Abstract:
Systems and methods are provided for efficient amplification of a signal utilizing a modified Doherty amplifier system. A modified Doherty amplifier system includes a nonlinear main amplifier and a nonlinear auxiliary amplifier. An impedance-inverting network separates the main amplifier from an associated load. A second quarter wave transmission line separates the auxiliary amplifier from an associated signal source. The signal source has an associated minimum signal power, such that the signal power never drops below a predetermined percentage of a peak signal power.

Description:
TECHNICAL FIELD  
       [0001]     The present invention relates to RF communication systems and is particularly directed to systems and methods for amplifying a signal using a modified Doherty amplifier.  
       BACKGROUND OF THE INVENTION  
       [0002]     In many applications, amplifier design requires a balance between linearity and efficiency. Linear (e.g., class A) amplifiers can provide a linear signal at a significant cost in efficiency. More efficient, nonlinear (e.g., class C) amplifiers are available, but these amplifiers tend to suffer from intermodulation and harmonic distortion. A compromise is found in the Doherty amplifier, which utilizes multiple forms of amplifiers to achieve fairly efficient, low distortion amplification of a signal over a wide range of signal power.  
         [0003]      FIG. 1  is a functional block diagram of a Doherty amplifier system  10 . The Doherty amplifier system  10  comprises a plurality of amplifiers  12  and  14  connected in parallel as to amplify a signal from a signal source  16 . The amplifiers include a linear main amplifier  12 , which is always operating, and a nonlinear auxiliary amplifier  14  that operates when the power from the signal source reaches a threshold level (e.g., one-quarter peak power). The auxiliary amplifier  14  is connected to the signal source  16  by a first quarter wave transmission line  20 , with its output provided to a load  22 . The main amplifier  12  is connected to the load through an impedance-inverting network  24  (e.g., quarter wave transmission line).  
         [0004]     When the signal power from the signal source  16  is low, the auxiliary amplifier  14  is disabled by a drive control, and the linear main amplifier  12  provides all of the amplification for the signal. When the power of the signal reaches the threshold level, a drive control  18  activates the auxiliary amplifier  14 . The activation of the auxiliary amplifier  14  lowers the overall impedance experienced by the main amplifier  12  and allows the signal power to be split between the two amplifiers. Accordingly, the overall efficiency of the system is increased without significantly impacting the linearity of the amplifier system.  
       SUMMARY OF THE INVENTION  
       [0005]     In accordance with an aspect of the present invention, a modified Doherty amplifier system is provided. The system includes a nonlinear main amplifier and a nonlinear auxiliary amplifier. An impedance-inverting network separates the main amplifier from an associated load. A second quarter wave transmission line separates the auxiliary amplifier from an associated signal source.  
         [0006]     In accordance with another aspect of the present invention, a method is provided for amplifying a signal of interest. The signal of interest is generated such that the signal of interest has a minimum power that is greater than a predetermined fraction of its peak power. A first portion of the signal power of the signal of interest is amplified at a first nonlinear amplifier operating at saturation. A second portion of the signal power from the signal of interest is amplified at a second nonlinear amplifier operating at saturation.  
         [0007]     In accordance with yet another aspect of the present invention, an amplifier system is provided for amplifying an in-band, on-channel digital audio broadcasting composite radio frequency signal from an associated signal source to provide an amplified RF signal to an associated load. A first nonlinear amplifier operates in a saturated region. A second nonlinear amplifier operates in a saturated region. The first and second amplifier circuits being connected together in parallel and being located intermediate to the signal source and the load. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]     The foregoing and other features of the present invention will become apparent to those skilled in the art to which the present invention relates upon consideration of the following description of the invention with reference to the accompanying drawings, wherein:  
         [0009]      FIG. 1  is a functional block diagram of a Doherty amplifier system;  
         [0010]      FIG. 2  is a functional block diagram of a modified Doherty amplifier system having enhanced efficiency;  
         [0011]      FIG. 3  is a chart illustrating a relationship between the input voltage of a modified Doherty amplifier system in accordance with an aspect of the present invention, and its associated output voltage;  
         [0012]      FIG. 4  is a functional block diagram of an exemplary IBOC digital audio broadcast system utilizing a modified Doherty amplifier in accordance with an aspect of the present invention; and  
         [0013]      FIG. 5  illustrates a methodology for amplifying a signal in accordance with an aspect of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0014]      FIG. 2  is a functional block diagram of a modified Doherty amplifier system  50  having enhanced efficiency. The Doherty amplifier system  50  comprises a plurality of nonlinear amplifiers  52  and  54 , connected in parallel as to amplify a signal from a signal source  56 . The amplifiers include a nonlinear main amplifier  52 , which is always operating, and a nonlinear auxiliary amplifier  54  that operates when the power from the signal source reaches a threshold level (e.g., one-quarter peak power).  
         [0015]     The auxiliary amplifier  54  is connected to the signal source  56  by a first quarter wave transmission line  60 , with its output provided to a load  62 . The main amplifier  52  is connected to a load through an impedance-transforming network  64  (e.g., quarter wave transmission line).  
         [0016]     In accordance with an aspect of the present invention, the signal source  56  can be enhanced such that its peak power never drops below the threshold level. It will be appreciated that the implementation of the enhanced signal source  56  does not necessarily require the use of additional hardware to maintain the minimum signal power. For example, in an exemplary system for in-band on-channel (IBOC) digital radio in the U.S., a digitally-modulated signal and a constant envelope analog carrier signal are uncorrelated and maintained at a sufficient FM to IBOC ratio such that their sum, a composite FM+IBOC signal, naturally remains at or above one-quarter of the peak signal power. Other applications for the illustrated system will be apparent to one skilled in the art.  
         [0017]     Accordingly, the main amplifier  52  for the modified Doherty amplifier system  50  will always operate around its saturation point. Since the main amplifier  52  does not drop below saturation, a nonlinear (e.g., class C) amplifier is utilized for the main amplifier to increase the efficiency of the system  50 . An activation voltage associated with the auxiliary amplifier  54  can be selected according to the minimum signal power, such that when the input voltage at the minimum signal power is detected at a drive control  68 , the auxiliary amplifier  54  can be active. Accordingly, both amplifiers  52  and  54  can be operated in saturation for all possible values of the signal power, allowing for efficient amplification of the signal over the desired range.  
         [0018]     In accordance with an aspect of the present invention, the respective gains, G m  and G a , of the main and auxiliary amplifiers  52  and  54  can be adjusted to maintain a region of linear operation through the expected range of signal power from the signal source  56 . In a typical Doherty amplifier, the gain ratio, G a/ G m , is equal to two. In an extended Doherty system, the amplifier system is adjusted to allow for a larger gain ratio between the main and auxiliary amplifiers  52  and  54 . An optimal gain ratio for such a system can be determined by known design equations to be equal to γ, where γ is a function of the high efficiency range, specifically: 
 
Range HE = 20[log 10 (γ)]dB  (Eq. 1) 
 
         [0019]     It will be appreciated that the activation point 1/β 1  for the nonlinear main amplifier  52  in the illustrated system  50  will be larger than a similar linear amplifier. Accordingly, the gain of the main amplifier  52  can be boosted to compensate for this increase in activation voltage. Similarly, the main amplifier should reach saturation at the point where the auxiliary amplifier is switched on. In the illustrated system, the auxiliary amplifier is turned on when the voltage of the signal reaches 1/γof a known maximum input voltage, V max . These conditions can be met by setting the gain of the main amplifier such that:  
               G   m     =       β   1         β   1     -   γ               (     Eq   .           ⁢   3     )             
 
         [0020]     The gain for the auxiliary amplifier at a voltage of V max/γ will be equal to γ, the same as an auxiliary amplifier in a classic Doherty system. Accordingly, the gain ratio for the illustrated system  50  can be expressed as:  
               G   ratio     =         G   a       G   m       =       γ   ⁡     (       β   1     -   γ     )         β   1                 (     Eq   .           ⁢   4     )             
 
         [0021]     Accordingly, the overall gain for the system  50  is zero for input voltages smaller than V max /β 1  and nonlinear for voltages between V max /β 1  and V max /γ. For voltages greater than V max /γ, however, the system  50  acts as a LINC (Linear Amplification using Nonlinear Components) system. Since the signal source  56  can be selected or enhanced such that it does not produce signals having a voltage smaller than V max/γ , the amplifier system  50  will operate linearly with a high degree of efficiency.  
         [0022]      FIG. 3  is a chart  80  illustrating a relationship between the input voltage or current  82  of a modified Doherty amplifier system in accordance with an aspect of the present invention, and a voltage or current at various points within the modified Doherty system  84 . The illustrated chart is normalized, such that the maximum value of both the input voltage, the input current, the output voltage of the system, and the output current is equal to one. The chart  80  illustrates the linear voltage response  86  produced by the modified Doherty amplifier over a limited region of operation.  
         [0023]     When the input voltage is below an activation voltage, 1/β 1 , the modified Doherty amplifier does not produce an output. Once the activation voltage, 1/β 1 , is reached, the main amplifier activates, producing a voltage  88  at the main amplifier that quickly ramps up to saturation as the input voltage increases to a value equal to the inverse of a selected gain ratio, 1/γ, for the system. The output voltage of the system  90  increases steadily to about one-half the peak output voltage at a voltage equal to the inverse of the gain ratio.  
         [0024]     Once the input voltage reaches this point, the auxiliary amplifier activates, reducing the load on the main amplifier and producing a current  92  that increases steadily until the auxiliary amplifier reaches saturation at a maximum input voltage. The combined outputs of the auxiliary amplifier and the saturated main amplifier approximates an ideal linear response  94  associated with the system between the voltage, 1/γ, at which the auxiliary amplifier activates and the maximum input voltage. Accordingly, by selecting an appropriate activation point and gain ratio for the system, a linear response for the modified Doherty amplifier system can be achieved with high efficiency over a desired range of operation. In one implementation, a nonlinear precorrection component (not shown) may be used to used linearize the system gain (e.g., achieve constant system gain and phase) over the entire voltage range.  
         [0025]      FIG. 4  is a functional block diagram of an exemplary IBOC digital audio broadcast (DAB) system  100  utilizing composite crest factor reduction in accordance with an aspect of the present invention. The transmitter system  100  comprises an encoder  102  that encodes an analog source signal into a digital audio signal. The digital signal is encoded as a quadrature phase shift keying (QPSK) such that a plurality of two-bit symbols are encoded as one of four vector states, each having an associated phase. The audio encoder  102  removes redundant information from the audio signal to reduce the bit rate and thus the bandwidth required to transmit the signal.  
         [0026]     The compressed bit stream is then provided to a forward error correction and interleaving component  104 . The forward error correction and interleaving component  104  codes the signal for later error correction to improve the reliability of the information transmitted in the digital signal. The forward error coding can include, for example, Reed-Solomon encoding and Trellis coding. The data interleaving spreads related data over time and frequency to mitigate the effects of burst errors in the transmitted signal. The coded signal is then provided to an orthogonal frequency division multiplexer  106  that assigns the interleaved data to various orthogonal subchannels and combines the subchannels into a modulated signal. This signal is then provided to an upconverter  108  that upconverts the signal to a radio frequency.  
         [0027]     An analog exciter  110  produces a frequency modulated (FM) analog signal from an analog carrier signal having an associated phase. The frequency modulated analog signal and the coded signal are provided to a multiplexer  112 . The multiplexer  112  combines the two signals to form a hybrid signal. The signals are combined in such a way as to minimize interference between the signals. The combined signal is then provided to a modified Doherty amplifier system  114  that amplifies the combined signal to a level appropriate for transmission.  
         [0028]     It will be appreciated that the combined signal comprises the sum of two uncorrelated signals, the constant envelope FM analog signal and the digital audio signal. In accordance with Ibiquity specifications for IBOC signals, the digital audio signal itself has a peak-to-average ratio of approximately 6 dB and is summed at a much lower level with respect to the FM carriers, typically 20 dB lower.  
         [0029]     Taking these two factors into account, and considering that the addition of these carriers will both add and subtract vectorially over time, the minimum to maximum excursion of the envelope voltage can be calculated to have a voltage ratio of approximately 1.5 or a power ratio of around (1.5) 2 =2.25. The envelope of an FM IBOC signal will thus never drop below 1/2.25, or 44%, of its maximum peak value.  
         [0030]     Accordingly, the nonlinear main amplifier in the Doherty amplifier can be run at saturation at all times. As an activation voltage of the auxiliary amplifier can be selected such that it is exceeded when the signal power is above one-quarter peak power, the auxiliary amplifier is also active, and the signal power is distributed between the two amplifiers. Both amplifiers can thus be operated at saturation for all values of the signal power to provide enhanced efficiency. The signal is then provided to an associated antenna  118  for transmission.  
         [0031]      FIG. 5  illustrates a methodology  150  for amplifying a signal in accordance with an aspect of the present invention. The illustrated methodology  150  allows for the efficient amplification of a signal over a limited range of amplitude modulation. Specifically, where a signal of interest is modulated such that the signal power does not drop below a fraction of the peak power, the signal can be amplified via a modified Doherty amplifier system that maintains two nonlinear amplifiers at saturation to maximize the efficiency of the system.  
         [0032]     The methodology  150  begins at step  152 , where an analog frequency modulated (FM) signal is generated. In an exemplary embodiment, a carrier signal can be frequency modulated to carry audio information. At step  154 , a digital in-band, on-channel (IBOC) signal can be generated, also representing audio information. In one example, the analog signal can comprising a redundant, slightly delayed representation of the audio information carried in the digital signal.  
         [0033]     At step  156 , the digital IBOC signal is combined with the analog FM signal to produce a composite signal of interest. This composite signal of interest will have a minimum power that remains above one-quarter of the peak power of the signal. At step  158 , a portion of the signal is amplified at a nonlinear main amplifier. At step  160 , a portion of the signal is amplified at a nonlinear auxiliary amplifier. It will be appreciated that the two amplifiers can be connected in a parallel Doherty arrangement between the source of the signal and an associated load such that steps  158  and  160  can occur simultaneously. The amplified signal is then transmitted at step  162 .  
         [0034]     From the above description of the invention, those skilled in the art will perceive improvements, changes, and modifications. Such improvements, changes, and modifications within the skill of the art are intended to be covered by the appended claims.