Abstract:
Differential power amplifier circuitry includes a differential transistor pair, an input transformer, and biasing circuitry. The base contact of each transistor in the differential transistor pair may be coupled to the input transformer through a coupling capacitor. The coupling capacitors may be designed to resonate with the input transformer about a desired frequency range, thereby passing desirable signals to the differential transistor pair while blocking undesirable signals. The biasing circuitry may include a pair of emitter follower transistors, each coupled at the emitter to the base contact of each one of the transistors in the differential transistor pair and adapted to bias the differential transistor pair to maximize efficiency and stability.

Description:
RELATED APPLICATIONS 
     This application claims the benefit of U.S. provisional patent application No. 61/679,293, filed Aug. 3, 2012, the disclosure of which is hereby incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE DISCLOSURE 
     The present disclosure relates to power amplifier circuitry, and specifically to differential power amplifiers for use in a mobile device. 
     BACKGROUND 
     Modern mobile devices continue to demand an increasing amount of operating time from a single battery charge. Accordingly, power management is a primary concern for many mobile device manufacturers. One major source of power consumption within a mobile device is the power amplifier used in the transmission of wireless signals. A well designed power amplifier may reduce the power requirements of the mobile device in which it is incorporated, thereby significantly extending the battery life of the mobile device. 
       FIG. 1  shows conventional single-ended power amplifier circuitry  10  for use in a mobile device. The conventional single-ended power amplifier circuitry  10  includes an amplifying transistor  12  including a collector contact  14 , an emitter contact  16 , and a base contact  18 , biasing circuitry  20 , an input capacitor  22 , an output capacitor  24 , and an output inductor  26 . The base contact  18  of the amplifying transistor  12  is coupled to the biasing circuitry  20 . The input capacitor  22  is coupled between the base contact  18  of the amplifying transistor  12  and an input terminal  28 . The emitter contact  16  of the amplifying transistor  12  is coupled to ground. The collector contact  14  of the amplifying transistor  12  is coupled to a supply voltage VCC through the output inductor  26 . The output capacitor  24  is coupled between an output terminal  30  and the collector contact  14  of the amplifying transistor  12 . 
     The conventional single-ended power amplifier circuitry  10  is adapted to receive an input signal V_INPUT at the input terminal  28  and produce at the output terminal  30  an amplified output signal V_OUTPUT. Although the conventional single-ended power amplifier circuitry  10  effectively amplifies the input signal V_INPUT, the circuitry suffers from a relatively low efficiency when compared to alternative power amplifier architectures. Due to the limited output voltage of the conventional single-ended power amplifier circuitry  10 , the output impedance of the conventional single-ended power amplifier circuitry  10  must be kept low, on the order of 2-3Ω for a power delivery of 2 W with a supply voltage VCC of 3.6 V and a saturation voltage of 100 mV. In order to match the low output impedance of the conventional single-ended power amplifier circuitry  10  with a load, the output capacitor  24  and the output inductor  26  are adapted to match the impedance of the load. Assuming a standard load impedance of 50Ω, the transformation ratio of the output capacitor  24  and the output inductor  26  should be around  16 . Because of the relatively high transformation ratio required by the conventional single-ended power amplifier circuitry  10 , the efficiency of the circuitry will suffer, thereby degrading the performance of a mobile device in which the conventional single-ended power amplifier circuitry  10  is incorporated. 
     One way to increase the efficiency of a power amplifier is to use a differential architecture.  FIG. 2  shows conventional differential power amplifier circuitry  32  for use in a mobile device. The conventional differential power amplifier circuitry  32  includes a differential transistor pair  34  including a first transistor TR1 and a second transistor TR2, an input transformer  36 , an output transformer  38 , and biasing circuitry  40 . The input transformer  36  includes a single-ended input terminal  42 , a first differential output terminal  44 , and a second differential output terminal  46 . The input transformer  36  is adapted to receive a single-ended input at the single-ended input terminal  42 , and produce a differential output signal at the first differential output terminal  44  and the second differential output terminal  46 . The output transformer  38  includes a first differential input terminal  54 , a second differential input terminal  56 , and a single-ended output terminal  50 . The output terminal is adapted to receive a differential input signal at a first differential input terminal  54  and a second differential input terminal  56 , and produce a single-ended output signal at the single-ended output terminal  50 . 
     The first transistor TR1 includes a collector contact  52 , an emitter contact  58 , and a base contact  60 . The collector contact  52  of the first transistor TR1 is coupled to the first differential input terminal  54  of the output transformer  38 . The emitter contact  58  of the first transistor TR1 is coupled to ground. The base contact  60  of the first transistor TR1 is coupled to the first differential output terminal  44  of the input transformer  36 . The second transistor TR2 also includes a collector contact  62 , an emitter contact  64 , and a base contact  66 . The collector contact  62  of the second transistor TR2 is coupled to the second differential input terminal  56  of the output transformer  38 . The emitter contact  64  of the second transistor TR2 is coupled to ground. The base contact  66  of the second transistor TR2 is coupled to the second differential output terminal  46  of the input transformer  36 . The biasing circuitry  40  is coupled at the midpoint of the differential output terminals  44  and  46  of the input transformer  36 . 
     The conventional differential power amplifier circuitry  32  is adapted to receive an input signal V_INPUT at the single-ended input terminal  42  of the input transformer  36 , and produce at the single-ended output terminal  50  of the output transformer  38  an amplified output signal V_OUTPUT. Due to the differential architecture of the conventional differential power amplifier circuitry  32 , the amplified output signal V_OUTPUT is increased by a factor of two over the conventional single-ended power amplifier circuitry  10 . Accordingly, the output impedance of the differential transistor pair  34  can be about four times higher than that of the conventional single-ended power amplifier circuitry  10  while maintaining substantially the same power output. As a result of the increased output impedance of the conventional differential power amplifier circuitry  32 , the transformation ratio of the output transformer  38  is reduced by a factor of four, resulting in a higher efficiency than what is achievable by the conventional single-ended power amplifier circuitry  10  shown in  FIG. 1 . 
     Although the conventional differential power amplifier circuitry  32  is capable of efficiently producing an amplified output signal V_OUTPUT from an input signal V_INPUT, the conventional differential power amplifier circuitry  32  may suffer from instability due to the high gain characteristics of the differential transistor pair  34  at low frequencies. Accordingly, a power amplifier is needed that is both efficient and stable in order to increase the operating time of a mobile device in which it is incorporated. 
     SUMMARY 
     Differential power amplifier circuitry includes a differential transistor pair including a first transistor and a second transistor, an input transformer, and biasing circuitry. The input transformer includes a single-ended input terminal, a first differential output terminal, and a second differential output terminal. The first transistor includes a base contact coupled to the first differential output terminal of the input transformer through a first coupling capacitor, an emitter contact coupled to ground, and an output terminal. The second transistor includes a base contact coupled to the second differential output terminal of the input transformer through a second coupling capacitor, an emitter contact coupled to ground, and an output terminal. The biasing circuitry is coupled to the base contact of the first transistor and the second transistor. The first coupling capacitor and the second coupling capacitor are designed to resonate with the input transformer at a desired frequency range in order to pass a signal to the base terminals of the differential transistor pair for amplification, while blocking signals outside of the desired frequency range from reaching the differential transistor pair. Accordingly, instability of the differential power amplifier circuitry at undesirable frequency ranges is avoided. 
     According to one embodiment, the biasing circuitry includes a first biasing transistor and a second biasing transistor. The first biasing transistor is configured as an emitter-follower, and includes a base contact coupled to a fixed voltage source, a collector contact coupled to a supply voltage, and an emitter contact coupled to the base of the first transistor. The second biasing transistor is also configured as an emitter-follower, and includes a base contact coupled to a fixed voltage source, a collector contact coupled to a supply voltage, and an emitter contact coupled to the base of the second transistor. By using separate biasing circuitry for the first transistor and the second transistor, signal losses as a result of the biasing circuitry are minimized, thereby allowing the differential power amplifier circuitry to achieve higher gains with a greater level of efficiency. 
     Those skilled in the art will appreciate the scope of the present disclosure and realize additional aspects thereof after reading the following detailed description of the preferred embodiments in association with the accompanying drawing figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
       The accompanying drawing figures incorporated in and forming a part of this specification illustrate several aspects of the disclosure, and together with the description serve to explain the principles of the disclosure. 
         FIG. 1  shows a schematic representation of conventional single-ended power amplifier circuitry. 
         FIG. 2  shows a schematic representation of conventional differential power amplifier circuitry. 
         FIG. 3  shows a schematic representation of an embodiment of differential power amplifier circuitry according to the present disclosure. 
         FIG. 4  shows a schematic representation of the differential power amplifier circuitry of  FIG. 3  including further details of the biasing circuitry. 
         FIGS. 5A-5F  show waveforms describing the voltage response at a plurality of locations in the differential power amplifier circuitry according to the present disclosure. 
         FIG. 6  shows a schematic representation of an additional embodiment of differential power amplifier circuitry according to the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The embodiments set forth below represent the necessary information to enable those skilled in the art to practice the embodiments and illustrate the best mode of practicing the embodiments. Upon reading the following description in light of the accompanying drawing figures, those skilled in the art will understand the concepts of the disclosure and will recognize applications of these concepts not particularly addressed herein. It should be understood that these concepts and applications fall within the scope of the disclosure and the accompanying claims. 
     It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of the present disclosure. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the disclosure. As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises,” “comprising,” “includes,” and/or “including” when used herein specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. It will be further understood that terms used herein should be interpreted as having a meaning that is consistent with their meaning in the context of this specification and the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 
     Turning now to  FIG. 3 , a schematic representation of differential power amplifier circuitry  68  is shown according to one embodiment of the present disclosure. According to this embodiment, the differential power amplifier circuitry  68  includes a differential transistor pair  70  including a first transistor TR1 and a second transistor TR2, an input transformer  72 , an output transformer  74 , and biasing circuitry  76 . The input transformer  72  includes a primary winding  78  coupled between ground and a single-ended input terminal  80  and a secondary winding  82  coupled between a first differential output terminal  84  and a second differential output terminal  86 . The input transformer  72  is adapted to receive a single-ended input signal at the single-ended input terminal  80  and produce a differential output signal at the first differential output terminal  84  and the second differential output terminal  86 . The output transformer  74  includes a primary winding  88  coupled between a first differential input terminal  90  and a second differential input terminal  92  and a secondary winding  94  coupled between a single-ended output terminal  96  and ground. The output transformer  74  is adapted to receive a differential input signal at the first differential input terminal  90  and the second differential input terminal  92  and produce a single-ended output signal at the single-ended output terminal  96 . According to one embodiment, a supply voltage VCC is coupled at the midpoint of the primary winding  88  of the output transformer  74 . 
     The first transistor TR1 includes a collector contact  98 , an emitter contact  100 , and a base contact  102 . The collector contact  98  of the first transistor TR1 is coupled to the first differential input terminal  90  of the output transformer  74 . The emitter contact  100  of the first transistor TR1 is coupled to ground. The base contact  102  of the first transistor TR1 is coupled to the first differential output terminal  84  of the input transformer  72  through a first coupling capacitor CB1. The second transistor TR2 also includes a collector contact  104 , an emitter contact  106 , and a base contact  108 . The collector contact  104  of the second transistor TR2 is coupled to the second differential input terminal  92  of the output transformer  74 . The emitter contact  106  of the second transistor TR2 is coupled to ground. The base contact  108  of the second transistor TR2 is coupled to the second differential output terminal  86  of the input transformer  72  through a second coupling capacitor CB2. 
     The biasing circuitry is coupled to the base contact  102  of the first transistor TR1 and the base contact  108  of the second transistor TR2. The biasing circuitry is adapted to set the quiescent operating characteristics of the differential transistor pair  70 , as will be discussed in further detail below. 
     The differential power amplifier circuitry  68  is adapted to receive an input signal V_INPUT at the single-ended input terminal  80  of the input transformer  72 , and produce at the single-ended output terminal  96  of the output transformer  74  an amplified output signal V_OUTPUT. Due to the topology of the differential power amplifier circuitry  68 , a high level of efficiency and stability can be achieved, thereby extending the battery life of a mobile device in which the differential power amplifier circuitry  68  is incorporated, as will be discussed in further detail below. 
     According to one embodiment, the first coupling capacitor CB1 and the second coupling capacitor CB2 are designed to resonate with the secondary winding  82  of the input transformer  72  about one or more passing frequency bands. When resonant with the secondary winding  82  of the input transformer  72 , the first coupling capacitor CB1 and the second coupling capacitor CB2 form a substantially low impedance path between the secondary winding  82  of the input transformer  72  and the differential transistor pair  70 . Accordingly, signals about the one or more passing frequency bands will be delivered to the differential transistor pair  70  for amplification. When not resonant with the secondary winding  82  of the input transformer  72 , the first coupling capacitor CB1 and the second coupling capacitor CB2 form a substantially high impedance path between the secondary winding  82  of the input transformer  72  and the differential transistor pair  70 . Accordingly, signals outside of the one or more passing frequency bands will be blocked from reaching the differential transistor pair  70 , and will not be amplified. By designing the first coupling capacitor CB1 and the second coupling capacitor CB2 such that they are resonant with the secondary winding  82  of the input transformer  72  about one or more passing frequency bands, instability of the differential power amplifier circuitry  68  at undesirable frequency ranges can be avoided. 
       FIG. 4  shows the differential power amplifier circuitry  68  of  FIG. 3  including further details of the biasing circuitry  76  according to one embodiment of the present disclosure. According to this embodiment, the biasing circuitry  76  includes a biasing current source I_BIAS, a first diode connected transistor TD1, a second diode connected transistor TD2, a first biasing transistor TB1, and a second biasing transistor TB2. The first diode connected transistor TD1 includes a collector contact  110 , an emitter contact  112 , and a base contact  114 . The second diode connected transistor TD2 also includes a collector contact  116 , an emitter contact  118 , and a base contact  120 . The emitter contact  118  of the second diode connected transistor TD2 is coupled to ground. The collector contact  116  of the second diode connected transistor TD2 is coupled to the emitter contact  112  of the first diode connected transistor TD1. The collector contact  110  of the first diode connected transistor TD1 is coupled to the biasing current source I_BIAS. The base contact  114  of the first diode connected transistor TD1 and the base contact  120  of the second diode connected transistor TD2 are coupled to the collector contact  110  of the first diode connected transistor TD1 and the collector contact  116  of the second diode connected transistor TD2, respectively. Accordingly, a first biasing voltage V_BIAS is generated across the first diode connected transistor TD1 and the second diode connected transistor TD2. According to one embodiment, the first diode connected transistor TD1 and the second diode connected transistor TD2 are diodes. 
     The first biasing transistor TB1 includes a collector contact  122 , an emitter contact  124 , and a base contact  126 . The collector contact  122  of the first biasing transistor TB1 is coupled to a battery voltage V_BATT. The base contact  126  of the first biasing transistor TB1 is coupled between the collector contact  110  of the first diode connected transistor TD1 and the biasing current source I_BIAS such that the base contact  126  of the first biasing transistor TB1 receives the first biasing voltage V_BIAS. The emitter contact  124  of the first biasing transistor TB1 is coupled to the base contact  102  of the first transistor TR1 through a first biasing resistor RB1 in order to deliver a first biasing current IB1 to the base contact  102  of the first transistor TR1. 
     The second biasing transistor TB2 also includes a collector contact  128 , an emitter contact  130 , and a base contact  132 . The collector contact  128  of the second biasing transistor TB2 is coupled to the battery voltage V_BATT. The base contact  132  of the second biasing transistor TB2 is coupled to the collector contact  110  of the first diode connected transistor TD1 and the biasing current source I_BIAS such that the base contact  132  of the second biasing transistor TB2 receives the first biasing voltage V_BIAS. The emitter contact  130  of the second biasing transistor TB2 is coupled to the base contact  108  of the second transistor TR2 through a second biasing resistor RB2 in order to deliver a second biasing current IB2 to the base contact  108  of the second transistor TR2. 
     In a quiescent state of operation, wherein no input signal V_INPUT is present, a DC collector current IC will flow in the collector contact of the first transistor TR1 and the second transistor TR2. The collector current IC is determined by the ratio of the respective areas of the first diode connected transistor TD1, the second diode connected transistor TD2, the first biasing transistor TB1, the second biasing transistor TB2, the first transistor TR1, and the second transistor TR2, as well as the biasing current source I_BIAS. 
     For input signals V_INPUT having a small amplitude, the operating conditions will remain substantially similar to those where no input signal V_INPUT is present. Further, the input impedance will be equal to the parallel combination of the base-emitter junction of the first transistor TR1 and the first biasing resistor RB1, or the base-emitter junction of the second transistor TR2 and the second biasing resistor RB2. 
     As the amplitude of the input signal V_INPUT increases, the differential power amplifier circuitry  68  will behave as indicated in  FIGS. 5A-5F . The input signal V_INPUT presented to the single-ended input terminal  80  of the input transformer  72  is split into a differential signal. The non-inverted portion of the input signal V_INPUT is delivered through the first coupling capacitor CB1 to the base contact  102  of the first transistor TR1. The inverted portion of the input signal V_INPUT is delivered through the second coupling capacitor CB2 to the base contact  108  of the second transistor TR2. During the positive half cycle of the input signal V_INPUT shown in  FIG. 5A , the non-inverted portion of the input signal V_INPUT causes the voltage at the base contact  102  of the first transistor TR1 to rise, thereby increasing the collector current IC1 of the first transistor TR1. This causes the voltage at the collector contact  98  of the first transistor TR1 to fall, as is shown in  FIG. 5B-1 . The increase in the voltage at the base contact  102  of the first transistor TR1 also causes an increase in the voltage at the emitter contact  124  of the first biasing transistor TB1, as is shown in  FIG. 5C-1 . This causes the first biasing transistor TB1 to turn off, thereby lowering the first biasing current IB1 to zero, as is shown in  FIG. 5D-1 . Additionally, this causes the first biasing transistor TB1 to present a substantially high impedance to the base contact  102  of the first transistor TR1, as is shown in  FIG. 5E-1 . 
     During the same positive half cycle of the input signal V_INPUT shown in  FIG. 5A , the inverting portion of the input signal V_INPUT causes the voltage at the base contact  108  of the second transistor TR2 to fall, thereby turning off the second transistor TR2 and essentially lowering the collector current IC2 of the second transistor TR2 to zero. This causes the voltage at the collector contact  104  of the second transistor TR2 to rise, as shown in  FIG. 5B-2 . The decrease in the voltage at the base contact  108  of the second transistor TR2 also causes a decrease in the voltage at the emitter contact  130  of the second biasing transistor TB2, as shown in  FIG. 5C-2 . This causes the second biasing transistor TB2 to pass more current, thereby increasing the second biasing current IB2, as shown in  FIG. 5D-2 . Additionally, this causes the second biasing transistor TB2 to present a substantially low impedance to the base contact  108  of the second transistor TR2, as is shown in  FIG. 5E-2 . Because the second transistor TR2 is in an off state, the current flows into and charges the second coupling capacitor CB2. 
     During the negative half cycle of the input signal V_INPUT shown in  FIG. 5A , the first transistor TR1 will behave in a substantially similar manner to the second transistor TR2, and vice versa.  FIG. 5F  shows the input impedance of the differential power amplifier circuitry  68  during both the positive and negative half cycle of the input signal V_INPUT. As is shown, the input impedance of the differential power amplifier circuitry  68  at high drive levels is about equal to the base-emitter impedance of the first transistor TR1 and the second transistor TR2. This is greater than the input impedance seen at low drive levels of the input signal V_INPUT, where the input impedance is equal to the base-emitter impedance of the first transistor TR1 in parallel with the first biasing resistor RB1 and the base-emitter impedance of the second transistor TR2 in parallel with the biasing resistor RB2. Thus, it is shown that the input impedance increases with the drive level of the input signal V_INPUT. The relationship of the input impedance to the drive level of the input signal V_INPUT minimizes the effect of net gain decreases as the drive level increases (due to effects such as compression, where the output voltage becomes limited by the supply voltage VCC). Accordingly, the differential power amplifier circuitry  68  may operate at higher power levels without losing linearity. Further, the biasing current requirements of the circuitry may be reduced while maintaining the same linearity at a given maximum output power level, thereby saving power in a mobile device in which the differential power amplifier circuitry  68  is incorporated. 
       FIG. 6  shows the differential power amplifier circuitry  68  of  FIG. 3  including further details of the biasing circuitry  76  according to one embodiment of the present disclosure. According to this embodiment, the biasing circuitry  76  includes a first biasing current source I_BIAS1, a second biasing current source I_BIAS2, a first diode connected transistor TD1, a second diode connected transistor TD2, a third diode connected transistor TD3, a fourth diode connected transistor TD4, a first biasing transistor TB1, and a second biasing transistor TB2. The first diode connected transistor TD1 includes a collector contact  134 , an emitter contact  136 , and a base contact  138 . The second diode connected transistor TD2 also includes a collector contact  140 , and emitter contact  142 , and a base contact  144 . The emitter contact  142  of the second diode connected transistor TD2 is coupled to ground. The collector contact  140  of the second diode connected transistor TD2 is coupled to the emitter contact  136  of the first diode connected transistor TD1. The collector contact  134  of the first diode connected transistor TD1 is coupled to the first biasing current source I_BIAS1. The base contact  138  of the first diode connected transistor TD1 and the base contact  144  of the second diode connected transistor TD2 are coupled to the collector contact  134  of the first diode connected transistor TD1 and the collector contact  140  of the second diode connected transistor TD2, respectively. Accordingly, a first biasing voltage V_BIAS1 is generated across the first diode connected transistor TD1 and the second diode connected transistor TD2. 
     Similarly, the third diode connected transistor TD3 includes a collector contact  146 , an emitter contact  148 , and a base contact  150 . The fourth diode connected transistor TD4 also includes a collector contact  152 , an emitter contact  154 , and a base contact  156 . The emitter contact  154  of the fourth diode connected transistor TD4 is coupled to ground. The collector contact  152  of the fourth diode connected transistor TD4 is coupled to the emitter contact  148  of the third diode connected transistor TD3. The collector contact  146  of the third diode connected transistor TD3 is coupled to the second biasing current source I_BIAS2. The base contact  150  of the third diode connected transistor TD3 and the base contact  156  of the fourth diode connected transistor TD4 are coupled to the collector contact  146  of the third diode connected transistor TD3 and the collector contact  152  of the fourth diode connected transistor TD4, respectively. Accordingly, a second biasing voltage V_BIAS2 is generated across the third diode connected transistor TD3 and the fourth diode connected transistor TD4. 
     The first biasing transistor TB1 includes a collector contact  158 , an emitter contact  160 , and a base contact  162 . The collector contact  158  of the first biasing transistor TB1 is coupled to a battery voltage V_BATT. The base contact  162  of the first biasing transistor TB1 is coupled between the collector contact  134  of the first diode connected transistor TD1 and the first biasing current source I_BIAS1 such that the base contact  162  of the first biasing transistor TB1 receives the first biasing voltage V_BIAS1. The emitter contact  160  of the first biasing transistor TB1 is coupled to the base contact  102  of the first transistor TR1 through a first biasing resistor RB1 in order to deliver a first biasing current IB1 to the first transistor TR1. 
     The second biasing transistor TB2 includes a collector contact  164 , an emitter contact  166 , and a base contact  168 . The collector contact  164  of the second biasing transistor TB2 is coupled to a battery voltage V_BATT. The base contact  168  of the second biasing transistor TB2 is coupled between the collector contact  146  of the third diode connected transistor TD3 and the second biasing current source I_BIAS2 such that the base contact  168  of the second biasing transistor TB2 receives the second biasing voltage V_BIAS2. The emitter contact  166  of the second biasing transistor TB2 is coupled to the base contact  108  of the second transistor TR2 through a second biasing resistor RB2 in order to deliver a second biasing current IB2 to the second transistor TR2. 
     In a quiescent state of operation, wherein no input signal V_INPUT is present, a DC collector current IC will flow in the collector contact of the first transistor TR1 and the second transistor TR2. The collector current IC is determined by the ratio of the respective areas of the first diode connected transistor TD1, the second diode connected transistor TD2, the third diode connected transistor TD3, the fourth diode connected transistor TD4, the first biasing transistor TB1, the second biasing transistor TB2, the first transistor TR1, and the second transistor TR2, as well as the first biasing current source I_BIAS1 and the second biasing current source I_BIAS2. For input signals V_INPUT having a small amplitude, these conditions will remain substantially similar. 
     As the amplitude of the input signal V_INPUT increases, the differential power amplifier circuitry  68  will behave as indicated in  FIGS. 5A-5F , as described above. Once again, due to the input impedance characteristics of the differential power amplifier circuitry  68 , the effects of net gain decreases as the drive level of the input signal V_INPUT increases (due to effects such as compression, where the output voltage becomes limited by the supply voltage VCC) are reduced. Accordingly, the differential power amplifier circuitry  68  may operate at higher power levels without losing linearity. Further, the biasing current requirements of the circuitry may be reduced while maintaining the same linearity at a given maximum output power level, thereby saving power in a mobile device in which the differential power amplifier circuitry  68  is incorporated. 
     Those skilled in the art will recognize improvements and modifications to the preferred embodiments of the present disclosure. All such improvements and modifications are considered within the scope of the concepts disclosed herein and the claims that follow.