Abstract:
An amplifying circuit and associated method that outputs a distortion-free signal. The circuit includes an output amplifier driven by two intermediate amplifiers having differing gain factors. The intermediate amplifiers are coupled via an attenuator, a phase shifter, a signal splitter, and signal combiner and produces a predistortion signal that cancels the distortion signal generated from the output amplifier. Another similar amplifying circuit includes an additional combiner and splitter loop, and independently cancels the distortion signal generated by the first intermediate amplifier and the distortion signal generated by the output amplifier.

Description:
BACKGROUND 
     1. Field of the Invention 
     The present invention relates to a circuit for improving the distortion performance of amplifiers, and more particularly, to a linearization circuit to improve the distortion performance of broadband hybrid RF amplifiers. 
     2. Description of Related Art 
     In transferring and distributing signals, most cable television (CATV) networks employ hybrid fiber/coax (HFC) networks. A typical HFC network includes optical transmitter and nodes, and distribution amplifiers to provide optical and electrical paths for the signals, respectively. (An optical node converts between the optical and RF domains.) Effective transfer of the signals along the optical and electrical paths requires output stage signal amplification at the optical transmitter and nodes, and the distribution amplifiers. For this amplification, hybrid RF amplifiers, which typically employ push-pull and power-doubling techniques to improve their distortion performance, are commonly used. Such hybrid RF amplifiers are commercially available from vendors such as Philips and Motorola. 
     When the coaxial cable portion of an HFC network is passive, that is, there is no distribution amplifier in the network, the distortion from the node output stage amplifier may dominate the total system distortion performance. At such a passive coaxial cable portion, it is desirable to drive the output stage amplifier in the node at maximum power level in order to transmit the signal further. However, higher output power can produce worse distortion performance. Therefore, the optical nodes need an enhanced output stage amplifier to improve the distortion performance at high output power. 
     There are several known linearization techniques that improve distortion performance of output stage amplifiers. Among them are predistortion and feedforward circuits which are schematically shown in FIGS. 1A and 1B, respectively. Even though both techniques can improve the distortion performance, they have their own limitations. 
     Referring to FIG. 1A, the feedforward circuit includes an error amplifier  20  to cancel the distortion produced by an output amplifier  10 . A first directional coupler  1  splits an input signal onto two paths  12  and  14 . On path  12 , output amplifier  10  amplifies the signal, and then a second directional coupler  2  splits the amplified signal onto paths  12  and  16 . On path  16 , an attenuator  3  attenuates the signal, and a phase-shifter  4  phase-shifts the signal by 180°. At a third directional coupler  7 , the signal from path  16  and the signal from path  14  through a first delay line  5  combine together. As a result, fundamental portions of the signals from paths  14  and  16  cancel each other, and distortion portions of the signals are coupled at the output of third directional coupler  7 . Then, error amplifier  20  amplifies the combined signal which is a distortion signal. 
     The signal from second directional coupler  2  on path  12  passes through a second delay line  6 , and is combined with the amplified signal from error amplifier  20  at a third directional coupler  30 . When the output signals combine together at a combiner  30 , the distortions in both output are equal in magnitude but opposite in phase, so that the distortions cancel each other. This feedforward technique is known for its high distortion cancellation capability. For example, this feedforward technique can cancel all orders of distortions whereas other linearization techniques only cancel one type of distortion. Thus, the feedforward technique is widely used in broadband applications where reactive components have different effects at different frequencies. However, the feedforward technique has several shortcomings. For example, multiple output amplifiers cannot be coupled to a single feedforward circuit, and there can be signal loss at the output stage. Driving the output amplifier at a higher level to overcome the loss at the output stage may introduce more distortions. 
     The predistortion circuit shown in FIG. 1B does not introduce any loss at the output stage of output amplifier  10 . This circuit employs a distortion generator  40  to produce predistortion from a signal that was split from an input signal through a first directional coupler  22 , and provides the predistortion into the input stage of error amplifier  20 . At a second directional coupler  24 , the other signal that was split from an input signal through a first directional coupler  22  passes through a delay line  25  combines with the signal from error amplifier  20 . Then, the combined signal is inputted into output amplifier  10 . 
     The predistortion from distortion generator  40  through error amplifier  20  cancels the distortion from amplifier  10  at the output stage of output amplifier  10 . Typically, distortion generator  40  does not have similar distortion characteristics as does amplifier  10 , limiting the circuit&#39;s cancellation capability. 
     Diode-based distortion generators, which are widely used for predistortion, have disadvantages when employed in hybrid RF amplifier linearization. First, different orders of distortions may require separate, different distortion generators. That is, separate distortion generators are required to cancel second and third order distortions. This increases cost and circuit complexity. Second, when the hybrid RF amplifiers are driven at high output power, higher (fourth and fifth) order distortions become more pronounced, and overlap on lower order distortion frequencies. Such diode distortion generators cannot effectively cancel the high order distortions. 
     U.S. Pat. No. 5,258,722, incorporated herein by reference in its entirety, discloses a distortion cancellation circuit including a distortion generating circuit  50 , as shown in present FIG.  2 . Distortion generator  50  includes a first intermediate amplifier  36 , which is ideally identical to each of four output amplifiers  60 - 1  to  60 - 4 , and a second intermediate amplifier  42 . In distortion generator  50 , a first splitter  53  splits an input signal onto two paths  32  and  34 . The signal on path  32  passes through amplifier  36 , a first attenuator  54 , and a phase-shifter  57 , and the signal on path  34  passes through amplifier  42  and a second attenuator  55 . The signals on both paths  32  and  34  combine together at a combiner  56 , and the combined signal is inputted into output amplifiers  60 - 1  to  60 - 4  through a second splitter  58 . 
     Amplifiers  36  and  60 - 1  to  60 - 4  are intended to be driven at the same output power. Theoretically, distortion generating circuit  50  may generate a predistortion that can cancel the distortion generated from output amplifiers  60 - 1  to  60 - 4 . However, distortion generating circuit  50  may have several shortcomings in practice. 
     For effective distortion cancellation, output amplifiers  60 - 1  to  60 - 4  must be of high gain (30 dB or more) in order to have amplifier  42  driven at far lower power level than are output amplifiers  60 - 1  to  60 - 4 , so as to compensate for the excessive signal loss in distortion generating circuit  50 . Otherwise, amplifier  42  must be driven at much higher power level. The distortion generated from amplifier  42  will become dominant, and the magnitude of the predistortion generated at combining point  56  will not be proper to cancel the distortion generated from output amplifiers  60 - 1  to  60 - 4 . 
     Commercially available hybrid RF amplifiers used for forward transmission typically have gain of 18 to 21 dB. If such amplifiers are used for amplifier  36 , amplifier  42  will be driven at only a few dB below the output power level of output amplifiers  60 - 1  to  60 - 4 . In addition, higher gain amplifiers are very difficult to make, and not commercially available. 
     Another shortcoming is that the fundamental signals from paths  32  and  34  must be subtracted from each other at combining point  56  to obtain the out-of-phase distortion component. The fundamental signal power difference at combining point  56  is 6 dB. When the signal power difference is small as in this case, a slight imbalance in phase and magnitude of signals will degrade frequency response flatness which may affect the optical node frequency response flatness requirement. In addition, the poor flatness will cause the distortion generated by output amplifiers  60 - 1  to  60 - 4  to be less similar to the distortion generated by amplifier  36 , and in turn will result in incomplete distortion cancellation. 
     SUMMARY 
     The present invention is directed to an amplifying circuit to improve distortion performance of broadband RF (radio frequency) amplifiers. A distortion generating amplifier and an error amplifier produce predistortions that cancel the distortions of the output amplifier. That is, the invention is directed to a distortion cancellation circuit which includes both feedforward and predistortion circuits. 
     An embodiment of the present invention is an amplifying circuit, which includes an output amplifier, two intermediate amplifiers, two signal splitters, two signal attenuators, two combiners, and a phase-shifter. The first splitter splits an input signal onto a first path and a second path, and the first intermediate amplifier amplifies the signal on the first path and produces a distortion signal. The second splitter splits the amplified signal and the distortion signal from the first intermediate amplifier onto a third path and a fourth path. 
     Then, the first attenuator and phase-shifter respectively attenuates and phase-shifts the signal and the distortion signal by 180 degrees. The first combiner combines the signal and the distortion signal on the third path, and the signal on the second path, so that the signal on the third path and the signal on the second path cancel each other, and couples the resulting distortion signal to a fifth path. On the fifth path, the second intermediate amplifier amplifies the distortion signal from the first combiner. 
     The second combiner combines the signal and the distortion signal on the fourth path and the distortion signal on the fifth path. At the second combiner, the distortion signal on the fifth path has a magnitude that is twice that of the distortion signal on the fourth path, and a sign opposite to that of the distortion signal on the fourth path. Then, the second attenuator attenuates the signal and the distortion signal from the second combiner, and an output amplifier amplifies the signal and the distortion signal from the second attenuator. In the output amplifier, the distortion signal amplified by the output amplifier cancels the distortion signal generated by the output amplifier. 
     The circuit in one embodiment further includes multiple output amplifiers and a distributor for distributing the signal and the distortion signal from the second attenuator to the output amplifiers. In one embodiment, the first intermediate amplifier and the output amplifier are driven at the same amplification level. The signal gain from the input terminal of the first intermediate amplifier to the input terminal of the output amplifier is zero. 
     Another amplifying circuit according to the present invention includes an additional combiner and splitter in addition to the elements of the abovedescribed circuit. In this circuit, the distortion signal from the second intermediate amplifier is split into two paths. The distortion signal on one path is used to cancel the distortion signal generated by the first intermediate amplifier, and the distortion signal on the second path is used to cancel the distortion signal generated by the output amplifier. 
    
    
     BRIEF DESCRIPTIONS OF THE DRAWING 
     FIG. 1A is a block diagram of a conventional amplification circuit including a feedforward distortion cancellation circuit. 
     FIG. 1B is a block diagram of a conventional amplification circuit including a predistortion distortion cancellation circuit. 
     FIG. 2 is a block diagram of another conventional amplification circuit including a distortion cancellation circuit. 
     FIG. 3 is a block diagram of an amplification circuit including a distortion cancellation circuit in accordance with the present invention. 
     FIG. 4 is a block diagram of an embodiment of the amplification circuit of FIG.  4 . 
     FIG. 5 is a block diagram of another embodiment of the amplification circuit of FIG.  4 . 
     FIG. 6 is a block diagram of another amplification circuit including a distortion cancellation circuit in accordance with the present invention. 
     FIG. 7 is a block diagram of another embodiment of the amplification circuit of FIG.  6 . 
     Use of the same reference symbols indicates similar or identical items. 
    
    
     DETAILED DESCRIPTION 
     The present invention is directed to a linearization circuit to improve the distortion performance of broadband RF amplifiers used, e.g., in the forward direction (downstream) of a CATV network. In accordance with the invention, an embodiment of the linearization circuit, upstream of an output amplifier, employs a distortion generating amplifier and an error amplifier to produce distortions that cancel the distortions of the output amplifier. By using a feedforward circuit as a predistorter, the linearization circuit achieves effective distortion cancellation and minimum signal loss at output stage. 
     FIG. 3 illustrates an amplification circuit  100  including a distortion cancellation circuit, which is a feedforward predistortion circuit, in accordance with the present invention. The magnitudes of signals and distortions are presented in voltages: ‘S’ denotes signal voltage, and ‘D’ denotes distortion voltage Referring to FIG. 3, an input signal S from a signal source (not shown) is applied to a terminal  110  where input signal S splits into a first path  160  and a second path  170 . Connected in first path  160  is a distortion generating amplifier  112 , which amplifies input signal S and outputs signal aS+D. Then, signal aS+D at node  122  splits into first path  160  and a third path  180 . Connected in third path  180  are a first attenuator  114  and a phase shifter  116 . On third path  180 , signal aS+D is thereby attenuated by a factor of 1/a and phase-shifted by 180 degrees to produce signal −(aS+D)/a. Then, signal −(aS+D)/a combines with signal S on second path  170  at a first summing point  118  (preferably without any signal loss), canceling fundamental signal portion (−S) and resulting in signal −D/a. That is, only distortion portion (−D/a) of signal −(aS+D)/a remains. 
     Then, an error amplifier  120  on second path  170 , downstream of summing point  118 , amplifies signal D/a by a factor of 2a, and outputs signal −2D. Signal −2D combines with signal aS+D on first path  160  at a second summing point  124  (preferably without any signal loss), resulting in signal aS−D. In other words, error amplifier  120  is to have a gain such that the magnitude of phase-reversed distortion at the entry of summing point  124  is exactly twice that of the distortion from distortion generating amplifier  112 . Accordingly, downstream of second summing point  124 , only the distortion magnitude (+D) of signal aS+D on from path  160  is phase-shifted to −D by 180 degrees. 
     A second attenuator  126 , which is connected downstream of second summing point  124 , attenuates signal aS−D, resulting in signal S−D/a. A 4-way splitter  128 , which is connected downstream of second attenuator  126 , distributes signal S−D/a to four output amplifiers  130 A to  130 D. Each of output amplifiers  130 A to  130 D has the same amplification characteristics as does distortion generating amplifier  112 . Thus, when output amplifiers  130 A to  130 D amplify signal S−D/a, distortion portion −D/a of signal S−D/a is amplified to −D, and this amplified (pre)distortion −D cancels a distortion D produced from output amplifiers  130 A to  130 D. Accordingly, output signal aS, which is amplified from input signal S by a times with no distortion, is output from each of output amplifiers  130 A to  130 D. 
     In summary, in the amplification circuit of FIG. 3, a distortion generating portion  102  generates a predistortion signal that is in theory identical (or in practice very similar) to the distortion signals generated by output amplifiers  130 A to  130 D, so that the distortion signals from output amplifiers  130 A to  130 D are completely canceled. Thus, there is no need for separate distortion generators for different orders of distortion signals. In addition, as illustrated in FIG. 3, multiple output amplifiers can be driven by a single predistortion circuit. 
     Unlike the prior art amplification circuit of FIG. 2, which must employ high gain output amplifiers for effective distortion cancellation, the amplification circuit of FIG. 3 does not require high gain output amplifiers. Output amplifiers  130 A to  130 D have lower gain than does error amplifier  120 . 
     FIG. 4 illustrates an amplification circuit  100 A which is an embodiment of the FIG. 3 amplification circuit  100  especially intended for broadband RF applications such as CATV video distribution. 
     Referring to FIG. 4, a first directional coupler  132  and a second directional coupler  142  are employed at nodes  110  and  118  of FIG.  3 . (Such couplers are commercially available, e.g., as part no. EMDC-10-1-75 from M/A-COM in Lowell, Massachusetts. They are transformer-based.) First directional coupler  132  transfers 90% and 10% of input signal (fundamental signal) from a signal source (not shown) to paths  160  and  170 , respectively, and prevents feedback from path  170  to path  160 . Second directional coupler  142  combines the signals from paths  170  and  180 , and also prevents the signal from path  180  from flowing back to first directional coupler  132 . In this circuit, although a resistive splitter can be used instead of first directional coupler  132 , first directional coupler  132  prevents a possible signal leakage from second directional coupler  142  from flowing into amplifier  112 . 
     A resistive splitter  134  and a resistive combiner  144  are connected in FIG. 4 at nodes  122  and  124 , respectively, of FIG.  3 . Resistive splitter  134 , which includes resistors  134 A to  134 C, splits the signal from amplifier  112  into paths  160  and  180 . Resistive combiner  144 , which includes resistors  144 A to  144 C, combines the signals from resistive splitter  134  and amplifier  120 . The resistance values of resistors  134 A to  134 C and  144 A to  144 C are all 25 ohm. 
     A first delay line  140  and a second delay line  150  are respectively connected between directional coupler  132  and directional coupler  142  on path  170  and between resistive splitter  134  and resistive combiner  144  on path  160 . Examples of delay lines  140  and  150  are lengths of coaxial cable or microstrip lines. The delays in delay lines  140  and  150  are 2.17 nanoseconds and 2.32 nanoseconds, respectively. In addition, an attenuator  152  is connected between directional coupler  142  and amplifier  120  in order to attenuate the signal from directional coupler  142 . 
     The gains or attenuations at components of circuit  100 A are shown in FIG.  4 . (These values are exemplary.) The difference between the attenuation factor from the output terminal of amplifier  112  to the input terminal of resistive splitter  144  along path  160  and the attenuation factor from the output terminal of amplifier  112  to the input terminal of resistive splitter  144  along path  180  is controlled such that the magnitude of the distortion signal from amplifier  120  is, with opposite sign, twice the magnitude of the distortion signal from delay line  150 . In FIG. 4, the attenuation factor (−6 dB) from the output terminal of amplifier  112  to the input terminal of resistive splitter  144  along path  160  is larger than the attenuation factor (−3 dB) from the output terminal of amplifier  112  to the input terminal of resistive splitter  144  along path  180  by 3 dB. However, this attenuation factor difference can be varied inclusively between 3 dB and 6 dB depending on the magnitude of signal loss in the circuit. 
     From the input terminal of distortion generating amplifier  112  to the output terminal of 4-way splitter  128 , the total signal gain is zero as shown, and only predistortion is introduced to the signal so that the predistortion cancels the distortion generated from output amplifiers  130 A to  130 D. Accordingly, each of output amplifiers  130 A to  130 D outputs a signal amplified by +21 dB with no distortion. 
     In circuit  100 A, the output power of distortion generating amplifier  112 , which is a hybrid RF amplifier, is high, and thus the input power level of the predistortion at each of output amplifiers  130 A to  130 D is still high. Accordingly, the noise of output amplifiers  130 A to  130 D does not degrade the carrier-to-noise ratio performance of amplification circuit  100 A. 
     Although amplification circuit  100 A includes four output amplifiers  130 A to  130 D, a similar circuit with a single output amplifier can be implemented. FIG. 5 shows such an amplification circuit  100 B which is the same as circuit  100 A of FIG. 4, but includes a single output amplifier  130 . No 4-way splitter is required. Instead, a third attenuator  126  attenuates the signal by −9 dB upstream of output amplifier  130 . 
     FIG. 6 shows another amplification circuit  200  including a distortion cancellation circuit in accordance with the present invention. In circuit  200 , the distortion at the output terminal of an error amplifier is split into two parts. The first part cancels the distortion generated by a distortion generating amplifier (a true feedforward cancellation), and the second part cancels the distortion generated by an output amplifier. This approach allows independent control of the two cancellations when the distortion performances of the distortion generating and the output amplifiers are not identical to each other. 
     Amplification circuit  200  is similar to amplification circuit  100  of FIG.  3 . Referring to FIG. 6, an input signal S from a signal source (not shown) splits at node  210  into a first path  260  and a second path  270 . Connected in first path  260  is a distortion generating amplifier  212 , which amplifies signal S input into distortion generating amplifier  212  and outputs signal aS+D. Then, signal aS+D splits at node  222  into first path  260  and a third path  280 . Connected in third path  280  are a first attenuator  214  and a phase shifter  216 . On third path  280 , signal aS+D is attenuated by a factor of 1/a and phase-shifted by 180 degrees to produce signal −(aS+D)/a. Then, signal −(aS+D)/a combines with signal S on second path  270  at a first summing point  218  with no signal loss, canceling fundamental signal portion (−S) and resulting in signal −D/a. That is, only distortion portion (−D/a) of signal −(aS+D)/a remains. 
     An error amplifier  220  on second path  270  downstream of summing point  218  amplifies signal D/a by a factor of a, and outputs signal −D. Signal −D is split at a node  252  to a fourth path  270 A and a fifth path  270 B. Then, signal −D on path  270 A combines with signal aS+D on first path  160  at a second summing point  224  without any signal loss, resulting in signal aS. Signal −D on path  270 B combines with signal aS from second summing point  224  at a third summing point  228  with no signal loss, resulting in signal aS−D. In other words, the distortion signal (−D) on path  270 A cancels the distortion signal generated from distortion generating amplifier  212 , and the distortion signal(−D) on path  270 B is a predistortion signal for canceling the distortion signal to be generated from an output amplifier  230 . When the distortion signal from output amplifier  230  is different from the distortion signal from distortion generating amplifier  212 , the distortion signal(−D) on path  270 B can be modified by an attenuator, an amplifier, a magnitude equalizer, and/or a phase equalizer for effective distortion cancellation at output amplifier  230 . 
     A second attenuator  226 , which is downstream of third summing point  228 , attenuates signal aS−D by a factor of 1/a, resulting in signal S−D/a. Then, output amplifier  130  amplifies signal S−D/a, such that distortion portion −D/a of signal S−D/a is amplified to −D, and this amplified predistortion signal −D cancels a distortion signal D from output amplifier  130 . Accordingly, output signal aS, which is amplified from input signal S by a factor of a with no distortion, is output from output amplifier  130 . 
     FIG. 7 illustrates an amplification circuit  200 A which is an embodiment of amplification circuit  200  especially intended for broadband RF application. 
     Referring to FIG. 7, a first directional coupler  232  and a second directional coupler  242  are connected at nodes  210  and  218  of FIG.  6 . The functions of directional couplers  232  and  242  are respectively identical to the functions of directional couplers  132  and  142  of FIG.  4 . 
     A resistive splitter  234 , a first resistive combiner  244 , and a second resistive combiner  254  are connected at splitting and summing points  222 ,  224 , and  226 , respectively, of FIG.  6 . Resistive splitter  234 , which includes resistors  234 A to  234 C, splits the signal from amplifier  212  into paths  260  and  280 . Resistive combiner  244 , which includes resistors  244 A to  244 C, combines the signals from resistive splitter  234  and amplifier  220  on path  270 A. Resistive combiner  254 , which includes resistors  254 A to  254 C, combines the signals from resistive combiner  244  and amplifier  220  on path  270 B. 
     A first delay line  240  and a second delay line  250  are respectively connected between directional coupler  232  and directional coupler  242  on path  270  and between resistive splitter  234  and resistive combiner  244  on path  260 . Examples of delay lines  240  and  250  are a length of coaxial cable or microstrip line. The delays in delay lines  140  and  150  are 2.17 nanoseconds and 2.32 nanoseconds, respectively. In addition, an attenuator  252  is connected between directional coupler  242  and amplifier  220  in order to attenuate the signal from directional coupler  242 . Although not shown in FIG. 7, additional attenuators, amplifiers, magnitude equalizers, and/or phase equalizers can be employed on paths  270 A and  270 B to adjust the magnitudes of the distortion signals on both paths  270 A and  270 B. 
     The gains or attenuations at components of circuit  200 A are also shown in FIG.  7 . The difference between the attenuation factor from the output terminal of amplifier  212  to the input terminal of resistive splitter  244  along path  260  and the attenuation factor from the output terminal of amplifier  212  to the input terminal of resistive splitter  244  along paths  280  and  270 A is controlled such that the magnitude of the distortion signal from amplifier  220  is, with opposite sign, identical to the magnitude of the distortion signal from delay line  250 . In FIG. 7, each of the attenuation factors from the output terminal of amplifier  212  to the input terminal of resistive splitter  244  along path  260  and from the output terminal of amplifier  212  to the input terminal of resistive splitter  244  along paths  280  and  270 A is identically −6 dB. 
     From the input terminal of distortion generating amplifier  212  to the input terminal of output amplifier  230 , the total signal gain is zero, and only predistortion is introduced to the signal so that the predistortion cancels the distortion generated from output amplifier  230 . Accordingly, output amplifiers  230  outputs a signal amplified by +21 dB with no distortion. 
     Although the invention has been described with reference to particular embodiments, the description is only an example of the inventor&#39;s application and should not be taken as limiting. Various adaptations and combinations of features of the embodiments disclosed are within the scope of the invention as defined by the following claims.