Abstract:
A method and apparatus for linear amplification of a modulated carrier signal or multi-carrier signal is disclosed. The linearity of the amplifier is improved by employing dynamic load line adjustments. A second current source coupled to the amplifier output load is turned on just before the amplifier reaches a nonlinear regime and reduces the effective load to prevent the amplifier allowing sufficient power to reach the nonlinear regime near saturation. The technique is particularly advantageous for amplification of a signal with large peak to average ratio.

Description:
RELATED APPLICATION INFORMATION 
     The present application claims priority under 35 USC 119(e) of provisional application Ser. No. 60/468,309 filed May 6, 2003, the disclosure of which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is related to radio frequency (RF) amplifiers. More particularly, the present invention is related to radio frequency power amplifiers used in wireless communication applications such as cellular base stations where signals with high peak to average ratios are generated and amplified. 
     2. Description of the Prior Art and Related Background Information 
     Most digitally modulated carrier signals used in modern telecommunication systems have an amplitude envelope showing a large peak to average ratio. In such systems, to preserve signal integrity and prevent transmitter spurious emissions, the amplifying device has to maintain linearity by having sufficient headroom for the signal peaks, albeit producing a modest average output power and therefore having a low efficiency. Hence, the amplifier efficiency and its linearity are practically mutually exclusive. 
     Even from the early days of AM broadcasting and in more recent complex transmission systems such as satellite communications, cable TV applications and cellular telephony, the carrier amplifiers have been mostly used in conjunction with some means of linearization to achieve the required performance. Feedback and in RF frequency bands, feedforward linearization are widely used linearization techniques. Analog predistortion has been used since the early days of satellite communication where frequency division multiple access (FDMA) systems were employed for sharing transponder bandwidth. In recent years, with the advent of digital signal processing (DSP), digital predistortion has received much attention. 
     Nonetheless, despite the significant efforts directed to linearization of RF power amplifiers, such techniques typically come at the expense of amplifier efficiency. Therefore, it is desirable to have additional techniques to achieve linearity and improve efficiency in RF amplifiers. Also, it is desirable to provide linearization techniques which may have reduced costs compared to the above known techniques, or which may improve performance, employed alone or in combination with the above techniques. 
     SUMMARY OF THE INVENTION 
     In a first aspect the present invention provides a power amplifier adapted for amplifying an RF input signal comprising an input for receiving an RF input signal and an amplifier device coupled to the input which receives the input signal and provides an amplified output signal. A dynamically varying output load is coupled to the amplifier device, including a variable impedance device coupled to the amplifier device so as to vary the impedance across the amplifier device as a function of the input signal. The impedance across the amplifier device is substantially constant through a first portion of the input signal power range corresponding to the lower major portion of the input signal power range and drops substantially for a second higher power portion of the input signal power range. The variable impedance device has approximately zero impedance through the first portion of the input signal power range and a substantially greater impedance through the second portion of the input signal power range. The power amplifier further comprises an output coupled to the output load for outputting the amplified output signal. 
     In a preferred embodiment of the power amplifier, the second portion of the input signal power range comprises the signal power range greater than about 6–10 dB below the amplifier device saturation region. The impedance across the amplifier device in the second portion of the input signal power range is preferably about 50% of the impedance across the amplifier device in the first portion of the input signal power range. The variable impedance device presents negligible load to the first device in the first portion of the input signal power range. The variable impedance device may have an impedance peak in a lower power region of the second portion of the input signal power range and a substantially constant impedance in a higher power region of the second portion of the input signal power range. The substantially constant impedance of the variable impedance device is approximately equal to the impedance across the amplifier device in the higher power region of the second portion of the input signal power range. The dynamically varying output load preferably further comprises a fixed load and the variable impedance device is coupled in parallel with the amplifier device to the fixed load. The amplifier device may comprise a field effect transistor biased in class A or class AB. The dynamically varying output load may further comprise transformer means for transforming the impedance of the fixed load by a factor of about 1.5–3. The dynamically varying output load may also further comprise means for adjusting the relative phase of the signals applied to the fixed load from the variable impedance device and the amplifier device. 
     According to another aspect the present invention provides a power amplifier circuit comprising an input for receiving an input signal and a coupler for receiving the input signal and splitting the input signal on two signal paths. A first amplifier device having a first turn-on threshold is coupled to the coupler on a first of the two signal paths and receives the input signal and provides a first amplified signal. A second amplifier device is coupled to the coupler on a second of the two signal paths and receives the input signal and provides a second amplified signal. The second amplifier device has a second turn-on threshold and an impedance near zero when the input signal is below the second turn-on threshold. An output load is coupled to the first and second amplifier devices. The power amplifier circuit further comprises a DC power supply and a first bias circuit coupled to the first amplifier device and the DC power supply. The first bias circuit provides a first bias to the first amplifier device setting the first turn-on threshold of the first amplifier device. A second bias circuit is coupled to the second amplifier device and the DC power supply and provides a second bias to the second amplifier device setting the second turn-on threshold of the second amplifier device. The second turn-on threshold is set at a substantially higher level than the first turn-on threshold, the second turn-on threshold corresponding to a peak power region of the input signal. An output is coupled to the first and second amplifier devices via the output load and provides an amplified output signal. 
     In a preferred embodiment of the power amplifier circuit, the second turn-on threshold of the second amplifier device is about 6–10 dB below device saturation of the first amplifier device. The second amplifier device may have a transitional region after the turn-on threshold and a fully turned on region at higher power and the real component of the impedance across the first and second amplifier devices are preferably substantially equal when the second amplifier device is in the fully turned on region. The first and second amplifier devices may be field effect transistors having respective source and gate terminals, and are coupled to receive the input signal applied to their respective gate terminals. The first and second bias circuits are coupled to the respective gate terminals of the first and second amplifier devices and provide first and second fixed voltage bias levels to the respective gate terminals of the first and second amplifier devices to set the respective turn-on thresholds of the first and second amplifier devices. The first and second bias circuits may preferably comprise first and second resistor networks coupled to the DC supply and the respective gate terminals of the first and second amplifier devices. The first and second resistor networks may comprise one or more common resistors. The first and second bias circuits may further comprise first and second low pass matching circuits coupled between the DC supply and the respective gate terminals of the first and second amplifier devices. The power amplifier circuit may further comprise a 90 degree phase inverting circuit coupled between the output load and the first or second amplifier device. The phase inverting circuit may be a K inverter circuit. The coupler may comprise a 90 degree hybrid coupler and the phase inverting circuit preferably adjusts the relative phase of the signals from the first and second amplifiers to compensate for the effect of the 90 degree hybrid coupler and relative phase shifts introduced by the amplifier devices and bias circuits. A transformer may also be provided coupled to the first and second amplifier devices in parallel with the output load. 
     According to another aspect the present invention provides a method for linear and efficient amplification of an RF input signal. The method comprises receiving an RF input signal and sampling the input signal to provide a sampled input signal. The method further comprises amplifying the input signal with a first amplifier device and applying the amplified signal across a load to provide an output signal. The method also comprises amplifying the sampled input signal with a second amplifier device and applying the amplified sampled input signal to the output load in parallel with said amplified signal. The method further comprises dynamically varying the impedance of the second amplifier device from a first substantially constant impedance near zero value over the lower major portion of the input signal power range to a second higher impedance in a peak power range of the input signal to substantially reduce the load of the first amplifier device when the input signal approaches the peak power region while maintaining the load substantially constant over the lower major portion of the input signal power range. 
     In a preferred embodiment of the method for linear and efficient amplification of an RF input signal, the load of the first amplifier device in the peak power region is reduced by at least about 50% from the load below the peak power region. The first amplifying device has a saturation power level and the peak power region preferably comprises the input signal power range greater than about 6–10 dB below the saturation power level. The second higher impedance of the second amplifier device may vary from a peak impedance value at a lower power level of the input signal to a second substantially constant impedance value at a higher power level of the input signal. The second amplifier device has a positive real impedance in at least a portion of the peak region of the input signal. The method preferably further comprises adjusting the relative phase of the amplified signal and the amplified sampled signal so as to be in phase at the output load. Sampling the input signal may comprise providing a 90 degree phase shifted sample of the input signal and adjusting the relative phase of the amplified signal and the amplified sampled signal compensates for the 90 degree phase shifting. 
     Adjusting the relative phase of the amplified signal and the amplified sampled signal may comprise passing the amplified sampled signal through a K inverter circuit. 
     Further features and advantages of the present invention will be appreciated from the following detailed description of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation of a voltage controlled current source. 
         FIG. 2  is a graphical representation of current vs. voltage characteristics of an amplifier device showing the amplifier load line. 
         FIG. 3  is a schematic drawing of an amplifier circuit in accordance with one embodiment of the invention. 
         FIG. 4  is a schematic drawing of an alternate embodiment of the invention employing combiner circuitry at the RF frequency range. 
         FIG. 5  is a schematic drawing of another embodiment of the invention employing an alternative amplifier combining arrangement. 
         FIG. 6A  is a graphical illustration of the load dynamics across the main amplifier device in the embodiment of  FIG. 3 . 
         FIG. 6B  is a graphical illustration of the load dynamics across the linearizing device in the embodiment of  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention provides a linearized high efficiency RF power amplifier and a method for linear amplification of an RF signal. A detailed circuit schematic of a preferred implementation of the amplifier is shown in  FIG. 3 , described below. First, however, the basic operational characteristics of amplifier devices employed in such circuit will be described in relation to  FIG. 1  and  FIG. 2 . 
     The basic structure of a controlled current source as embedded in an amplifier circuit  10  is shown in  FIG. 1 . This structure is a representation of a voltage controlled current source and is a simplified representation of solid-state devices such as a Field Effect Transistor (FET). The mechanisms responsible for the active device (transistor) nonlinearity are multifold. The device transconductance, the input and the output nonlinearities, all contribute to the amplifier distortion and are well known to those skilled in the art. The following description of the invention is equally applicable to other devices such as bipolar transistor technology. 
     Referring to  FIG. 1 , the amplifier circuit  10  includes a bias network  20  coupled to an active device which may be modeled as a plurality of current sources  16 . An input signal v in  is applied to input  12  and an output is provided via output load  18 . The parasitic gate to source capacitance  14  is also shown. In this simple model of the device, the drain to source current is given by:
 
 I   DS   =g   0   +g   1   v   in   +g   2   v   2   in   +g   3   v   3   in   + . . . +g   n   v   n   in .  (1)
 
     In this near-ideal representation of the active device, the output power limitation is caused by the drain saturation current (I DSS ), which is a device physical limitation, and also the load resistance, once the device is embedded in an amplifier circuit. The point where the drain saturation current is reached is thus determined by v in  assuming a fixed load  18 . 
     In an amplifying circuit, the signal distortion is most pronounced when the device is driven into the saturation region by large v in . That region is where the output signal will be clipped causing severe signal distortion. This situation arises when the RF voltage (the current supplied by the active device multiplied by the load value) exceeds the dc supply rail. Although in such mode of operation, the amplifier is very nonlinear, its efficiency is high. Hence, in applications with large signal envelope, the amplifier is normally operated in the back off region to avoid distortion, and therefore, its efficiency is fairly low. 
     The present invention provides an approach to amplifier linearization, where, for a given input excitation, the load value is dynamically changed and controlled by the envelope of the modulated signal. In this fashion, output clipping is avoided. 
     In  FIG. 2 , the drain to source current IDS for a typical FET solid state device is shown as a function of drain to source voltage V DS  for various gate to source voltages V GS . The line AB is determined by the output load  18  (shown in  FIG. 1 ). In accordance with the present invention, the slope of line AB (the load line) is changed dynamically and in tune with the envelope of the carrier signal. When the envelope is small, the load is set to be larger than the nominal value to generate a larger RF voltage swing, just short of dc rail. This leads to an improvement of the dc to RF conversion efficiency. As signal envelope is increased, dynamic load pulling reduces the load to avoid distortion. 
     The power added efficiency η, as known to those skilled in the art, is defined as:
 
η=(output  RF  power−input  RF  power)/ dc  input  (2)
 
     In class AB mode of operation, the dc power (I DS ×VDC) is dependant on quiescent current, and the efficiency of such amplifier is improved by maximizing the RF power in back off and this can be achieved by increasing the amplifier load. However, this will lead to nonlinearity and severe distortion at higher input levels. Therefore, when the input signal envelope goes through its peaks, the RF load will have to be reduced to prevent output clipping and maintain fidelity. 
     The linearization method of the present invention provides such a load pulling mechanism. A preferred implementation achieves this task by using a 2 nd  current source (solid state device) that is activated to adjust the load dynamically, in accord with the signal envelope and avoids the output signal hitting the dc rail. One specific circuit implementation employing dynamic adjustment of the amplifier ac load line is schematically shown in  FIG. 3 . 
     Referring to  FIG. 3 , the amplifier circuit  100  includes an input  102  for receiving an RF input signal. The input signal is provided by input signal source  104  applied to the input and an input load  106 . This input signal is divided into two paths  110 ,  112  by coupler  108 , which may be a 90 degree hybrid coupler (HYB) with an isolation port coupled to a termination load  109 . The signal on main path  110  is applied along line  118  to a main amplifying device  114  via capacitor  132 , which is a dc block. In this implementation a radio frequency MOS device  114 , such as an LDMOS device, is being used as the main amplifier device. A first bias circuit comprising the network of resistors  124 ,  126 ,  128  (values R 2 , R 3  and R 4 ) supply the required gate bias to amplifier device  114  from DC power supply  122 . These resistor values are adjusted to operate the device preferably in class A or AB mode of operation. DC feed circuit  130  acts as a low pass filter to stop the RF signal from leaking into the dc lines. 
     The power supply to the main amplifier device is provided from power supply  122  via DC feed circuit  138  which also blocks RF signals from the DC feed lines. The output of main amplifier device  114  is connected to output load  146  via phase shifter (PS)  144 . Load  146  may comprise a conventional fixed load  148  and an inductive load  150 , shown by a schematic representation of an RF transformer (TF). The role of the phase shifter  144  and its functionality will be discussed shortly. DC blocking capacitors  142 ,  152  are also shown. The combination of the inductive load  150  and the phase shifter  144  transform the load impedance of fixed load  148  into an appropriate level. Impedance scaling by a factor of k (1.5&lt;k&lt;3 larger than the nominal load value) will be suitable for typical applications. The load value seen by main amplifier device  114  will cause amplifier output clipping to happen at typically 6–10 dB input back off from device saturation. Hence, at this region, large output voltage swings are possible and high efficiency will be the result. Nonetheless, beyond this point, the amplifier output clipping leads to severe distortion if the load impedance value remains high. To avoid output clipping, the load of the main amplifier device is reduced as signal envelope increases. 
     Still referring to  FIG. 3 , a sample of the RF input is derived via input directional coupler  108  and provided to the second (auxiliary) signal path  112 . The sampled signal is amplitude adjusted and phase conditioned to the appropriate level before it is combined with the main amplifier device current. More specifically, in the illustrated preferred embodiment the sampled input signal is provided to second (auxiliary) amplifier device  116  along line  120  via DC blocking capacitor  136 . A second bias circuit comprising resistor network  124 ,  126 ,  128  coupled to DC power supply  122  sets the turn-on threshold of auxiliary amplifier  116 . DC feed line  134  acts as a low pass filter blocking RF energy from the DC feed lines. The current produced by the auxiliary amplifier device is thus proportional to the envelope of the signal, i.e. this device will only supply current to the load  146  above a certain input threshold (e.g., 6–10 dB back off). The current from auxiliary amplifier device  116  is combined with the main device  114  output current before it is applied to output device  146 . The addition of this (envelope controlled) current to the load results in the dynamic control of the load. The role of the phase shifter  144  is to introduce phase change and impedance inversion. Therefore, above the turn-on threshold of device  116 , the load impedance experienced by device  114  is reduced. As a result linearization of the main amplifier device  114  is achieved by avoiding output clipping. The load current is thus composed of two in-phase components leading to higher peak power at amplifier output  154  resulting in improved overall efficiency at back off. 
     In  FIG. 3 , the two amplifying devices  114 ,  116  will normally be used with input and output matching circuits. The inclusion of distributed or lumped matching circuits will introduce phase changes, leading to load impedance inversions. In such circumstances, the role of devices  114 ,  116  may need to be exchanged, but the principle of operation remains unchanged. 
       FIGS. 4 and 5  depict alternative embodiments employing other combining arrangements. In the embodiments of  FIGS. 4 and 5 , as in the embodiment of  FIG. 3  the signal in the auxiliary path is combined with the main path to provide dynamic load adjustment as described above. 
     In the embodiment of  FIG. 4 , an RF input signal is applied to input  102  and provided to sampling circuit  156 , including termination load  158 . Sampling circuit  156  may be any suitable sampling circuit known to those skilled in the art, including a hybrid coupler as described in relation to  FIG. 3 . The input signal and sampled input signal are provided along main and auxiliary paths  110 ,  112 , respectfully, as in the embodiment of  FIG. 3 . An RF combiner  160  is then employed to combine the two signal paths and the output signal is provided to output  154  via RF load  162 . The RF combiner  160  may be any suitable RF combiner of a type known to those skilled in the art. In this realization, the two arms are designed to have different transfer characteristics. While the main amplifier is designed to have a load for maximum efficiency at some back off signal level (6–10 dB), the 2nd amplifying branch is designed to have maximum peak power at full power. 
     In the embodiment of  FIG. 5 , the input signal at input  102  is similarly sampled by sampling circuit  166 , including termination load  164 , and provided along main and auxiliary paths  110 ,  112  to combiner  176  and to output  154  via RF load  168 . 
     The arrangement shown in  FIG. 5  can offer broadband response and ease of implementation. In this configuration the required phase shift between auxiliary path  112  and main path  110  is provided by a K-inverter  174 , for example as described in Matthaei G., Young L. and Jones E. M. T., Microwave Filters, Impedance Matching, and Coupling structures, Artech House, ISBN: 0-89006-099-1, the disclosure of which is incorporated herein by reference. 
     For both the embodiments of  FIGS. 4 and 5 , as well as  FIG. 3 , the addition of the auxiliary arm output to the main signal path is equivalent to lowering the impedance of the load or to a change in the slope of the load line ( FIG. 2 ). It should be noted that provided that a good phase balance is preserved between the main signal path and the auxiliary path, the power delivered to the load will be enhanced. Therefore, for all practical purposes, the circuit is configured to have a load impedance value, presented to the active device in the main path that is large compared to nominal load value. This load is gradually reduced, as the signal envelope increases above a threshold, and therefore, the device in the main path is loaded with an optimum load to avoid distortion. By decreasing the load at high input signal levels, the amplifier output voltage swing is lowered, preventing the excessive nonlinearity which would be the result otherwise. The control circuitry can take different forms and one mechanism for the control of the two current sources in the two signal paths is the bias of each stage as described above. As the main device will have to be active at lower envelope power levels, it will preferably be biased at class A or AB. The device in the auxiliary arm will be biased with smaller quiescent current, in which case, the drive signal level can turn this device on and allow the current to flow across the device and into the load. Other approaches to the control of the two current sources in the two signal paths may also be employed, however. For example, the envelope of the input signal can be extracted by using an envelope detector circuit. This information can be used for the control of the second current source in the auxiliary path  112 . 
     Referring to  FIGS. 6A and 6B  the results from a computer modeling of the circuit of  FIG. 3  showing the dynamics of load variation with signal level are illustrated.  FIGS. 6A and 6B  show the real and imaginary components of the impedance across the main and auxiliary amplifier devices, respectfully, as a function of input signal power (in dBm).  FIG. 6A  shows that the real part of the load impedance is larger at the lower power region  180 , and it drops through a transitional region  182  as the input signal level is increased above the turn-on threshold (dashed line) of the auxiliary device and the load pulling is activated. The main device impedance then stabilizes at a substantially lower real part of impedance value (e.g. about 50% of maximum impedance) at a higher power region  184 . As shown in  FIG. 6B , in the auxiliary signal path the load impedance measured across the auxiliary device terminal is approximately zero (but looks slightly negative) in the region  190  when the auxiliary device is inactive, i.e., below the turn-on threshold (dashed line). This is indicative of the fact that this device absorbs very small RF power in this mode (this loss of output power is outweighed by the improvements of main path efficiency). As the input signal level is increased, the auxiliary device is turned on and starts supplying current into the load. At some intermediate level, a relatively large impedance  192  is observed across the auxiliary device (little or no current flow into the 2 nd  arm). At larger powers, the load impedance observed by the 2 nd  arm stabilizes in region  194 . The auxiliary device impedance in region  194  is substantially the same as the impedance value experienced by the main arm in region  184 . In this region the imaginary component of the auxiliary device impedance  196  is negative. The comparison of the two graphs  6 A and  6 B thus shows that the load across the main device is dynamically changed (reduced) to improve the linearity and prevent output clipping/distortion. 
     The foregoing descriptions of preferred embodiments of the invention are purely illustrative and are not meant to be limiting in nature. Those skilled in the art will appreciate that a variety of modifications are possible while remaining within the scope of the present invention.