Abstract:
The present invention relates generally to an amplifier such as that with the radio frequency (RF) spectrum having a nonlinear feedback loop to cancel out distortions in the input signal, and method therefor.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Technical Field  
           [0002]    The present invention relates generally to an amplifier such as that with the radio frequency (RF) spectrum having a nonlinear feedback loop to cancel out distortions in the input signal, and method therefor.  
           [0003]    2. Related Art  
           [0004]    In amplifier design, a balance exists between distortion performance and efficiency. Linear amplifiers which operate under certain conditions create little distortion but are inefficient. Conversely, nonlinear amplifiers are reasonably efficient but are known to introduce significant distortion. While both efficiency and distortion are important considerations in amplifier design, efficiency becomes increasingly important at high power levels. Because of their efficiency, nonlinear amplifiers are largely preferred, leaving the problem of distortion to be solved.  
           [0005]    Radio frequency (RF) amplifiers known in the art can be equipped with linearization circuitry. The linearization circuitry is typically placed in front of (i.e., predistortion), around (i.e., feedback), or after (i.e., feedforward) the amplifier. However, in each of these techniques, undesirable losses and, in some cases, increased power dissipation are incurred.  
           [0006]    There is thus a need for a linearization circuit which can enhance amplifier linearity for both broadband and narrowband signals, while eliminating undesirable losses and reducing unwanted power dissipation.  
         SUMMARY OF THE INVENTION  
         [0007]    It is therefore a feature of the present invention to overcome the above shortcomings related to increased losses and power dissipation of typical linearization circuits by providing a novel linearization circuit that modifies the amplifier without increasing either losses or power dissipation, and which has the additional benefit of not reducing the gain of the amplifier. The disclosed linearization circuit can nevertheless be applied to broadband technologies, and as such, is applicable to the entire cable television (CATV) frequency range. The disclosed linearization circuit is also expandable in frequency range so as to be useful in narrowband applications.  
           [0008]    The present invention eliminates losses and power dissipation problems due to the linearization circuit by utilizing the knowledge that in most operating regimes the distortion in a RF amplifier is significantly smaller than the overall output power. For this reason, a feedback loop with nonlinear characteristics can be designed to cancel out these distortions without significantly affecting the response or the stability of the RF amplifier.  
           [0009]    In high bandwidth linear amplifiers, feedback is typically provided in a single transistor stages composed of a few transistors only. Thus, the phase is well controlled and a nonlinear feedback scheme is devisable with a controlled phase as is required for broadband cancellation. This scheme reduces the amount of feedback at large signal excursions, which increases the gain at those signals and which offsets the saturation behavior of the RF amplifier that would otherwise reduce the gain. Both the small and large signals&#39; gains are kept equal to the small signal gain. Thus, the RF amplifier linearity is enhanced in a simple scheme without adding losses from power dissipation.  
           [0010]    The present invention is also drawn to a method of utilizing the nonlinear feedback linearizer disclosed herein by combining the nonlinear feedback linearizer circuit of the present invention with an existing RF amplifier.  
           [0011]    In a first general aspect, the present invention provides an apparatus for enhancing amplifier linearity over a frequency range, said apparatus comprising: an input frequency signal, said input frequency signal subject to signal excursions from a nominal signal level; circuitry which includes a circuit for generating a corrective signal having nonlinear characteristics; and a system for receiving the input frequency signal, said system including a circuit for applying a corrective signal to the input frequency signal.  
           [0012]    In a second general aspect, the present invention provides a method for enhancing amplifier linearity over a frequency range, said method comprising: providing an input frequency signal, said input frequency signal subject to signal excursions from a nominal signal level; receiving the input frequency signal; generating a corrective signal having nonlinear characteristics; and applying the corrective signal to the input frequency signal.  
           [0013]    In a third general aspect, the present invention provides an apparatus for reducing distortion in a transmitted signal, said apparatus comprising: at least one radio frequency amplifier; and at least one nonlinear feedback linearizer circuit operationally connected to said amplifier.  
           [0014]    The foregoing and other features and advantages of the invention will be apparent from the following more particular description of embodiments of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    The features and inventive aspects of the present invention will become more apparent upon reading the following detailed description, claims, and drawings, of which the following is a brief description.  
         [0016]    [0016]FIG. 1 is a block diagram representation of a normal feedback apparatus as embodied in the related art.  
         [0017]    [0017]FIG. 2 is a block diagram of an exemplary nonlinear feedback linearization circuit in an embodiment of the present invention.  
         [0018]    [0018]FIG. 3 is an exemplary nonlinear feedback linearization circuit of an embodiment of the present invention 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0019]    The following is a detailed explanation of the nonlinear feedback linearization apparatus, and a method for using the improved nonlinear feedback linearization apparatus in embodiments of the present invention, with reference to the attached drawings. It should be noted that the same reference numbers are assigned to components having approximately the same functions and structural features in the following explanation and the attached drawings to preclude the necessity for repeated explanation thereof.  
         [0020]    [0020]FIG. 1 shows a block diagram representation of a normal feedback apparatus as embodied in the related art. The block diagram  100  includes an input node  110 , for receiving an input frequency signal, connected to the positive input of amplifier  120 . The input frequency signal is characterized by having a nominal signal level, from which the input frequency signal may deviate due to electrical noise or other system faults which can cause excursions from the nominal input frequency signal level. Amplifier  120  also has an output node  150 . The negative input to amplifier  120  is connected via feedback loop  140  to the output node  150 . Feedback loop  140  includes a gain network  130 .  
         [0021]    Referring now to FIG. 2, a block diagram representation of an exemplary embodiment of a nonlinear feedback linearization circuit of the present invention is shown. The block diagram  200  includes an input node  210  connected to the positive input of amplifier  220 . Amplifier  220  also has an amplifier output node  250 . The negative input to amplifier  220  is connected via feedback loop  240  to the output node  250 . Feedback loop  240  includes a nonlinear gain network circuit  230  of the present invention. Nonlinear gain network circuit  230  has a network input node  270  and a network output node  260 . Network input node  270  is connected to amplifier output node  250 , while network output node is connected to the negative amplifier input node.  
         [0022]    Nonlinear gain network circuit  230  provides a corrective signal, having a nonlinear characteristic, at network output node  260  which is applied to the negative input to amplifier  220 . The nonlinear gain network circuit  230  can operate in broadband frequencies such as, inter alia, 54 MHz to 870 MHz. The nonlinear gain network circuit  230  can also operate in narrowband frequencies such as, inter alia, 900 MHz to 1 GHz. Further, nonlinear gain network circuit  230  may produce a controlled phase signal. The nonlinear gain network circuit  230  may provide a large signal gain such as, inter alia, &gt;30 dB, and a small signal gain such as, inter alia, &lt;10 dB. The nonlinear gain network circuit  230  may be operated such that these two gains, large and small, are kept substantially equal to each other during periods of signal excursions.  
         [0023]    Referring now to FIG. 3, a circuit diagram of one embodiment of a nonlinear feedback linearization network circuit of the present invention is shown. The circuit diagram  300  depicts one possible embodiment of the circuitry connected between network input node  270  and network output node  260 . In the embodiment shown, a first resistor  210  and a second resistor  220  are connected in series between network input node  270  and network output node  260 . Node  215  represents a point between the series connected resistors  210  and  220 . A first diode  260  and a second diode  270  are connected in parallel, with reversed polarity, between node  215  and ground. First and second diodes may be replaced by another type of electronic, optical, or electro-optical switch such as, inter alia, a transistor element.  
         [0024]    Embodiments of the present invention have been disclosed. A person of ordinary skill in the art would realize, however, that certain modifications would come within the teachings of this invention. Therefore, the following claims should be studied to determine the true scope and content of the invention.