Abstract:
High efficiency and high power output are attained in a multi-tone FF RF amplifier by eliminating fundamentals in a first loop and then operating on the IMD products only in a second loop. Since lower power tones are adjusted in the IMD product reduction, less complex, lower power processing circuitry is required and higher efficiency is realized since both amplifiers add in-phase and approximately equally at the RF output.

Description:
FIELD OF THE INVENTION 
     This invention relates to feed forward amplifiers for achieving low distortion during signal amplification and in particular to high efficiency feed forward RF amplifiers. It particularly concerns a feed forward amplifier having a closed loop control of the amplifier to counteract time and temperature variations. It specifically involves RF (radio frequency) amplifiers used in wireless applications. Multi-tone DMT (discrete multi-tone) feed forward RF amplifiers are particularly amenable to the feed forward process described. 
     BACKGROUND OF THE INVENTION 
     Feed Forward (FF) amplifiers are special amplifiers that include circuitry for reducing distortion (i.e., in DMT RF amplifiers, intermodulation distortion is caused by the beating of the multi-tone signals together) normally introduced by the amplification process. They (FF amplifiers) are widely used in applications where reduction of signal distortion is critical to achieving performance objectives related to amplified signal clarity. Reduction of distortion is accomplished by use of correction or canceling signals, which are derived from the very signal components causing the distortion and then summed with the amplified signal. 
     FF RF (i.e., low distortion) amplifiers are very valuable in amplifying multi-tone signals such as are used in high frequency radio signals. Typically, such FF RF amplifiers have two substantially parallel signal processing/amplification paths. A first path (i.e., the amplification path) amplifies the multi-tone signal and in doing so creates inter-modulation distortions (IMD) in the resultant amplified signal. A portion of the multi-tone input signal is separated from the primary amplification path and channeled into a secondary path (i.e., the feed forward path) and combined with the signal as amplified in the first path including the IMD. In the signal combining the IMD signal is amplified to equal the IMD of the amplified signal in the first path. These IMD signals are subtracted from the amplified signal to recover a substantially distortionless amplified multi-tone signal. 
     While following these generalized principals, practical FF RF amplifiers are considerably more complex in structure and operation. This complexity is needed in part because of the high frequency operation at RF, which accentuates the IMD products. This complexity often takes the form of added control loops or/and added signal processing circuitry. This complexity however often compromises the basic purpose of the FF RF amplifier by introducing inefficiencies, which reduce its overall amplification capability. Hence, it is important to maintain amplification capability as well as reduce IMD. 
     One example of a widely used FF RF amplifier is described in U.S. Pat. No. 5,917,375 issued to Lisco et al Jun. 29, 1995. This particular FF RF amplifier uses a correction amplifier in a feed forward loop to amplify the signal (i.e., increase output power) as well as providing the IMD cancellation signal to reduce distortion. 
     Another FF RF amplifier is disclosed in the U.S. Pat. No. 6,111,462, issued to Mucenieks et al on Aug. 29, 2000, uses predistortion in the feed forward path to include the same RF carrier component as that in the amplification path. The amplified signals in both paths are combined (i.e., summed) to produce a high power amplified signal without the IMD components. The circuitry however is complex and power consuming causing inefficiencies. In particular, Quadrature detectors are a required component of the distortion correction circuitry. 
     SUMMARY OF THE INVENTION 
     High efficiency and high power output are attained in a multi-tone FF RF amplifier by eliminating fundamentals in a first loop and then operating on the IMD products only in a second loop. Since lower power tones are adjusted in the IMD product reduction, less complex, lower power processing circuitry is required and higher efficiency and lower costs are realized. 
     In an exemplary embodiment a multi-tone high frequency signal is applied to a first main amplification path, which amplifies a multi-tone high frequency input signal. In the process of amplification, by the first main amplification path, IMD is introduced into the amplified signal. 
     Input to the main amplification path is sensed at the input and channeled into a secondary amplification path (i.e., sometimes known as a feed forward path), which initially delays the multi-tone high frequency signal. This delayed multi-tone signal is combined with a feedback version of the amplified signal of the main amplification path output to add the distortion tones and permit further phase and amplitude adjustment. Phase adjustment is designed to invert the IMD and align the inverted IMD of the first loop path such that the IMD is out-of-phase with the IMD in the secondary path and combines this IMD in the secondary path through a coupler. This secondary amplification path signal combining the IMD and the multi-tone input is amplified so that the inverted distortion terms are equal in absolute value as to the distortion terms of the main amplification path output. With judicious amplitude, and signal delay distortion products (IMD) are canceled at the output and the multi-tone signal amplitude is enhanced by addition of the two amplification path outputs. This results in a clean (i.e., distortion free) multi-tone signal and improved amplifier efficiency due to the additive enhancement of the output multi-tone signal. 
     It is readily apparent that the forgoing feed forward system cancels IMD as well as increasing multi-tone amplitude by adding signal output at both the primary and secondary amplifiers, providing improved amplification efficiency and improved low distortion. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a block diagram illustrating principles of the invention; and 
     FIG. 2 is a schematic of a feed forward system incorporating the principles of the invention. 
    
    
     DETAILED DESCRIPTION 
     A feed forward (FF) RF amplifier implementing the principles of the invention that significantly cancels distortion while amplifying the DMT (i.e., multi-tone) input signal. As shown in the functional diagram of FIG. 1 a feed forward amplifier embodying the principles of the invention includes three distinct closed loops. An input high frequency multi-tone signal is applied to input coupler  101  and is coupled to a first amplification path  105  via a coupler  103 . Input multi-tones are relayed by coupler  101  to a second amplification path  107  via a delay circuit  109  and coupler  123 . A first adjust path  111  couples to the input via coupler  103  and adjusts the phase of the input signal, in phase adjuster  111  by 180° degrees and delays the signal in delay circuit  113  to match corresponding delays in the first amplification path  105 . 
     An output of the first amplification path is coupled to coupler  115 , and so is the adjusted and delayed signal output of delay circuit  113 . The coupler combines and applies these two signals to the IMD inversion and phase adjustment circuit  117 . Circuit  117  applies these signals, just prior to amplification processes, into the second amplification path  107 . Phase adjustment and inversion adjusts the delayed input which when combined with the output of the first amplification path has a multi-tone component in phase with the delayed two tone input applied to the second amplification path. IMD distortion components are inverted with respect to the IMD output of the first amplification path. The outputs of the two amplification paths are combined in output coupler  121 . 
     The output of the first amplification path is delayed in delay circuit  119  to correspond to delays introduced into the signal by the second amplification path  107  and is applied to output coupler  121 . The two processing loops including amplitude and phase adjustment circuits of  111  and  117  accomplish signal adjustments so that the multi-tone signal is increased in amplitude and the IMD is canceled at the output coupler  121 . 
     The amplification system of FIG. 1 is shown in schematic form in the FIG. 2 in which the controlling loops of the system are shown as well as the circuit components. The system is configured so that the main terms in the first and second amplification paths are of equal phase and are additive to increase and maximize RF power output. The IMD terms are of opposite phase so that they completely cancel. 
     A high frequency multi-tone RF signal is applied to the input  201  of the coupler  211  and is divided into two signal amplification paths. Each amplification path includes an identical RF multi-tone amplifier  221  and  231  respectively. Amplifier  221  receives the multi-tone RF and amplifies it and in the process generates signal distortions (i.e., IMD). This signal is applied, via delay circuit  202 , to an output coupler  223  where it is combined with a processed RF signal from the second amplification path that enhances the multi-tone RF signal and cancels the IMD. 
     Amplifier  231  receives an input from the coupler  211 , which has been delayed by delay circuit  212 . A distortion correcting input is also applied to the input of amplifier  231 . A distortion cancellation circuit (described below) supplies this correcting input. 
     Input to distortion control circuitry is derived from multiple control points of the circuitry. A sensing coupler  242 , connected at the input to amplifier  221 , applies a signal representative of the multi-tone input to a first loop IMD adjuster device  252  that controls a magnitude and phase of the sensed signal. This adjusted signal is applied to a delay circuit  254  to compensate for delays occurring in the amplifier  221 . This path may be disconnected by opening a normally closed switch  247  if adding the RF input to the signal processing of the loop is not desired. 
     This adjusted signal is combined with a signal sensed by sensing coupler  244  at the output of amplifier  221 . The multi-tone signal plus IMD terms from coupler  244  are combined with the multi-tone signal from delay element  254  in coupling device  256  such that the multi-tone signals cancel, leaving only IMD at the output of coupler  256 . The amplitude and phase adjustment circuit from device  262  operates to invert the distortion terms sensed from the output of amplifier  221 . This modified signal is applied to the input of the amplifier  231  via coupler  246 . At this point, the input signal to amplifier  231  comprises a multi-tone RF signal accompanied by an inverted multi-one distortion signal. The inverted IMD of the input signal counteracts the IMD introduced by amplifier  231  (i.e., amplifiers  221  and  231  are identical). 
     The output of amplifier  231  is an amplified multi-tone RF signal with an accompanying inverted multi-tone distortion signal. When this signal is combined with the output of amplifier  221  as delayed by delay circuit  202  the distortion terms cancel and the multi-tone signals are additive. Hence, the overall result is a distortion less multi-tone RF signal output at very high efficiency due to the additive effects from both amplifiers,  221  and  231 . 
     It is readily apparent that the various delays must be selected so that the multi-tone RF signals and multi-tone distortion terms are in exact phase alignment whereby the various summing and inversions may properly cancel the distortion terms and enhance the amplitude of the dial tone RF signal. Variable attenuators subject to control signals and variable phase shifting circuits subject to control signals may accomplish amplitude and phase adjustment. 
     A signal sensing point is provided at the node  253  to monitor the signal level applied the second RF amplifier. Adjustment of the signal component to a minimum at this point assures that the distortion circuitry is operating properly. 
     As shown in the FIG. 2 schematic, the FF RF amplifier includes inputs for injecting spreading signals into the amplification process. A spreading tone injection point  207  accepts a spreading tone just prior to the feedback coupler  244 . This injected spreading tone provides a mechanism to ensure that the adjustment of circuitry  262  provides proper IMD cancellation. A spreading tone is also injected into the output of the amplifier  231  at injection point  245 . Despreading of the spread tones is performed at the output at detection point  247  by a despreading circuit  249 . By nulling the spread tone signals at the output of despreading circuit  249 , the nulling of the distortion products is assisted. Spreading tones are provided by an oscillation system so that spreading tones provided at points  207  and  237  are synchronized with each other as well as the despreading circuit  249 . 
     While a specific embodiment of the invention has been disclosed, it is understood that those skilled in the art thereof may devise variations without departing from the spirit and scope of the invention.