Abstract:
An amplifier provides two or more selectively enabled amplifier segments allowing a source signal to be amplified with a selectable output power. A bias circuit responsive to a bias control signal enables selectable combinations of one or more amplifier segments, thus allowing selection of a desired output power. Selecting a desired output signal power via the bias control signal corresponds to selecting an overall amplifier quiescent current that decreases with decreasing output signal power. Thus, the amplifier permits a controlling system, such as a mobile terminal (e.g., cellular telephone) to amplify a transmit signal with a selectable transmit signal output power and corresponding level of amplifier quiescent power consumption. The amplifier is particularly advantageous for inclusion in systems designed for digital communications requiring variable transmit power levels, such as the digital cellular standard IS-95. Preferably, each segment comprises one or more transistor amplification stages which, when enabled by the bias circuit, are biased to permit maximum power linear amplification for the transmit signal.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention generally relates to communications systems, and particularly relates to controlling the output power and corresponding current consumption of a power amplifier.  
         BACKGROUND OF THE INVENTION  
         [0002]    Mobile wireless communications systems have numerous and oftentimes challenging design requirements not commonly shared with other types of communications equipment. For example, mobile terminals, such as cellular telephones, are expected to operate on battery power for significant intervals of time between re-charging. Battery life represents a key performance benchmark used by the consuming public to evaluate the relative desirability of available mobile terminals. Indeed, “talk time” performance, which refers to the total length of time the mobile terminal will operate from a fully charged battery, is critical to many users. As designers continue reducing mobile terminal size and, consequently, battery capacity, commensurate reductions in mobile terminal power consumption are paramount.  
           [0003]    Other challenges arise from the wireless communications standards themselves. IS-95, for example, is a Code-Division Multiple-Access (CDMA) 800 MHz digital cellular standard that requires mobile terminals to dynamically control their maximum transmit signal power over a defined range during operation, in accordance with changing conditions within the communications system. (Wide-band CDMA systems have similar power control requirements.) Transmit signal linearity must be maintained over the range of required transmit signal power. Both IS-95 and TIA/EIA-136 (an 800/1900 MHz Time-Division Multiple-Access digital cellular standard) require linear transmit signal amplification to meet strict signal fidelity requirements, and to avoid interference with other mobile terminals simultaneously active within the same service area.  
           [0004]    Adjacent channel power ratio (ACPR) represents a key benchmark in evaluating transmitter performance and is commonly used to assess a mobile terminal&#39;s potential for interference. Mobile terminals typically use some form of power amplifier for transmit signal amplification. To meet the aforementioned linearity requirements, transmit signal power amplifiers are commonly biased at an operating point providing linear amplification at the maximum required transmit signal power. This ensures transmit signal linearity when the mobile terminal operates at maximum transmit signal power, but results in relatively high levels of power amplifier quiescent current (bias current).  
           [0005]    Such levels are power amplifier quiescent current are needlessly high when the mobile terminal is not required to operate at maximum transmit power. In light of the demanding battery life requirements imposed on mobile terminals, such inefficiency is particularly significant. Indeed, the transmitter in a typical mobile terminal represents a dominant component of operating current consumption, and thus represents a key area of concern in ensuring competitive “talk time” performance ratings.  
           [0006]    Reducing amplifier quiescent current as a function of output power represents one solution to this problem. Such schemes commonly employ a control voltage that varies amplifier quiescent current, as supplied by a bias network. This approach requires some form of analog control based on feeding back a signal proportionate to output power, which results in undesirable circuit complications. More importantly, varying the quiescent current of the power amplifier can result in degraded linearity, which results from changing the operating point(s) of the transistor(s) comprising the power amplifier. This can have serious implications for transmitted signal fidelity. Strict limits on ACPR allowed by standards such as IS-95 become a significant problem if linearity is compromised.  
           [0007]    Another existing approach to quiescent current control entails using a switched-gain, multi-stage amplifier where the final stage of the amplifier is shut off and bypassed for low power operation. This action reduces the quiescent current of the amplifier at low power. This approach has several drawbacks, including large gain discontinuities and compromised linear output power adjustment.  
           [0008]    Because overall amplifier gain depends on the final gain stage, switching it out for low power operation represents a substantial transmit signal gain discontinuity when changing from full power to low power operation. Also, because such approaches typically leave active only a driver stage in the low power mode, the available linear output power can be severely limited. A further drawback of the switched-stage approach is that switching out the final stage causes a large change in amplifier conduction angle, thus leaving this approach unsuitable for signaling applications requiring phase continuity to maintain the integrity of transmitted information.  
           [0009]    Thus, there remains a need for a power amplifier that provides linear signal amplification over a range of selectable output power, while also providing relatively consistent signal gain and phase conduction angle over this range. Further, the needed power amplifier operates with a quiescent current proportionate to its selected output power, thereby reducing its quiescent current consumption at lower levels of output power.  
         SUMMARY OF THE INVENTION  
         [0010]    The present invention provides both methods and apparatus for controlling the maximum output signal power and quiescent operating current of a power amplifier. A segmented power amplifier comprises two or more parallel amplifier segments that may be selectively enabled by a controlling system. When enabled, an amplifier segment provides an amplified output signal based on amplifying a common source signal received by the segmented power amplifier. These amplified source signals combine to form the final amplifier output signal from the segmented power amplifier. Overall amplifier quiescent current and maximum amplifier output signal power are both dependent upon the number of simultaneously enabled amplifier segments.  
           [0011]    Exemplary embodiments of the segmented power amplifier of the present invention operate on radio frequency signals. Thus, a mobile terminal or other wireless communications device may advantageously use the segmented power amplifier of the present invention for transmit signal amplification. This allows the mobile terminal to transmit at different output power levels based on selectively enabling one or more amplifier segments in differing combinations. Because the quiescent current of the segmented power amplifier also depends upon the number of simultaneously enabled amplifier segments, the mobile terminal may reduce its operating current by enabling fewer amplifier segments in accordance with its transmit signal power requirements.  
           [0012]    Preferably, each amplifier segment comprises a like number of one or more transistor amplification stages, with each stage enabled or disabled via a bias signal. A bias circuit, comprising a portion of the segmented power amplifier, selectively provides these bias signals to each amplifier segment in accordance with a bias control signal. Thus, a controlling system may disable all amplifier segments, or selectively enable one or more segments in a desired combination via the bias control signal. In exemplary embodiments, the bias circuit comprises a plurality of bias networks, with groups of bias networks associated with each amplifier segment. The bias control signal controls these groups of bias networks such that all amplification stages within a given amplifier segment may be commonly enabled or disabled.  
           [0013]    In exemplary embodiments, the bias circuit biases all of the amplification stages in an amplifier segment either on or off (enabled or disabled), as detailed above, in response to the bias control signal. Preferably, when an amplifier segment is enabled, each amplification stage is biased to an operating point providing maximum linear signal amplification for that individual stage. Thus, when biased on, the overall amplifier segment provides linear signal amplification of the source signal. While each amplifier segment preferably provides similar signal gain, the segments may have substantially different levels of maximum linear output power. This allows a controlling system more flexibility in optimizing transmit signal power and quiescent current consumption, as given amplifier segments may be optimized for efficient operation at different levels of segment output power. Design of the individual transistor amplification stages comprising each segment provides this opportunity to set the maximum output power and quiescent current for each segment.  
           [0014]    In exemplary embodiments, the segmented power amplifier of the present invention avoids large signal gain discontinuities between its possible combinations of enabled amplifier segments. That is, the segmented power amplifier of the present invention provides substantially the same signal gain whether one, all, or a combination of parallel amplifier segments are simultaneously enabled. Thus, while the maximum output power available from the segmented power amplifier of the present invention varies as a function of which parallel amplifier that simultaneously enabled, the signal gain does not vary appreciably. Further, the segmented power amplifier of the present invention additionally avoids significant changes in conduction angle between its different levels of maximum available output power. These benefits, as well as other advantages and features, will become apparent in the following detailed discussion. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    [0015]FIG. 1 illustrates an exemplary communications system  10  that may advantageously use the segmented power amplifier of the present invention.  
         [0016]    [0016]FIG. 2 provides more detail regarding the mobile terminal of FIG. 1, and illustrates an exemplary transmit signal chain location for the segmented power amplifier of the present invention.  
         [0017]    [0017]FIG. 3 provides a simplified block diagram for an exemplary embodiment of the segmented power amplifier of the present invention.  
         [0018]    [0018]FIG. 4 provides a more detailed block diagram for an exemplary embodiment of the segmented power amplifier of the present invention.  
         [0019]    [0019]FIG. 5 graphs maximum output power versus bias control signal value for an exemplary two-segment segmented power amplifier.  
         [0020]    [0020]FIG. 6 is a simplified flow diagram illustrating operating logic for an exemplary embodiment of the segmented power amplifier of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]    [0021]FIG. 1 depicts a communications system  10 , including a mobile terminal  100  advantageously including the segmented power amplifier of the present invention. Communications system  10  further includes a communications tower  12 , a base station  14 , and a mobile switching center  16 . A user of mobile terminal  100  communicates with other system users, or users of other communications networks, via wireless signaling between mobile terminal  100  and communications tower  12 .  
         [0022]    [0022]FIG. 2 depicts an exemplary mobile terminal  100 . From a top-level perspective, mobile terminal  100  includes a user interface  150 , a transmitter  110 , a receiver  140 , an antenna assembly  170 , a system controller  180 , and frequency synthesizer  182 . In operation, user interface  150  receives audio input including user voice, which is processed by transmitter  110  to form a transmit signal broadcast to communications tower  12  by antenna assembly  170 . Receiver  140  decodes signals received from communications tower  12  via antenna assembly  170  to recover speech and other audio information, which is output to user interface  150 .  
         [0023]    User interface  150  includes a microphone  154  for receiving speech and other sounds from the user. A buffer amplifier  112  in transmitter  110  provides analog-to-digital converter (ADC)  114  with a signal suitable for digitization. A signal processor  116  operates on the digitized output from ADC  114  to form an encoded baseband digital signal. Modulator  118  includes a baseband signal generator/encoder and signal modulators (not shown), which operate on the baseband digital signal to form a transmit signal having a desired carrier frequency determined by an output from frequency synthesizer  182 . The segmented power amplifier  120  of the present invention amplifies the transmit signal before being broadcast from antenna  174  included in antenna assembly  170 .  
         [0024]    In many types of communications systems, such as those governed by the IS-95 specifications, transmit signal power must be carefully controlled to minimize the interference between mobile terminals  100  simultaneously operating within a given service area of communications system  10 . Segmented power amplifier  120  allows mobile terminal  100  to select a desired transmit signal power while simultaneously controlling amplifier quiescent current. Segmented amplifier  100  is designed such that lower transmit signal powers correspond with lower levels of amplifier quiescent current, thus allowing mobile terminal  100  to achieve significant power savings when operating segmented amplifier  120  at lower power levels.  
         [0025]    Receiver  140  receives signals from communications tower  12  through antenna assembly  170 . Receiver/amplifier  142  typically includes signal conditioning, such as filtering, amplification, and down-conversion (not shown). A receiver processor  144 , which may be a DSP or ASIC, processes the output from receiver/amplifier  142  to extract signaling and speech information from the received signal. Signaling data includes control information, such as channel assignments and required transmit power adjustments. Such control information may be relayed to system controller  180  for further action or control output adjustment. Speech information is converted from digital to analog format using digital-to-analog converter (DAC)  146 . An output audio amplifier  148  may buffer the DAC output to provide adequate drive for loudspeaker  152 . Thus, loudspeaker  152  reproduces speech and other sounds from a remote user based on information extracted from the received signal.  
         [0026]    [0026]FIG. 3 provides a simplified illustration of an exemplary embodiment of the segmented power amplifier of the present invention. Segmented amplifier  120  comprises two or more parallel amplifier segments  124 . A common input terminal (source input)  122  provides an input connection to each amplifier segment  124 . A segment output from each amplifier segment  124  connects together to form a final output  128 . Thus, each active (enabled) amplifier segment  124  amplifies a source signal received through source input  122 , and the amplified output signal from each active amplifier segment  124  combines with the output from the other simultaneously active amplifier segments  124  to form the final amplifier output signal provided on final output  128 . Bias circuit  126  selectively provides bias signals to each amplifier segment according to the bias control signal. Bias circuit  126  preferably provides multiple bias signals, one per transistor amplification stage  200  (not shown in FIG. 3) in each amplifier segment  124 , but controls these bias signals in a manner allowing the bias control signal to enable or disable an entire amplifier segment  124 .  
         [0027]    [0027]FIG. 4 provides a more detailed illustration of an exemplary segmented power amplifier  120 . FIG. 4 depicts a segmented power amplifier  120  comprising two parallel amplifier segments  124 -it should be understood that the present invention may be practiced with essentially any number of parallel amplifier segments  124  numbering two or greater. Each amplifier segment  124  comprises a like number of transistor amplification stages  200 . While FIG. 4 illustrates two amplification stages  200  per amplifier segment  124 , the present invention may be practiced with essentially any number of one or more amplification stages  200  per amplifier segment  124 .  
         [0028]    When more than one amplification stage  200  is used per amplifier segment  124 , successive stages  200  within an amplifier segment  124  are wired in series fashion. Thus, the signal gain for a given segment  124  depends upon the overall gain of its series amplification stages  200 . Preferably, the individual amplifier segments  124  have similar signal gains such that the overall amplifier signal gain from source input  122  to final output  128  does not vary significantly as the controlling system selectively enables varying combinations of amplifier segments  124 . Of course, the maximum output power available from a given amplifier segment may vary significantly between segments.  
         [0029]    In an exemplary embodiment, each amplification stage  200  includes a stage input  200 , a stage output  208 , a bias input  206 , and a transistor amplifier arrangement  204  disposed between the stage input  202  and stage output  208 . The bias input  206  receives a bias signal from the bias circuit  126 . This bias signal establishes the bias or operating point of transistor amplifier arrangement  204 . Preferably, transistor amplifier arrangement  204  comprises a single Hetero-junction Bipolar Transistor (HBT), as shown in FIG. 4, and the bias circuit  126  selectively provides a bias signal that biases the HBT comprising transistor amplifier arrangement  204  for linear signal amplification at maximum transistor output power. Thus, in exemplary embodiments of the present invention, the transistor amplifier arrangements  204  in each amplification stage  200  within an enabled amplifier segment  124  are each biased for maximum power linear signal amplification. Of course, when an amplifier segment  124  is disabled, its transistor amplifier arrangement(s)  204  are biased off.  
         [0030]    Indium Phosphide/Indium Gallium Arsenide (InP/InGaAs) HBTs are readily fabricated using conventional optical lithography techniques and offer excellent fabrication uniformity and high operating cutoff frequencies. While these attributes make HBTs desirable for use in some exemplary embodiments of the present invention, other types and arrangements of transistors may be used in other embodiments of the present invention. For example, those skilled in the art will readily appreciate that dual-gate FETs may be used in place of HBTs, and further that each transistor amplification stage  204  may comprise two or more transistors arranged in various amplifier topologies. Thus, the present invention is not limited to using a single, common-collector HBT transistor per transistor amplification stage  200 .  
         [0031]    As earlier explained, controlling amplifier quiescent current represents a key approach in minimizing unnecessary power dissipation in battery-operated equipment, such as mobile terminals. In existing systems with conventional power amplifiers, the power amplifier may be initially biased such that its operating point yields linear signal amplification at maximum output power. In response to decreased output signal power requirements, such existing systems reduce power amplifier quiescent current in proportion to the reduced power requirements. Altering the quiescent current, however, changes the operating point of the transistors comprising the conventional power amplifier. Changing the operating point of the power amplifier can result in undesirable changes in signal amplification characteristics. For example, as earlier noted, the conventional power amplifier may operate at a point where its transistors are close to their cutoff point. This can result in significant and unwanted signal amplification non-linearity.  
         [0032]    In an exemplary embodiment of the present invention, each amplification stage  200  in each parallel amplifier segment  124  is either biased on to operate linearly at a maximum output power, or is biased off to avoid interfering with a signal amplification of any enabled amplifier segments  124 . Unlike a conventional power amplifier, the segmented power amplifier  120  of the present invention can include multiple parallel amplifier segments  124 , with each segment  124  including amplification stages  200  where the transistor amplifiers  204  are fabricated for efficient operation at a desired output power level.  
         [0033]    For example, even though segmented power amplifier  120  operates at differing levels of overall maximum output power based on which amplifier segments  124  are simultaneously enabled, each amplifier segment  124  may be fabricated such that the transistors comprising its one or more amplification stages  200  operate with roughly the same emitter current density, regardless of the particular combination of enabled amplifier segments  124 . A segmented power amplifier  120  might include two parallel amplifier segments  124  (as illustrated in FIG. 4). One segment may be designed as a lower power segment  124 , while the remaining segment may be designed as a higher power segment  124 .  
         [0034]    For lower power operation, only the lower power segment  124  is enabled. This lower power segment  124  may comprise amplification stages  200  including transistors with relatively small active emitter areas as compared to the emitter area size of transistors comprising the amplification stages  200  in the higher power segment  124 . Thus, even though the absolute current output from segmented power amplifier  120  during low power operation-with the low-power segment  124  enabled-is less than during higher power operation-with at least the higher power segment  124  enabled-the transistor current densities may be similar for the lower power segment  124  and higher power segment  124 .  
         [0035]    Thus, in exemplary embodiments, the segmented power amplifier  120  of the present invention avoids the problems associated with varying a transistor&#39;s operating point by operating all transistors in an enabled amplifier segment  124  at a fixed, linear operating point. Overall quiescent current of segmented power amplifier  120  is a function of the number of simultaneously enabled amplifier segments  124 . Thus, a controlling system selects a desired level of amplifier quiescent current by choosing which if any, parallel amplifier segments  124  comprising segmented power amplifier  120  are enabled.  
         [0036]    Selecting different levels of maximum output power based on simultaneously enabling differing numbers and/or combinations of parallel amplifier segments  124  rather than employing the existing approach of switching out one or more gain stages or varying a transistor&#39;s operating point, allows the segmented amplifier of the present invention to offer relatively consistent signal gain regardless of the number of segments  124  that are enabled. Further, because the transistor(s) comprising the amplification stages  200  in a given amplifier segment  124  are, in exemplary embodiments, always biased to a consistent operating point when the given segment  124  is enabled, the conduction angle of the segmented power amplifier  120  of the present invention does not appreciably change, regardless of the selected combination of enabled amplifier segments  124 .  
         [0037]    Preferably, as shown in FIG. 4, corresponding stage inputs  202  and stage outputs  208  are interconnected with each other between the amplifier segments  124 . Further, the stage input  202  of the first amplification stage  200  in each amplifier segment  124  preferably includes a source impedance matching circuit  202 A. This circuit matches the input impedance of the amplifier segments  124  to the source impedance of the input signal. If more than one amplification stage  200  is used per amplifier segment  124 , successive amplification stages  200  include inter-stage impedance matching circuits  202 A, matched to the impedance of the preceding amplification stage outputs  208 . Also, each stage output  208  preferably includes a phase compensation circuit  208 A to minimize the relative phase difference of amplified signals within all enabled amplifier segments  124 .  
         [0038]    [0038]FIG. 4 depicts bias circuit  126  as providing a plurality of bias signals, with one bias signal per amplification stage  200 . In such an arrangement, bias circuit  126  may comprise individual bias networks, with groups of these bias networks associated with each amplifier segment  124 . FIG. 4 depicts individual enable signals (e.g., ENABLE 11 . ENABLE 12 , ENABLE 21 . ENABLE 22 ), but other embodiments of the present invention may use common enable lines for an entire group of bias circuits  126  associated with a given amplifier segment  124 . As previously explained, bias circuit  126  uses its output bias signals to selectively enable or disable different combinations of amplifier segments  124  in response to an external control signal.  
         [0039]    Exemplary embodiments of the present invention include a bias circuit  126  responsive to a digital bias control signal. Preferably, bias circuit  126  includes a digital signal input for each amplifier segment  124 . In this configuration, bias circuit  126  provides bias signals to each amplifier segment  124  corresponding to each asserted digital signal input. Thus, a segmented power amplifier  120  having four parallel amplifier segments  124  might include a bias circuit  126  having four digital input lines, with each line enabling/disabling a different amplifier segment  124 .  
         [0040]    In other embodiments, segmented power amplifier  120  includes a bias circuit  126  having a digital interface suited for receiving binary values corresponding to a desired combination of enabled amplifier segments. Such an interface might be configured for access via a conventional microprocessor data/memory bus, or might be configured for access via serial interface lines. These types of digital interfaces are well understood in the art. In still other embodiments of the present invention, segmented power amplifier  120  may include a bias circuit having an analog interface that permits selectively enabling different amplifier segments  124  based on varying the level of the input voltage or current.  
         [0041]    [0041]FIG. 5 plots maximum output power versus a binary-valued bias control signal for an exemplary segmented power amplifier  120 . P OUT     —     MAX  represents the maximum output signal power available from segmented power amplifier  120  for a given bias control signal value. FIG. 6 assumes a segmented power amplifier  120  having first and second amplifier segments  124 . When the bias control signal has a value of “11,” both amplifier segments  124  are enabled (biased on) and segmented power amplifier  120  provides its maximum available output signal power, based on the combined power contributions of the first and second amplifier segments  124 . Adjusting the bias control signal to a value of “10” disables the first amplifier segment  124  and the maximum output power of segmented power amplifier  120  equals the maximum output power of the second amplifier segment  124 . Further adjusting the bias control signal to a value of “01” disables the second amplifier segment  124  and re-enables the first segment  124 . At this point, the maximum output power of segmented power amplifier  120  is the maximum power available from just the first segment  124 . Setting the bias control signal to a value of “00” disables both first and second segments  124 , thereby disabling segmented power amplifier  120 .  
         [0042]    Note that, in the illustrated example, the maximum power available from the first segment  124  is less than that available from the second segment  124 . Of course, a greater number of amplifier segments  124  would provide a greater number of available levels of output power. Since the maximum output power available from each segment  124  may be different, the overall maximum output power of the segmented power amplifier  120  may change slightly if a low-power segment  124  is enabled or disabled, or may change significantly if a high-power segment  124  is enabled or disabled.  
         [0043]    [0043]FIG. 6 provides a greatly simplified logic flow diagram associated with an exemplary embodiment of the present invention. FIG. 6 depicts the operation of both a controlling system (mobile terminal  100 ) and an exemplary embodiment for the segmented power amplifier  120  of the present invention. Operation begins ( 510 ) with the segmented power amplifier  120  disabled ( 512 ). Disabling segmented power amplifier  120  may entail adjusting the bias control signal provided to it such that none of its parallel amplifier segments  124  are enabled (biased on). Mobile terminal  100  continuously determines whether transmitter operations are required (NO from  514 ). If mobile terminal  100  does determine that must actively transmit (YES from  514 ), it determines a desired maximum transmit signal power level ( 516 ), and generates a bias control signal corresponding to this desired level ( 518 ). From the perspective of segmented power amplifier  120 , it must “read” the bias control signal it receives from mobile terminal  100  ( 520 ). Of course, reading the bias control signal may simply mean that segmented power amplifier  120  adjusts its bias circuit  126  accordingly in response to changes in the input control signal. In some embodiments, the segmented power amplifier  120  may indeed read a binary or analog value from the bias control signal, and set bias circuit  126  accordingly. In any case, segmented power amplifier  120  enables and disables a specific combination of parallel amplifier segments  124  in accordance with the present setting of the bias control signal ( 522 ).  
         [0044]    Thereafter, segmented power amplifier  120  “monitors” the bias control signal to determine if different a combination of enabled and disabled amplifier segments  124  has been selected ( 524 ). Monitoring may imply periodically checking the value of the bias control signal, or may simply mean that segmented power amplifier  120  remains responsive to changes in the bias control signal during operation. If there are no changes in the bias control signal (NO from  526 ), bias circuit  126  of segmented power amplifier  120  maintains its bias signal outputs corresponding to the value of the bias control signal. If the bias control signal changes (YES from  526 ), bias control circuit  126  response by enabling/disabling a new combination of parallel amplifier segments  124  corresponding to the value of the newly changed bias control signal.  
         [0045]    As discussed earlier, segmented power amplifier  120  may include one or more parallel amplifier segments  124  optimized for efficient linear operation at the lower current signal currents associated with lower transmit signal output power, and other segments  124  optimized for efficient operation at high output power. Thus, the mobile terminal  100  may have a set of defined combinations of amplifier segments  124  that are selectively enabled, based on whether low, high, or intermediate levels of transmit signal output power are required.  
         [0046]    The present invention allows tremendous flexibility in both implementation and use. Thus, the foregoing discussion and accompanying illustrations simply present exemplary embodiment of the present invention, and should not be construed as limiting the scope of the invention. Those skilled in the art will immediately appreciate the many possible variations afforded by the present invention. Such variations do not depart from the scope and spirit of the present invention and, indeed, are embraced herein.  
         [0047]    As noted, the segmented power amplifier  120  of the present invention allows a controlling system, such as a mobile terminal  100 , to configure the segmented power amplifier  120  for two or more levels of maximum amplifier output power based on enabling different combinations of parallel amplifier segments  124  comprising the segmented power amplifier  120 . Each amplifier segment comprises a like number of amplification stages  200  (transistor-based amplification stages). The present invention may be practiced with two or more parallel amplifier segments  124 , and the segments  124  may comprise like numbers of one or more amplification stages  200 .  
         [0048]    Although exemplary embodiments of the present invention use HBTs in the amplification stages  200 , this is not a limitation of the invention. Dual-gate FETs and other types of transistor amplifiers may be used as indicated by the particular needs of a given design or fabrication technology. Further, the present invention is not limited to inclusion in mobile terminals  100 . Indeed, many other types of systems, including other communications equipment, may advantageously use the segmented power amplifier  124  of the present invention. Thus, the scope of the present invention is limited only by the attached claims and all equivalents thereto.