Abstract:
Generally, the invention relates to a method and apparatus to improve the performance of radio frequency power amplifiers. More particularly, the present invention discloses the use of a power booster in conjunction with a radio-frequency linear power amplifier to reduce intermodulation distortion of an amplified signal.

Description:
RELATED APPLICATION  
       [0001]    This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/181,345 filed on Feb. 9, 2000 (Attorney Docket No. 004711.P001Z) and U.S. Provisional Patent Application Ser. No. 60/185,311 filed on Feb. 28, 2000 (Attorney Docket No. 004711.P002Z). 
     
    
     
       FIELD  
         [0002]    The present invention relates generally to power amplifier systems, and more particularly to a method and apparatus to increase the linearity of radio frequency power amplifiers by minimizing intermodulation distortion.  
         BACKGROUND OF THE INVENTION  
         [0003]    Communication services providers, such as cellular system operators, are subject to very strict bandwidth usage spectrum constraints imposed by the Federal Communications Commission (FCC). The FCC licenses transmission channels in the radio frequency spectrum and requires that signals be confined within certain emission limit masks in order to prevent interference caused by signals straying or spilling into adjacent transmission channels. The “emission mask” is a power spectrum density envelope. The maximum emitted power allowed varies as a function of the frequency offset from the nominal allocation center frequency. In other words, the emission mask determines the maximum power which may be emitted at a specific frequency for each frequency within the channel allocation. This requires that sideband spillover, the amount of energy outside the licensed channel, be sharply attenuated.  
           [0004]    Meeting these emission mask requirements is specially difficult when implementing modern, digitally-based, modulation formats, such as Code Division Multiple Access (CDMA), or Time Division Multiple Access (TDMA). Attenuating the sidebands to meet FCC requirements using such modulation requires very linear signal processing systems and components. Designing linear components, and in particular power amplifiers, at radio frequencies is costly and challenging to achieve.  
           [0005]    Radio-frequency (RF) linear power amplifiers (LPA) are typically used in digital cellular base stations to boost the power of a transmitted signal. RF power amplifiers for cellular communications typically operate in the Megahertz (MHz) and Gigahertz (GHz) frequency regions. Boosting a transmitted signal usually requires a LPA with a high ratio of peak-to-average power output (dynamic headroom), typically of at least 10 dB. The challenge is to design LPAs which can provide such dynamic headroom while minimizing sideband spillover without distorting the boosted signal.  
           [0006]    Typically, conventional LPA designs have relied on increasing the number of additional RF power transistors in order to achieve the increasingly higher dynamic headroom required by newer implementations. However, using additional RF power transistors has the highly undesirable effect of raising the overall parts count, the manufacturing costs, and the DC current consumption of the LPA.  
           [0007]    A fundamental problem in designing linear RF power amplifiers is that power amplifiers are inherently non-linear devices and generate unwanted intermodulation distortion (IMD).  
           [0008]    Linearity refers to a characteristic of power amplifiers where there is a substantially constant (linear) gain between an input signal and an output signal. Typically, power amplifiers only exhibit linear gain for a range of input signal voltage levels. This range is often called the linear region of the power amplifier. If the input signal voltage is below the minimum voltage for the linear region or above the maximum voltage for the linear region, then intermodulation distortion occurs.  
           [0009]    Intermodulation distortion manifests itself as spurious signals in the amplified RF output signal, separate and distinct from the RF input signal. IMD occurs when different frequencies from the input signal mix to produce sum and difference frequencies which did not exist in the input signal. It is the result of the behavior of amplifier components when operating outside the linear region.  
           [0010]    Therefore, a linear RF power amplifier is desired which has substantially linear characteristics, minimizes both sideband spillover and intermodulation distortion, and does not increase the DC current consumption over conventional LPAs.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 is a system-level illustration of a first embodiment of the present invention to increase the linearity of the RF power amplifiers.  
         [0012]    [0012]FIG. 2 illustrates the transfer characteristic of a linear power amplifier operating at 30 watts RF power output.  
         [0013]    [0013]FIG. 3 illustrates the transfer characteristic of a linear power amplifier of operating at 30 watts RF power output and supplemented by the power booster of the present invention.  
         [0014]    [0014]FIG. 4 is a system-level illustration of a second embodiment of the present invention to increase the linearity of the RF power amplifiers.  
         [0015]    [0015]FIG. 5 is a system-level illustration of a third embodiment of the present invention to increase the linearity of the RF power amplifiers.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]    [0016]FIG. 1 is a first embodiment of the present invention disclosing a conventional RF linear power amplifier (LPA)  102  with a signal input port  104 , a signal output port  106 , and a supply voltage port  108 . Also disclosed in FIG. 1 is a power detector  110  to sample the overall output power of the LPA  102  and coupled to a microprocessor  112 . The microprocessor  112  is configured to instruct a power booster  114  to increase the DC voltage from a first port  116  to a second port  118  based on the output power measurement received by microprocessor  112  from the power detector  110 . The second port  118  is then coupled to the supply voltage port  108  of the LPA  102 . In one embodiment, the power detector  110  may be a root-mean-square power detector.  
         [0017]    According to one embodiment of the invention, the power booster  114  may be capable of both increasing and decreasing the DC voltage from a first port  116  to a second port  118 . Thus, the microprocessor  112  may also be configured to instruct the power booster  114  to increase or decrease the DC voltage. In one embodiment of the invention, the power booster may be adaptively and dynamically adjusted to provide a greater or lesser source voltage to the power amplifier.  
         [0018]    In another embodiment of the invention, a machine-readable medium may contain instructions to adjust the power booster&#39;s configuration. That is, when the instructions in the machine-readable medium are executed by a processor, it causes the processor to configure the power booster to increase or decrease the source voltage to the power amplifier.  
         [0019]    LPAs typically comprise RF power transistors configured to amplify a signal between an input port and an output port. Conventionally, cellular base stations supply a power source of +26 to +28 Volts DC to communication equipment within a base station. This limits the peak-to-average power output capability (dynamic headroom) an RF power amplifier may provide for transmissions.  
         [0020]    According to one embodiment of the present invention, the LPA  102  may be comprised of one or more RF lateral diffusion metal oxide semiconductor (LDMOS) power transistors. One characteristic of MOS devices is that they behave like voltage-controlled current sources. Thus, the power output of a MOS device increases approximately as a factor of the square of the bias voltage. Consequently, raising the bias voltage of LDMOS transistors within an LPA increases their dynamic headroom thereby increasing their output linearity and improving the overall IMD performance of the LPA.  
         [0021]    [0021]FIG. 2 illustrates the performance plot of a LPA, like that of FIG. 1, without the power booster circuit  114 . For purposes of providing an example, a true CDMA modulated waveform or signal is depicted by the graph. However, the present invention is not limited to any particular transmission frequency, modulation scheme, or signal type. Thus, this particular graph and data thereon are not to be construed as limitations upon the present invention. In FIG. 2, the signal is illustrated at three different stages. The first region  202  depicts the un-amplified reference signal. The second region  204  shows the signal as it transitions from an un-amplified to amplified state. The third region  206  depicts the signal after it has been amplified by an LPA. At the third region  206 , the signal power is 30 Watts with the LPA operating near its maximum power output. As measured at the marker  208 , the signal is centered about 1.9468 GHz and has an IMD performance measured at −41.33 dBc.  
         [0022]    [0022]FIG. 3 illustrates the performance plot of a LPA, like that of FIG. 1, including the power booster circuit  114 . As with FIG. 2, a true CDMA modulated waveform is depicted by the graph. The first region  302  depicts the un-amplified reference signal. The second region  304  shows the signal as it transitions from an un-amplified to an amplified state. The third region  306  depicts the signal after it has been amplified by an LPA supplemented by the power booster  114 . At the third region  306 , the signal power is 30 Watts. As measured at the marker  308 , the signal is centered about 1.9468 GHz and has an improved IMD performance measured at −48.67 dBc. This is an improvement of 7 dB over the LPA without the power booster shown in FIG. 2.  
         [0023]    Thus, a setup as shown in FIG. 1 may be used to boost the voltage to LPAs in existing cellular stations thereby achieving the desired reduction in the IMD of the amplified signal. In various embodiments of the invention, the power booster  114  (FIG. 1) may increase the source voltage (V+ in FIG. 1) by an additional 1 to 20 volts to achieve the desired improvement in IMD. In one embodiment of the invention, the power booster  114  (FIG. 1) increases the source voltage (V+ in FIG. 1) by an additional 2 to 15 volts.  
         [0024]    A power booster may be designed in numerous conventional configurations known to those of ordinary skill in the art. For example, in one embodiment the power booster may comprise a DCto-DC converter capable of boosting or reducing a voltage from a first port to a second port.  
         [0025]    [0025]FIG. 4 illustrates another embodiment of the present invention, with an input port  404  to receive a signal, a conventional LPA  402  to amplify the input signal, and an output port  406  to provide the amplified signal. The LPA  402  is coupled to a power booster  414  which is in turn coupled to a voltage source (V+) via a first port  416 . The power booster  414  acts to raise the voltage (V+) from the first port  416  and supply an increased voltage (V++) to the supply voltage port  408  of the LPA  402 . As previously noted, the increased voltage serves to reduce the IMD of the amplified signal observed at the output port  406  of the LPA  402 .  
         [0026]    To configure the power booster  414 , the power booster  414  comprises a third port  422  and a fourth port  424 .  
         [0027]    According to one embodiment of the invention, the output voltage (V++) of the power booster  414  may be set by a resistive load  420  between the third port  422  and the fourth port  424 . In one embodiment of this invention, the resistive load  420  may be a manually adjustable impedance component such as a potentiometer. For a given resistive value between the third port  422  and the fourth port  424 , the amount that the DC voltage output (V++) increases over the DC voltage input (V+) will remain fixed.  
         [0028]    In one embodiment of the invention, the power booster  414  detects the voltage differential between the third port  422  and the fourth port  424  in order to set its DC voltage output (V++). In another embodiment, the power booster  414  sets its DC voltage output (V++) by measuring the current through the resistive load  420  between third port  422  and the fourth port  424 .  
         [0029]    [0029]FIG. 5 illustrates another embodiment of the present invention. The DC output voltage (V++), at a second port  518  of the power booster  514 , is fixed and constant even when the DC input voltage (V+), at a first port  516 , may vary. The power booster  514  may regulate the DC output voltage (V++) at the second port  518  by comparing the voltage at a third port  522  to a reference voltage at a fourth port  524 . The voltage present at the third port  522  is the result of a voltage divider formed by a first resistor  526 , coupled between DC voltage output port  518  and the third port  522 , and a second resistor  528 , coupled between the third port  522  and ground  530 . A reference voltage  510  is coupled to the fourth port  524  of the power booster  514 . By comparing the reference voltage at the fourth port  524  and the voltage at the third port  522 , the power booster  514  may be instructed to hold its DC output voltage (V++), at the second port  518 , to a predetermined constant value. The predetermined constant value may be directly proportional to the reference voltage  510  coupled to the fourth port  524 .  
         [0030]    While the invention has been described and illustrated in detail, it is to be clearly understood that this is intended by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of this invention being limited only by the terms of the following claims.