It should be noted that minute differences in timing between these elements can impair the effectiveness of a feedforward system. While a manufacturer can carefully match components prior to shipment, as feedforward components age, the correcting signal and processed signal can become mistimed if not properly compensated. 
     SUMMARY OF THE INVENTION 
     The invention relates to an apparatus and method for reducing out-of-band frequency components of an amplified, multichannel RF signal able to have at least two frequency bandwidth limited signal channels, each channel having a carrier frequency which is not known in advance. Each channel of the amplified RF signal has both in-band frequency components and out-of-band frequency components. The apparatus features a network for amplifying an input signal for producing the RF signal, the network having adjustable electrical characteristics, and a control system connected to the network for locating a frequency within the bandwidth of one of the channels of the RF signal and for detecting energy in the out-of-band frequency components in the one located channel for producing a control signal related to the energy in the out-of-band frequency components of that one channel. The control signal is coupled to the network to adjust the electrical characteristics of the network to reduce the energy in the out-of-band frequencies of all the channels. 
     In a preferred embodiment of the invention, the network has a predistorter having adjustable characteristics controlled by the control signal. In another aspect, the network can include a power amplifier having adjustable characteristics controlled by the control signal. In this instance, at least one adjustable characteristic is a bias point parameter of the amplifier. The network, in other aspects, can include a feed forward network and the signal can be, for example, a multichannel CDMA signal, having well known characteristics. 
     The method of the invention features locating an in-band frequency component of one of the channel signals having out-of-band frequency components of the RF signal, detecting energy at frequencies which are at a predetermined offset from the located in-band frequency component, and adjusting network electrical characteristics to reduce the out-of-band frequency energy of the located channel. Thereby, out-of-band frequency components of the other out-of-band channel signals are reduced. 
     In particular aspects, the method features measuring the energy at a first frequency, measuring the energy at a second frequency, and determining whether the energy measured at the second frequency exceeds the energy measured at the first frequency by more than a selected threshold. In particular aspects, the method heterodynes the multichannel signal having out-of-band frequencies to baseband. 
     In one particular aspect of the invention, the apparatus includes a predistorter connected to receive an amplified, multichannel input signal and having its output coupled to a power amplifier. The predistorter has a nonlinear output signal versus input signal transfer level characteristic which can be selectably adjusted in accordance with an out-of-band feedback control signal provided by a feedback loop. The result is a substantially linear amplifier output versus input signal transfer characteristic across the multiple channels. The feedback loop, in this embodiment, features a control signal connected to the power amplifier output for locating the carrier frequency of one of the channel signals of the input signal and for producing a feedback control signal related to the energy in the distortion frequency components outside the bandwidth at that located channel signal. The feedback control signal is coupled to the predistorter for adjusting the electrical characteristics of the predistorter to reduce the energy in the out-of-band frequency components of the located channel in the power amplifier output. As a result, the out-of-band frequency components of each other channel of the input signal are reduced. 
     The method and apparatus thus advantageously reduce distortion in a multichannel amplified RF signal, for example, where a Class A/B RF amplifier is used for efficiency and high power output. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Reference is made to the following drawings, in which: 
     FIG. 1 is a graph illustrating amplifier output regions according to the PRIOR ART; 
     FIGS. 2A and 2B are diagrammatical sketches of a signal having in-band and out-of-band frequency components according to the PRIOR ART; 
     FIG. 3 is a diagrammatical sketch of an amplification system according to the PRIOR ART; 
     FIG. 4 is a diagrammatical sketch of another amplification system according to the PRIOR ART; 
     FIG. 5 is a diagrammatical sketch of an amplification system having a predistorter with adjustable electrical characteristics according to the invention; 
     FIGS. 6A-6C are diagrammatical sketches of frequency spectra of signals produced in the amplification system of FIG. 5; 
     FIG. 7 is a diagrammatical sketch of the amplification system of FIG. 5, a control system of such amplification system being shown in more detail; 
     FIG. 8 is a flow chart of the process used by the control system in FIG. 7 to produce control signals based on energy in out-of-band frequency components; 
     FIG. 9 is a flow chart of the process used by the control system of FIG. 7 to determine frequency components of a signal produced in the amplification system of FIG. 7; 
     FIG. 10 is a diagrammatical sketch of an amplification system having a predistorter with adjustable electrical characteristics according to another embodiment of the invention; 
     FIG. 11 is a diagrammatical sketch of a mixer configured as a four quadrant multiplier biased into a linear operating region, such mixer being adapted for use in the amplification system of FIG. 10; 
     FIG. 12 is a diagrammatical sketch of the amplification system of FIG. 10, a control system of such amplification system being shown in more detail; 
     FIG. 13 is a diagrammatical sketch of an amplification system according to another embodiment of the invention, such amplification system having a cancellation network configured to increase dynamic range of out-of-band signal components; 
     FIG. 14 is a diagrammatical sketch of an amplification system, such amplification system having a cancellation network configured to increase dynamic range of out-of-band signal components according to another embodiment of the invention; 
     FIG. 15 is a diagrammatical sketch of an amplification system, such amplification system having a cancellation network configured to increase dynamic range of out-of-band signal components according to another embodiment of the invention; 
     FIG. 16 is a diagrammatical sketch of an amplification system having adjustable characteristics being controlled by the control system of FIG. 5 according to the invention; 
     FIG. 17 is a diagrammatical sketch of an amplifier having adjustable characteristics being controlled by the control system of FIG. 10 according to the invention; 
     FIG. 18 is a diagrammatical sketch an amplification system having a feedfoward network with adjustable electrical characteristics controlled by the control system of FIG. 5 according to the invention; 
     FIG. 19 is a diagrammatical sketch of an amplification system having a feedforward network having adjustable characteristics being controlled by the control system of FIG. 10 according to the invention; 
     FIG. 20 is a diagrammatical sketch of an amplification system having a control system adapted to control the adjustable electrical characteristics of the feedforward network of FIG. 18; 
     FIG. 21 is a diagrammatical sketch of an amplification system having a control system controlling multiple components according to the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 5, an amplification system  100  is shown amplifying an input signal fed thereto on a line  101 . More particularly, the system  100  provides an amplified output signal on a line  103 . The system  100  includes an amplifier  102 , a control system  104 , the details being shown in FIG.  7 ), and a predistorter  105 , all arranged as shown. The input signal on line  101 , in this embodiment, is a received CDMA signal. The received signal has a predetermined, a priori known, bandwidth “BW”; however, the carrier frequency f c  of such received signal may be any one of a plurality of available carrier frequencies and is not known in advance. 
     The amplifier  102  is biased to Class A/B, and thus has a non-linear amplification characteristic. Therefore, non-linear amplification by the amplifier  102  will introduce amplitude and phase distortion into the amplified output signal. Thus, passing a signal through the amplifier  102 , operating with a non-linear output power versus input power transfer characteristic produces frequency components outside the bandwidth BW (that is, out-of-band frequency components). 
     In this illustrated embodiment, however, the output signal produced by the amplifier  102  is fed, using the control system  104 , to the predistorter  105 . The predistorter  105  has adjustable electrical characteristics, for example, adjustable bias characteristics and parameters. The predistorter  105  receives the input signal on line  101  and the output of the control system  104 , over line(s)  109 . The output of the predistorter  105  is fed to the amplifier  102 . The predistorter  105  has a non-linear gain versus input signal level characteristic selected in accordance with an out-of-band feedback control signal (the signals over line(s)  109 ) to enable the amplification system  100  to provide a substantially linear amplifier output power versus input signal power transfer characteristic to the input signal  101 . Thus, in the steady-state, the output on line  103  is an amplification of the input signal on line  101  without, or with reduced, out-of-band frequency components. As will be described, any out-of-band frequency energy in the output signal on line  103 , as the result of drift in the amplifier  102 , for example, is detected and is fed to the predistorter  105  using the control system  104  to enable the system  100  to again produce, in the steady-state, an output signal on line  103  with little, or no, out-of-band frequency components. 
     More particularly, a feedback loop  107  is provided wherein the control system  104  receives the output of the amplifier  102  and produces the feedback control signal on line  109  for the predistorter  105 . The control system  104  analyzes the signal produced by the amplifier  102  to locate a carrier frequency having the bandwidth BW of the received signal, here the carrier frequency of the input signal on line  101 , and to produce the feedback control signal on line  109  related to the energy in the distortion frequency components (that is, the energy out of the bandwidth BW) detected in the output signal on line  103 . In the illustrated embodiment, the control system  104  measures the energy of the distortion frequency components by measuring energy at a frequency or frequencies offset from the carrier frequency (for example, at frequencies 800 KHz and 1.25 MHz from the carrier frequency), the measurement frequency(s) being outside of the bandwidth of the input signal. The feedback control signal on line  109  is coupled to the predistorter  105  for adjusting characteristics of the predistorter  105  (for example gain and phase, or predistorter bias points) and thereby null (that is, reduce) the energy in the out-of-band signals on line  103 . 
     Referring again to FIG. 5, in one embodiment, the control system  104  heterodynes to baseband the amplified signal on line  103  with the carrier frequency of the received signal and measures the energy in the output signal on line  103  at one or more predetermined offsets from the carrier frequency. Referring also to FIGS. 6A-6C, the frequency spectrum  18  of the input signal on line  101  is shown in FIG.  6 A. The frequency spectrum of the output signal on line  103 , in a non-steady-state condition, that is before correction, is shown in FIG. 6B to have out-of-band frequency components  22   a,    22   b  resulting from the non-linear operation of amplifier  102 . The frequency spectrum resulting from heterodyning to baseband the output signal on line  103  with the carrier frequency of the input signal is shown in FIG.  6 C. 
     As shown in FIG. 6A, the input signal on line  101  is centered about carrier frequency f c  and has an a priori known bandwidth BW. In the case of a CDMA signal, BW will be 1.25 MHz. As shown in FIG. 6B, amplification by amplifier  102 , prior to steady-state, introduces out-of-band distortion components  22   a  and  22   b  to the output signal on line  103 . The control system  104  heterodynes the amplified signal on line  103  (FIG. 6B) to baseband, thus centering the signal about DC (zero frequency) as shown in FIG.  6 C. After heterodyning, the out-of-band distortion components appear at frequencies greater than an offset of BW/2 from DC or in the case of a CDMA signal at frequencies above 0.625 MHz. The control system  104  produces control signals based on amount of energy measured at, for example, 0.625 MHz or other predetermined frequency offsets. That is, control system  104  produces control signals based on the amount of out-of-band energy in components  22   a,    22   b.    
     More particularly, and referring to FIG. 7, in one embodiment, the control system  104  is shown in more detail and includes a microcontroller  124  that controls a frequency synthesizer  126  to heterodyne (here, to bring down to baseband) the signal produced by amplifier  102  on line  103 . A mixer  106  receives the output of the frequency synthesizer  126  and the amplifier output on line  103 , and delivers its output to a bandpass filter  108  that eliminates in-band frequency components of the heterodyned signal to enhance resolution of the out-of-band distortion components. An amplifier  110  receives the filtered signal and provides its amplified output to an analog-to-digital converter  120 , the digital output of which is delivered for digital signal analysis by a digital signal processor (DSP)  122 . The DSP is specially configured to effect a spectrum analysis on the digital input signal from the analog-to-digital converter  120 . The microcontroller  124 , executing firmware instructions  128 , queries the DSP  122  for the energy measurements at predetermined offsets. The microcontroller  124  analyzes past and present energy measurements to produce control signals over lines  109  that adjust the electrical characteristics, for example, a phase and gain, of the predistorter  105 . 
     Referring also to FIG. 8, in operation, the microprocessor instructions  128  continuously monitor distortion levels by querying the DSP  122  for measurement data describing the energy at offsets from the now baseband signal center frequency (step  132 ). After determining whether the current measurement process is operating satisfactorily (step  134 ) (that is, distortion is reduced to predefined minimum levels for the system), by analyzing past and current measurements, the microprocessor produces the control signals on lines  109  (step  136 ) that reduce or maintain the distortion level. The control signals on lines  109  adjust different electrical characteristics, for example, the phase and amplitude characteristics of the predistorter  105  or bias characteristics of the predistorter, to null any out-of-band frequency components  22   a,    22   b  in the output signal on line  103 . It should be noted that reducing distortion may require dynamic experimentation with different combinations of control signals before identifying a set of control signals that best minimize distortion. 
     Referring again to FIG. 7, in addition to generating control signals on lines  109 , the microcontroller  124  executes instructions that control the frequency fed to mixer  106  by frequency synthesizer  126 . Referring to FIG.  9 , in operation, the microcontroller  124  uses the frequency synthesizer  126  to incrementally sweep through the frequency spectrum to find the carrier frequency f c . The microcontroller  124  initiates the search for the carrier frequency f c  by setting the frequency synthesizer  126  to produce a low frequency (step  138 ). The microcontroller  124  queries the DSP  122  for a measure of the carrier energy at this frequency (step  140 ). This corresponds to a DC measurement of the signal output of mixer  106 . The microcontroller  124  compares the energy measurement with the measurement of energy at a previously selected carrier frequency produced by the frequency synthesizer (step  142 ). If the comparison (step  142 ) indicates a steep rise (step  144 ) in energy. characteristic of a signal having a predefined bandwidth, the microcontroller  124  can freeze the frequency synthesizer at this or a nearby frequency. If the comparison (step  142 ) does not indicate the presence of a signal (that is, very little energy in either the present or previous energy measurement), the microcontroller  124  will increment the frequency produced by the frequency synthesizer  126  (step  143 ). In a typical CDMA system, the frequency synthesizer will be incremented in 50 KHz steps. (Other, or random, search patterns can also be used.) Finding the carrier frequency usually needs only to be performed upon start-up as an allocated frequency usually remains constant. The search can be periodically repeated, however, to ensure proper calibration. The results of the search can be stored to obviate the need for searching each time the equipment is start-up. The instructions of the microcontroller  124  can be altered to search for different signals other than CDMA signals. 
     Referring to FIG. 10, in another particular embodiment, the control system  104 , here designated as control system  104 ′, has an alternate configuration for reducing distortion in the amplification system. Control system  104 ′ receives both the original input signal on line  101  (FIG. 6A) and the amplifier output signal on line  103  having, in the non-steady-state condition, distortion components introduced by the amplifier  102  (FIG.  6 B). By mixing the original input signal on line  101  with the signal on line  103  which can have distortion components, the control system  104 ′ quickly heterodynes the amplified signal on line  103  to baseband without scanning the frequency spectrum to determine the input signal&#39;s carrier frequency f c . That is, instead of searching for the carrier frequency of the input signal on line  101 , the input signal itself serves as the signal for a mixer  106 ′ (FIG. 12) in a homodyne arrangement. In any event, the control system  104 ′ thus locates a frequency within the bandwidth (BW), here the center frequency of the received signal, by automatically homodyning, mixing, and filtering as provided by mixer  106 ′ and low pass filter  108  (FIG.  12 ). Mixing a signal in this manner, however, imposes a constraint upon the mixer used by the control system. 
     More particularly, many mixers depend on a threshold amount of energy to multiply signals without introducing distortion. For example, diode mixers introduce distortion into an output signal if the energy in either of its two input signals falls below a level needed to keep the mixer diodes operating in their linear region. Many signals, including CDMA signals, sometimes fail to provide this minimum energy, thereby introducing distortion. 
     Referring to FIG. 11, many mixers, such as a Gilbert Cell mixer, remain linear even when the input signals have little energy. As shown, Gilbert Cell mixer  106 ′ includes active devices configured as a four quadrant multiplier biased into a linear operating region. These active devices form a differential amplifier  176   a - 176   b  that drives dual differential amplifiers  172   a - 172   b  and  174   a - 174   b.  The mixer output, on a line  177 , is thus available for filter  108 . 
     Referring to FIG. 12, an amplification system  101 ′, using a homodyning mixer  106 ′, is shown for measuring the energy in frequency bands of a received signal, such signal having an allocated frequency bandwidth and a carrier frequency. The system  101 ′ includes mixer  106 ′ having active devices configured as a four quadrant multiplier biased into a linear operating region for enabling the mixer to handle low input signal levels. The mixer  106 ′ receives a pair of signals, one of the signals being the received signal (that is, the original input signal on line  101 ) and the other signal, on line  103 ′, being a portion of the output on line  103  from a coupler  103 ″. The output of mixer  106 ′ is processed, as was the output of mixer  106  (FIG. 7) by the remaining components of the control system  104 ′ which detect energy in a frequency band at a predetermined offset from the baseband carrier (center) frequency as described above in connection with FIG.  7 . Note that in an alternate embodiment of those illustrated in FIGS. 7 and 12, the DSP  122  (and its related circuitry) can be replaced by bandpass filters, each adapted to pass signals only at selected offsets from the center frequency. Other circuitry would measure the energy from each filter and provide that data to the microprocessor. 
     Referring now to FIG. 13, the use of the four-quadrant linear multiplier (that is, mixer)  106 ′ can pose a dynamic range problem. However, cancelling in-band frequencies to isolate the out-of-band distortion components can increase the dynamic effective range of the mixer. Thus a cancellation network  146 , under microprocessor control, performs this isolation function, thereby in effect, increasing the dynamic range of the mixer. The operation and structure of cancellation network  146  is illustrated in FIGS. 14 and 15. 
     Referring now to FIG. 14, one embodiment of the cancellation network  146  is shown which uses a voltage controllable phase shifter  150  and a voltage controllable attenuator  148  to substantially cancel in-band frequency components in the amplifier output signal on line  103 . The microcontroller  124  adjusts the phase shifter  150  and attenuator  148  to modify the phase of a sample of the original input signal on line  101  (FIG. 6A) by 180° and thereby null the in-band frequency components of the signal on line  103 , from a coupler  149   a,  as they are coupled to the output of variable attenuator  148  using a coupler  149 . The microcontroller  124  can repeatedly adjust the phase shifter  150  and attenuator  148  until the in-band&#39;s signal cancellation is at maximum level. 
     Referring to FIG. 15, an alternative embodiment of the cancellation network  146 ′ uses an automatic gain control element (AGC)  158 , as is well known in the field, to effectively increase the dynamic range of the mixer. The AGC  158  controls an amplifier  156  to hold the local oscillator (LO) input of mixer  106 ′ over a line  159  constant so that the down converted output of the mixer  106 ′ is a linear function of the input over line  159   a  and no longer a multiplicative function of the inputs to the cancellation network  146 ′. The AGC  158  also matches the outputs of amplifiers  154  and  156 . Phase and gain network  160  enables the microcontroller  124  to adjust the signal fed into mixer  106 ′ and thereby increase dynamic range. 
     The control systems  104  and  104 ′ can control a wide variety of amplification system networks having adjustable characteristics other than predistorter  105 . For example, in particular, referring to FIGS. 16 and 17, corresponding to FIGS. 5 and 12, respectively, the control system  104 ,  104 ′ can control the amplification characteristics of amplifier  102  by altering the amplifier&#39;s bias point(s). While a predistortion circuit is not shown, it can be advantageously employed to further reduce unwanted distortion. As described in co-pending U.S. application Ser. No. 09/057,380, filed Apr. 8, 1998, incorporated herein, by reference, in its entirety, over long periods of time (for example, hundreds of hours) amplifiers frequently exhibit a drift in operating bias current. Amplification by an amplifier experiencing drift can introduce out-of-band distortion components into a signal. The control system  104 ,  104 ′ can generate control signals that control the bias of the amplifier based on out-of-band frequency energy to compensate for amplifier bias current drift. This method of compensation is particularly useful in connection with MOSFET devices, and in particular lateral MOSFETS where the gate bias is critical. 
     In addition, referring to FIGS. 18,  19 , the control system  104 ,  104 ′ can also reduce distortion by adjusting the characteristics of a feedforward network  160 . As noted with regard to FIGS. 16 and 17, a predistorer circuit can be advantageously used to further reduce unwanted distortion components under the control of, for example, a microprocessor. Referring to FIG. 20, an amplification system  161  is illustrated which reduces out-of-band frequency components of an input signal over line  101  which after passing through Class A/B amplifier  102  has both in-band frequency components and out-of-band frequency components. The amplification system includes a feedfoward network  160  having a combiner  162  that receives a pair of signals: the first signal (FIG. 6A) over a line  161  from delay element  161   a  having the in-band frequency components and a second signal (FIG. 6B) over a line  163  coupled to the amplifier  102  output, that has both in-band and out-of-band frequency components. Optimally, the combiner  162  subtracts the first signal from the second signal to produce a signal having only out-of-band frequency components. A variable gain-phase network  164 ,  166  receives the output of the combiner  162  and applies its output to an error amplifier  168 . Amplifier  168  amplifies the out-of-band frequency components. A second combiner  170  adds the output of amplifier  168  (that is, a signal having out-of-band distortion components shifted by 180°) to the signal having both out-of-band and in-band frequency components from a delay  169 . Ideally, combiner  170  produces an amplified signal substantially free of out-of-band distortion components as is well known in the field. 
     However, as mentioned above, changes in the feedforward network  160  components and the amplifier  102 , over time, can reduce the effectiveness of the feedforward network  160  in reducing distortion. Thus, the output of combiner  170  is coupled, in part, by a coupler  171  to a feedback loop having control system  104 . The control system  104 , described previously, detects energy in the out-of-band frequency components and produces a feedback control signal related to the measured energy in those out-of-band frequency components. The feedback control signals are coupled to and adjust, in this illustrated embodiment, the characteristics of the gain-phase network  164 ,  166  in accordance with out-of-band frequency components. 
     As noted above, a predistortion circuit, as illustrated in FIGS. 5 and 12, can be advantageously used to further reduce unwanted distortion components under the control of, for example, the microprocessor  122  of FIG.  20 . In addition, regarding both the illustrative examples of FIGS. 19 and 20, the microprocessor  122  can be used to adjust bias parameters of a predistorter, main amplifier  102 , gain-phase circuities or other devices to advantageously reduce distortions in the amplified output signal. 
     Referring now to FIG. 21, the control system  104  or  104 ′ can control multiple components of an amplifier system to produce an overall reduced distortion amplified output signal. As shown, the control system  104  controls the predistorter  105 , the bias point of the main amplifier  102 , and the characteristics of the feedforward network  106 . Thus, different individual amplification system networks (for example, the predistorter) combine to form a larger network (that is, predistorter and amplifier and feedforward network) having adjustable characteristics adjustably controlled by the control system in response to detected energy in the out-of-band frequency components. 
     Throughout this discussion it has been implicitly assumed that the input signal on line  101  was a single channel, bandwidth limited signal having a carrier frequency which was not known in advance. The distortion compensation circuitry described in connection, for example, with FIGS. 7 and 20, can also be employed when the input signal is a multi-channel signal, each channel having a bandwidth limited signal. When used with multi-channel inputs, the compensation system finds one channel, and minimizes the out-of-band frequency components for that channel as if the other channel(s) did not exist. Thereafter, the settings used for the one channel are used for all of the channels. 
     Thus the operative structure and method of operation of the FIGS. 7 and 20 embodiments remain the same. It further appears not to matter which channel was minimized so that the frequency generator  126  search pattern, established by microprocessor  124  can be the same as for a single channel, that is, for example, can be a linear or a random sweep. 
     Additions, subtractions, and other modifications of the disclosed embodiments will be apparent to those practiced in the field and are within the spirit and scope of the appended claims.