Patent Publication Number: US-9431969-B2

Title: Doherty power amplifier with tunable impedance load

Description:
RELATED APPLICATIONS 
     This application claims the benefit of provisional patent application Ser. No. 61/735,820, filed on Dec. 11, 2012, and provisional patent application Ser. No. 61/747,017, filed on Dec. 28, 2012, the disclosures of which are hereby incorporated herein by reference in their entireties. 
    
    
     FIELD OF THE DISCLOSURE 
     This disclosure relates generally to radio frequency (RF) amplification devices. 
     BACKGROUND 
     In radio frequency (RF) applications, a conventional Doherty amplification circuit typically includes a main RF amplifier coupled in parallel with a peaking RF amplifier. At low power levels, the main RF amplifier in the conventional Doherty amplification circuit is activated and biased for linear operation, while the peaking RF amplifier is deactivated. The peaking RF amplifier is activated once an RF signal reaches a particular signal level, which is generally at or near a compression point of the main RF amplifier. To increase power efficiency, quarter wave transmission line transformers or quarter wave transmission line inverters are often employed in conventional Doherty amplification circuits in order to provide the appropriate impedance transformations while the peaking RF amplifier is activated and deactivated. Unfortunately, quarter wave transmission line transformers/inverters have narrowband characteristics and thus do not allow for broadband operation. Furthermore, at the higher frequencies, the quarter wave transmission line transformers/inverters in these conventional Doherty amplification circuits degrade the power efficiency of the Doherty amplification circuit at backed-off power levels. Generally, this is due to the narrowband characteristics of the quarter wave transmission line transformers/inverters and, in addition, to the inability of the quarter wave transmission line transformers/inverters to correct for parasitic effects in the peaking RF amplifier at higher frequencies. 
     Accordingly, RF circuit designs that improve bandwidth performance and/or the power efficiency of the Doherty amplification circuit are needed. 
     SUMMARY 
     Radio frequency (RF) amplification devices, along with methods of operating the same, are disclosed that include Doherty amplification circuits configured to amplify an RF signal. The Doherty amplification circuits are dynamically tunable in order to increase power efficiency and/or to allow for broadband operation. In one embodiment, an RF amplification device includes a control circuit and a Doherty amplification circuit configured to amplify an RF signal. The Doherty amplification circuit includes a quadrature coupler having an isolation port and a tunable impedance load coupled to the isolation port of the quadrature coupler and configured to provide a tunable impedance. The control circuit is configured to tune the tunable impedance of the tunable impedance load dynamically as a function of RF power in the Doherty amplification circuit. In this manner, the control circuit can provide dynamic load modulation to a load impedance presented to the main RF amplifier, thereby increasing the power efficiency of the Doherty amplification circuit, particularly at backed-off power levels. The dynamic load modulation provided by the control circuit thereby allows the Doherty amplification circuit to provide broadband amplification in various RF communication bands. 
     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  illustrates an exemplary radio frequency (RF) amplification device having a Doherty amplification circuit and a control circuit, wherein the Doherty amplification circuit includes a first quadrature coupler that receives an RF signal; a main RF amplifier; a peaking RF amplifier; and a second quadrature coupler having an isolation port and a tunable impedance load coupled to the isolation port and configured to provide a tunable impedance. 
         FIG. 2  illustrates one embodiment of the RF amplification device shown in  FIG. 1 , wherein the control circuit is configured to detect RF power in the Doherty amplification circuit at an output port provided by the second quadrature coupler. 
         FIG. 3  illustrates another embodiment of the RF amplification device shown in  FIG. 1 , wherein the control circuit is configured to detect RF power in the Doherty amplification circuit at an input port provided by the first quadrature coupler. 
         FIG. 4  illustrates yet another embodiment of the RF amplification device shown in  FIG. 1 , wherein the control circuit is configured to detect RF power in the Doherty amplification circuit from a supply current provided to the peaking RF amplifier. 
         FIG. 5  illustrates one embodiment of the tunable impedance load shown in  FIG. 1 , where the embodiment of the tunable impedance load shown in  FIG. 5  is purely resistive. 
         FIG. 6  illustrates is a graph including power curves that describe power added efficiency (PAE) as a function of output power in the RF amplification device shown in  FIG. 2  using the tunable impedance load shown in  FIG. 5 , wherein each of the power curves demonstrates PAE versus output power for a different pairing of a carrier frequency of the RF signal and a tunable impedance of tunable impedance load where the carrier frequency/tunable impedance pairings correspond to (680 MHz, Znorm×10 (Znorm is a characteristic amplifier impedance of the Doherty amplification circuit), (850 MHz, Znorm×10), (1020 MHz, Znorm×10), and (850 MHz, Znorm). 
         FIG. 7  illustrates another embodiment of the tunable impedance load shown in  FIG. 1 , where the embodiment of the tunable impedance load shown in  FIG. 7  is resistive and inductive. 
         FIG. 8  is a graph including power curves that describe PAE as a function of output power in the RF amplification device shown in  FIG. 2  using the tunable impedance load shown in  FIG. 7 , wherein each of the power curves demonstrates PAE versus output power where the carrier frequency/tunable impedance pairings correspond to (680 MHz, Znorm×10), (850 MHz, Znorm×10), (1020 MHz, Znorm×10), and (850 MHz, Znorm). 
         FIG. 9  illustrates power curves that describe PAE as a function of output power in the RF amplification device shown in  FIG. 2 , wherein one of the power curves is produced using the tunable impedance load shown in  FIG. 5  and the other power curve is produced using the tunable impedance load shown in  FIG. 7 , and both power curves have a carrier frequency/tunable impedance pairing corresponding to (680 MHz, Znorm×10). 
         FIG. 10  illustrates power curves that describe PAE as a function of output power in the RF amplification device shown in  FIG. 2 , wherein one of the power curves is produced using the tunable impedance load shown in  FIG. 5  and the other power curve is produced using the tunable impedance load shown in  FIG. 7 , and both power curves have a carrier frequency/tunable impedance pairing corresponding to (1020 MHz, Znorm×10). 
         FIG. 11  illustrates power curves that describe PAE as a function of output power in the RF amplification device shown in  FIG. 2 , wherein two of the power curves are produced using the tunable impedance load shown in  FIG. 5  and have carrier frequency/tunable impedance pairings of (1020 MHz, Znorm×10) and (1020 MHz, Znorm×40), respectively, and the other power curve is produced using the tunable impedance load shown in  FIG. 7  and has a carrier frequency/tunable impedance pairing corresponding to (1020 MHz, Znorm×10). 
         FIG. 12  illustrates power curves that describe PAE as a function of output power in the RF amplification device shown in  FIG. 2 , wherein all of the power curves are produced using the tunable impedance load shown in  FIG. 5  and have carrier frequency/tunable impedance pairings of (850 MHz, Znorm×10), (1020 MHz, Znorm×10), and (1020 MHz, Znorm×30), respectively. 
         FIG. 13  illustrates yet another embodiment of the tunable impedance load shown in  FIG. 1 , where the embodiment of the tunable impedance load shown in  FIG. 13  is resistive and inductive, like the embodiment in  FIG. 7 , but includes inductors with varying physical sizes. 
         FIG. 14  illustrates still another embodiment of the tunable impedance load shown in  FIG. 1 , where the embodiment of the tunable impedance load shown in  FIG. 14  is resistive, inductive, and capacitive so as to form resonant tanks. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates one embodiment of a radio frequency (RF) amplification device  10  having a Doherty amplification circuit  12  and a control circuit  14 . As explained in further detail below, the control circuit  14  is operably associated with the Doherty amplification circuit  12  in order to improve the power performance and the Doherty characteristics of the Doherty amplification circuit  12 . For instance, the control circuit  14  may be used in for the Doherty amplification circuit  12  to obtain improved power efficiency at backed-off power levels. Additionally, the control circuit  14  can also be used to improve the bandwidth performance of the Doherty amplification circuit  12  and thus allow the Doherty amplification circuit  12  to provide broadband amplification. 
     The Doherty amplification circuit  12  is configured to amplify an RF signal  16 . To amplify the RF signal  16 , the Doherty amplification circuit  12  includes a main RF amplifier  18  and a peaking RF amplifier  20 . While a signal level of the RF signal  16  is below a threshold level, the peaking RF amplifier  20  is deactivated and the main RF amplifier  18  provides all of the amplification to the RF signal  16 . The RF signal  16  may be any type of RF signal depending on the type of communication device (e.g., smartphone, tablet, laptop, base station, etc.) in which the Doherty amplification circuit  12  is provided and the amplification operation being provided by the Doherty amplification circuit  12 . The main RF amplifier  18  is configured to amplify the RF signal  16  in accordance with a main amplifier gain of the main RF amplifier  18 . So long as the main RF amplifier  18  is within a linear operating range (i.e., not saturated and below a compression point), the peaking RF amplifier  20  is deactivated. 
     The specific characteristics of the Doherty amplification circuit  12  may vary in accordance with the communication device and technological environment in which the Doherty amplification circuit  12  is employed and, in addition, the performance parameters relevant to the operation of the Doherty amplification circuit  12  for the particular application(s) of the Doherty amplification circuit  12  within the communication device (or prospective communication device(s)) and the technological environment (or prospective technological environment(s)). It should be noted that the RF amplification device  10  may be configured for operation in any suitable communication device and technological environment. Thus, the RF signal  16  may be any type of RF signal. More specifically, the RF amplification device  10  shown in  FIG. 1  can be manufactured to meet the requirements of a wide variety of multiplexing schemes and RF communication standards. For example, the RF signal  16  may be an uplink signal transmitted to base stations and/or a downlink signal transmitted from base stations. Furthermore, the RF signal  16  may be encoded in accordance with any type of multiplexing scheme and/or RF communication standard. For example, the RF signal  16  may be multiplexed using time division multiplexing (TDM), frequency division multiplexing (FDM), space division multiplexing (SDM), code division multiple access (CDMA) multiplexing, orthogonal frequency division multiple access (OFDMA) multiplexing, and/or the like. Additionally, the RF amplification device  10  may be configured to provide duplexing for various RF communication standards. For example, the RF amplification device  10  may be configured to provide amplification for the RF signal  16  if the RF signal  16  is formatted in accordance with 2G Global System for Mobile Communications (GSM) standards, 3G standards, 4G Long Term Evolution (LTE) standards, and/or the like. Furthermore, the RF amplification device  10  may provide duplexing for one or more specifications within these RF communication standards, along with their RF communication bands. For instance, the RF signal  16  may be formatted in accordance with RF communication bands defined by specifications of the 2G GSM standard, such as a Digital Communication System (DCS) specification, a Personal Communications Service (PCS) specification, a GSM-850 specification, and a GSM-900 specification; specifications within the 3G standard, such as an Enhanced Data Rates for GSM Evolution (EDGE)-850 specification, an EDGE-950 specification, an EDGE-1800 specification, and an EDGE-1900 specification; and specifications within the 4G LTE standard, such as a Mobile Device Management (MDM) specification, a High Speed Packet Access (HSPA) specification, a Multiple-Input and Multiple-Output (MIMO) specification, and/or the like. 
     Note that relational terminology such as “substantially,” “approximately,” and/or the like should be interpreted objectively in accordance with the communication device and technological environment in which the RF amplification device  10  is employed and, in addition, the performance parameters relevant to the operation of the RF amplification device  10  for the particular application of the RF amplification device  10  within the communication device (or at least one prospective communication device) and the technological environment (or at least one prospective technological environment). 
     In this embodiment, the RF signal  16  may be an RF uplink signal for uplink to a base station from a mobile communication device (e.g., smartphone, tablet, laptop, etc.). The RF amplification device  10  may be within the mobile communication device and the mobile communication device may be using the Doherty amplification circuit  12  to amplify the RF signal  16  for transmission by an antenna (not shown). Alternatively, the RF amplification device  10  may be within the base station. Thus, the base station may be using the Doherty amplification circuit  12  for amplification upon reception of the RF signal  16  from the mobile communication device. In another embodiment, the RF signal  16  may be an RF downlink signal for downlink to the mobile communication device (e.g., smartphone, tablet, laptop, etc.) from the base station. In this case, the mobile communication device may be using the Doherty amplification circuit  12  to amplify the RF signal  16  after reception from the base station. Alternatively, the base station may be using the Doherty amplification circuit  12  to amplify the RF signal  16  for transmission by an antenna (not shown) to the mobile communication device. 
     Referring again to the Doherty amplification circuit  12  shown in  FIG. 1 , the output impedance of the peaking RF amplifier  20  is very high and the peaking RF amplifier  20  appears like an open circuit with respect to the main RF amplifier  18  while the main RF amplifier  18  is activated and the peaking RF amplifier  20  is deactivated. However, the peaking RF amplifier  20  is configured to activate in response to the signal level of the RF signal  16  reaching a threshold level. The Doherty amplification circuit  12  is configured such that the main RF amplifier  18  remains activated while the peaking RF amplifier  20  is activated. In some embodiments, the main RF amplifier  18  may be held at or near its peak power level (i.e., saturation). In general, this peak power level occurs just prior to the 1 dB compression point (i.e., a knee voltage) of the main RF amplifier  18  and thus when the main RF amplifier  18  is nearly saturated. 
     However, it should be noted that some embodiments of the RF amplification device  10  may hold the main RF amplifier  18  at backed-off power levels while the peaking RF amplifier  20  is activated. If the RF amplification device  10  holds the main RF amplifier  18  at backed-off power levels while the peaking RF amplifier  20  is activated, the control circuit  14  may be configured to reduce or prevent power-efficiency degradations in the Doherty amplification circuit  12  due to the main RF amplifier  18  operating at power levels backed off from the 1 dB compression point. This is typically desirable for modern communication systems. 
     As shown in  FIG. 1 , the main RF amplifier  18  and the peaking RF amplifier  20  are operably associated by a first quadrature coupler  22  and a second quadrature coupler  24 . With regard to the first quadrature coupler  22 , the first quadrature coupler  22  includes a first port  26 , a second port  28 , a third port  30 , and a fourth port  32 . It should be noted that throughout this disclosure, the term “port” refers to any type of definable circuit location and/or structure that is operable to receive and/or transmit electromagnetic signals to or from an electronic component. For example, a port may be a terminal, a contact, a pad, a pin, a wire, a conductive bond, a node, a group of more than one of the aforementioned elements (e.g., terminals for a differential signal), and/or the like. 
     The first port  26  of the first quadrature coupler  22  is operable to receive the RF signal  16 . For example, the first port  26  may be coupled to receive the RF signal  16  from upstream RF circuitry (not shown) that is exogenous to the RF amplification device  10 . Thus, a source impedance of the upstream RF circuitry may be presented to the Doherty amplification circuit  12  at the first port  26  of the first quadrature coupler  22 . 
     The second port  28  of the first quadrature coupler  22  is an isolation port. In this embodiment, an impedance load  34  is coupled to the first quadrature coupler  22  at the second port  28 . The impedance load  34  thus serves as a termination impedance of the first quadrature coupler  22 . 
     As such, the second port  28  is isolated from the first port  26 , the third port  30 , and the fourth port  32 . The third port  30  is coupled to an input terminal  36  of the main RF amplifier  18  while the fourth port  32  is coupled to an input terminal  38  of the peaking RF amplifier  20 . In this embodiment, a resistance and a reactance of the impedance load  34  is configured to be constant. As such, the impedance load  34  shown in  FIG. 1  is not tunable. For example, the terminating impedance of the impedance load  34  may be approximately equal to 50Ω. Also, in some embodiments, the impedance may be purely resistive and constant such that the impedance load  34  has no reactance. 
     The second quadrature coupler  24  shown in  FIG. 1  is also operably associated with the main RF amplifier  18  and the peaking RF amplifier  20 . The second quadrature coupler  24  includes a fifth port  40 , a sixth port  42 , a seventh port  44 , and an eighth port  46 . The fifth port  40  of the second quadrature coupler  24  is coupled to an output terminal  48  of the main RF amplifier  18 . Thus, the main RF amplifier  18  is coupled between the third port  30  of the first quadrature coupler  22  and the fifth port  40  of the second quadrature coupler  24 . The sixth port  42  is coupled to an output terminal  50  of the peaking RF amplifier  20 . Thus, the peaking RF amplifier  20  is coupled between the fourth port  32  of the first quadrature coupler  22  and the sixth port  42  of the second quadrature coupler  24 . An output impedance of the peaking RF amplifier  20  is thus presented at the sixth port  42  to the second quadrature coupler  24  and an input impedance of the peaking RF amplifier  20  is presented to the first quadrature coupler  22  at the fourth port  32 . 
     The second quadrature coupler  24  is operable to transmit the RF signal  16  at the seventh port  44  after the RF signal  16  has been amplified by the Doherty amplification circuit  12 . The seventh port  44  of the second quadrature coupler  24  is thus an output port of the Doherty amplification circuit  12 . For example, the seventh port  44  may be coupled to transmit the RF signal  16  to downstream RF circuitry (not shown) that is exogenous to the RF amplification device  10 . A load impedance of the downstream RF circuitry may thus be presented to the Doherty amplification circuit  12  at the seventh port  44  of the second quadrature coupler  24 . 
     The eighth port  46  of the second quadrature coupler  24  is an isolation port. In this embodiment, a tunable impedance load  54  is coupled to the second quadrature coupler  24  at the eighth port  46 . The tunable impedance load  54  is configured to provide a tunable impedance, which is provided as a termination impedance of the second quadrature coupler  24 . Thus, the tunable impedance load  54  may have a variable resistance and/or a variable reactance. Since the tunable impedance may be set by the variable resistance and/or the variable reactance, adjusting the variable resistance and/or the variable reactance of the tunable impedance load  54  thereby adjusts the termination impedance of the second quadrature coupler  24 . 
     The control circuit  14  is configured to detect RF power in the Doherty amplification circuit  12 . For example, in this embodiment, the control circuit  14  is configured to receive a feedback input  56  that indicates RF power in the Doherty amplification circuit  12 . The feedback input  56  may include one or more feedback signals that indicate the RF power in the Doherty amplification circuit  12 . The control circuit  14  may be analog, digital, or both, and thus may detect the RF power using analog techniques, digital techniques, and/or a mixture of both. When digital techniques are employed, digital-to-analog converters may be employed by the control circuit  14  such that the feedback input  56  can be converted into a digital reading related to RF power of the Doherty amplification circuit  12 . If analog techniques are employed, the feedback input  56  may result in an analog response controlled by the characteristics of the feedback input  56  that indicate the RF power in the Doherty amplification circuit  12 . Variations in topology and design for the control circuit  14  would be apparent to one of ordinary skill in the art provided one of ordinary skill in the art has a proper understanding of the control principles described in this disclosure. The control circuit  14  may thus also detect RF power in the Doherty amplification circuit  12  dynamically, as long as the Doherty amplification circuit  12  is providing amplification to the RF signal  16 . In this manner, the control circuit  14  can respond dynamically to both exogenous and endogenous changes that result in modifications in the RF power of the Doherty amplification circuit  12 . 
     The Doherty amplification circuit  12  has a characteristic amplifier impedance Znorm. The characteristic amplifier impedance Znorm is a source impedance of the Doherty amplification circuit  12 . As seen from an input side of the second quadrature coupler  24 , the characteristic amplifier impedance Znorm is a source impedance presented at the fifth port  40  of the second quadrature coupler  24  from the main RF amplifier  18 . The second quadrature coupler  24  provides an impedance transformation to the characteristic amplifier impedance Znorm and the characteristic amplifier impedance Znorm presented from an output side of the second quadrature coupler  24  at the seventh port  44  with the impedance transformation. Thus, the characteristic amplifier impedance Znorm is also presented as a source impedance at the seventh port  44 , but with the impedance transformation provided by the second quadrature coupler  24  relative to the input side at the fifth port  40 . 
     A load impedance is presented at the output side of the second quadrature coupler  24  at the seventh port  44 . For example, the downstream RF circuitry may be coupled to the seventh port  44  to present the load impedance at the seventh port  44 . At the seventh port  44 , the load impedance is seen from the output side of the second quadrature coupler  24 . From the input side of the second quadrature coupler  24 , the second quadrature coupler  24  provides an impedance transformation to the load impedance presented at the seventh port  44 . From the input side, the load impedance is presented by the second quadrature coupler  24  to the main RF amplifier  18 , but transformed relative to the output side of the second quadrature coupler  24  in accordance with the impedance transformation provided by the second quadrature coupler  24 . In this embodiment, the second quadrature coupler  24  presents the load impedance to the main RF amplifier  18  at the fifth port  40 . 
     To improve power efficiency, the control circuit  14  is configured to tune the tunable impedance load  54  dynamically as a function of the RF power detected in the Doherty amplification circuit  12 . This adjusts the impedance transformations provided by the second quadrature coupler  24  to the source impedances and the load impedances described above. Thus, by tuning the tunable impedance load  54 , the control circuit  14  can set the load impedance presented to the main RF amplifier  18  by the second quadrature coupler  24  at the fifth port  40  and the characteristic amplifier impedance Znorm presented at the seventh port  44  at or close to optimal values in different RF communication bands. 
     To provide Doherty amplification operation, the load impedance should approximately equal double the characteristic amplifier impedance Znorm while the peaking RF amplifier  20  is deactivated at the fifth port  40 . Thus, when the main RF amplifier  18  is approximately saturated and the peaking RF amplifier  20  is deactivated, the load impedance should equal approximately double the characteristic amplifier impedance Znorm at the fifth port  40 . However, while the peaking RF amplifier  20  is activated, the load impedance seen by the main RF amplifier  18  is decreased from approximately double the characteristic amplifier impedance Znorm to approximately the characteristic amplifier impedance Znorm. When both the main RF amplifier  18  and the peaking RF amplifier  20  are approximately at saturation, the load impedance seen by the main RF amplifier  18  should be equal to the characteristic amplifier impedance Znorm. 
     The control circuit  14  may be configured to tune the tunable impedance of the tunable impedance load  54  dynamically so as to provide a Doherty amplification operation in different RF communication bands. As mentioned above, while the main RF amplifier  18  is activated and the peaking RF amplifier  20  is being deactivated, the control circuit  14  is configured to tune the tunable impedance of the tunable impedance load  54  dynamically as the function of the RF power of the Doherty amplification circuit  12  such that the load impedance presented by the second quadrature coupler  24  to the main RF amplifier  18  at the fifth port  40  is approximately equal to double the characteristic amplifier impedance Znorm of the Doherty amplification circuit  12 . For example, if the load impedance at the seventh port  44  is 50Ω, the control circuit  14  tunes the tunable impedance of the tunable impedance load  54  so that the main RF amplifier  18  is presented a load impedance equal to 100Ω or double the characteristic amplifier impedance Znorm of the Doherty amplification circuit  12  at the fifth port  40 . In this case, an impedance of the peaking RF amplifier  20  is very high and the peaking RF amplifier  20  appears like an open circuit with respect to the main RF amplifier  18  and the seventh port  44 . 
     The load impedance is maintained approximately equal to double the characteristic amplifier impedance Znorm of the Doherty amplification circuit  12  at the fifth port  40  until the main RF amplifier  18  is approximately saturated and the peaking RF amplifier  20  is activated. While both the peaking RF amplifier  20  and the main RF amplifier  18  are activated, the control circuit  14  is configured to tune the tunable impedance of the tunable impedance load  54  smaller such that the load impedance presented by the second quadrature coupler  24  to the main RF amplifier  18  at the fifth port  40  is decreased as the RF power of the Doherty amplification circuit  12  is increased. In other words, the main RF amplifier  18  remains in saturation while the RF signal  16  is above the threshold level and the load impedance presented by the second quadrature coupler  24  to the main RF amplifier  18  at the fifth port  40  decreases as power to the peaking RF amplifier  20  is increased. The control circuit  14  tunes the tunable impedance load dynamically as the function of the RF power detected in the Doherty amplification circuit  12  such that the load impedance presented by the second quadrature coupler  24  to the main RF amplifier  18  has an impedance range from approximately double the characteristic amplifier impedance Znorm of the Doherty amplification circuit  12  to approximately the characteristic amplifier impedance Znorm of the Doherty amplification circuit  12 . The load impedance presented at the fifth port  40  therefore varies from approximately 2*Znorm to Znorm as the RF power of the Doherty amplification circuit  12  is increased while the peaking RF amplifier  20  is activated. As such, the control circuit  14  is further configured to tune the tunable impedance of the tunable impedance load  54  dynamically as the function of the RF power of the Doherty amplification circuit  12  such that the load impedance presented by the second quadrature coupler  24  to the main RF amplifier  18  at the fifth port  40  is set approximately to the characteristic amplifier impedance Znorm when both the main RF amplifier  18  and the peaking RF amplifier  20  are approximately saturated. 
     To increase the power efficiency of the Doherty amplification circuit  12 , the tunable impedance of the tunable impedance load  54  may have a variable real impedance (i.e., a variable resistance). Generally, the tunable impedance of the tunable impedance load  54  should be higher (i.e., by a factor of 5 or greater) than a real impedance of the load impedance presented at the seventh port  44  (i.e., the output port of the Doherty amplification circuit  12 ). Furthermore, to optimize the power efficiency in different RF communication bands, the tunable impedance of the tunable impedance load  54  may have different impedance levels at various frequencies for a given power level of the RF power and/or an imaginary impedance that compensates for non-ideal parasitic reactances of the output power load impedances of the peaking RF amplifier  20  and the main RF amplifier  18  across a wide frequency band. The control circuit  14  is configured to dynamically vary the real impedance and/or the imaginary impedance of the tunable impedance provided by the tunable impedance load  54  to provide the Doherty amplification operation described above. 
     With regard to the control circuit  14  shown in  FIG. 1 , the control circuit  14  is configured to generate an impedance control output  58  that sets the tunable impedance of the tunable impedance load  54 . The impedance control output  58  may include one or more control signals that are operable to set the variable resistance and/or the variable reactance of the tunable impedance load  54 . The function implemented by the control circuit  14  may thus map the RF power detected in the Doherty amplification circuit  12  to permutations of the impedance control output  58 . These permutations of the impedance control output  58  set the tunable impedance of the tunable impedance load  54  to different impedance levels. More specifically, different permutations of the impedance control output  58  can be selected by the control circuit  14  to adjust the variable resistance and/or variable reactance in the manner explained above. As such, the real impedance and/or the imaginary impedance of the tunable impedance provided by the tunable impedance load  54  are set by the control circuit  14  using the impedance control output  58 . The function implemented by the control circuit  14  may thus ultimately map the RF power detected in the Doherty amplification circuit  12  to desired impedance levels for the tunable impedance provided by the tunable impedance load  54 . 
     The tunable impedance load  54  may include networks of passive and/or active circuit components that are responsive to the impedance control output  58  such that the tunable impedance of the tunable impedance load  54  is set in accordance with the particular permutation of the impedance control output  58  provided by the control circuit  14 . The control circuit  14  may be configured to generate the impedance control output  58  such that the permutations of the impedance control output  58  are (at least partially) discrete permutations and/or (at least partially) continuous permutations. This may depend on a particular topology of the tunable impedance load  54 . For example, a non-saturated transistor (not shown) or a non-saturated network of transistors may be used so that the variable resistance and/or the variable reactance can be varied in a continuous manner based on the impedance control output  58 . In this case, the impedance control output  58  may be at least partially analog. However, the tunable impedance load  54  may also be provided by an impedance matching network in which switches selectively connect passive circuit components (i.e., resistors, capacitors, and/or inductors) so that the tunable impedance of the tunable impedance load  54  varies discretely. In this case, the tunable impedance load  54  may switch the switches on and off in response to the particular and discrete permutation of the impedance control output  58  from the control circuit  14 . 
     Additionally, the function implemented by the control circuit  14  may further depend on other variables besides the RF power detected. In this embodiment, the control circuit  14  is further configured to receive a control input  60 , which may include one or more control signals. The control input  60  is configured to indicate at least one operational frequency characteristic of the RF signal  16 . For example, an operational frequency characteristic of the RF signal  16  may be a carrier frequency of the RF signal  16  where the control input  60  may include one or more control signals identifying the carrier frequency. The function implemented by the control circuit  14  may thus not only depend on the RF power detected but also on the carrier frequency of the RF signal  16 . Alternatively or additionally, another operational frequency characteristic of the RF signal  16  may be a frequency band of the RF signal  16 . In one embodiment, to indicate the frequency band of the RF signal  16 , the control input  60  may indicate an RF communication specification (see above) used to format the RF signal  16 , and thus may ultimately indicate the frequency band in which the RF signal  16  operates. 
     Furthermore, the control circuit  14  may be operable in a detect mode and in a non-detect mode. In the detect mode, the control circuit  14  is configured to detect the RF power in the Doherty amplification circuit  12 , as described above. However, in the non-detect mode, the control input  60  may further indicate the RF power of the Doherty amplification circuit  12 . For example, the RF power may be at a known a priori given power setting for transmission or reception of the RF signal  16 . In this manner, the control circuit  14  may be configured to store a look-up table (in a non-transient computer-readable medium) and implement the look-up table, which indicates the permutation of the impedance control output  58  given the RF power and/or the operating frequency of the Doherty amplification circuit  12  that is indicated by the control input  60 . As such, the control circuit  14  is also configured to tune the tunable impedance of the tunable impedance load  54  in accordance with the RF power and/or operating frequency of the Doherty amplification circuit  12  indicated by the control input  60 . 
     Thus, in both the detect mode and the non-detect mode, the control circuit  14  may be configured to tune the tunable impedance of the tunable impedance load  54  dynamically based on the RF power of the Doherty amplification circuit  12  and the operational frequency characteristic(s) of the RF signal  16 . This is advantageous, since the characteristic amplifier impedance Znorm presented at the fifth port  40  generally may vary based on the operational frequency characteristic(s) of the RF signal  16 . Furthermore, the source impedance presented by the upstream RF circuitry (not shown) at the first port  26  and the load impedance presented by the downstream RF circuitry (not shown) at the seventh port  44  also vary based on the operational frequency characteristic(s) of the RF signal  16 . Finally, the imaginary impedance of the tunable impedance provided by the tunable impedance load  54  varies depending on the operational frequency characteristic(s) of the RF signal  16 . As such, since the function implemented by the control circuit  14  is both the function of the RF power of the Doherty amplification circuit  12  and of the operational frequency characteristic(s) of the RF signal  16 , the control circuit  14  can provide the Doherty amplification operation in different RF communication bands. The RF amplification device  10  is thus configured for broadband operation. Note that other relevant quantities, such as temperature and biasing levels, may be indicated by the feedback input  56  and/or the control input  60 . The function implemented by the control circuit  14  may further depend on these other relevant quantities so that the control circuit  14  can determine how the control circuit  14  tunes the tunable impedance load  54  based on a wider range of operational conditions. 
     As mentioned above, the Doherty amplification circuit  12  is configured to maintain the peaking RF amplifier  20  deactivated until the RF signal level of the RF signal  16  reaches the threshold level. Different embodiments of the Doherty amplification circuit  12  can be provided where the main RF amplifier  18  is simply a higher class of amplifier than the peaking RF amplifier  20 . In this embodiment, the main RF amplifier  18  is a Class A amplifier while the peaking RF amplifier is a Class C amplifier. The Doherty amplification circuit  12  shown in  FIG. 1  is configured to activate the peaking RF amplifier  20  when the main RF amplifier  18  is below saturation. However, in alternative embodiments, the peaking RF amplifier  20  may be activated before the main RF amplifier  18  is saturated or at least prior to the main RF amplifier  18  becoming fully saturated. For example, the peaking RF amplifier  20  may be configured to be activated before the main RF amplifier  18  reaches saturation if the main RF amplifier  18  is a Class A amplifier and the peaking RF amplifier  20  is a Class AB amplifier. In some applications, this helps linearize the Doherty amplification circuit  12  and increases power efficiency. 
     Since different classes of amplifier may be provided for both the main RF amplifier  18  and the peaking RF amplifier  20  in the Doherty amplification circuit  12 , the peaking RF amplifier  20  may or may not be off when the peaking RF amplifier  20  is deactivated. This may depend on the class of the peaking RF amplifier  20 . More specifically, the peaking RF amplifier  20  being activated and deactivated refers to whether the peaking RF amplifier  20  is providing amplification or not providing amplification. Depending on the class of amplifiers being used for the main RF amplifier  18  and the peaking RF amplifier  20 , the peaking RF amplifier  20  may not be off when the peaking RF amplifier  20  is deactivated. For example, if the peaking RF amplifier  20  is the Class C amplifier described above, the peaking RF amplifier  20  may not be truly off during operation, but may be deactivated because the peaking RF amplifier  20  is not providing amplification. In contrast, if the peaking RF amplifier  20  is a Class B amplifier, the peaking RF amplifier  20  may be turned off in order to deactivate the peaking RF amplifier  20 . Thus, how the peaking RF amplifier  20  is activated and deactivated may depend on a particular implementation of the Doherty amplification circuit  12 . 
     With regard to the Doherty amplification circuit  12  shown in  FIG. 1 , the first quadrature coupler  22  is configured such that the first port  26  is phase-aligned with the fourth port  32  and such that the second port  28  is phase-aligned with the third port  30 . In addition, the first quadrature coupler  22  is configured such that the first port  26  has a quadrature phase shift with respect to the third port  30 , and such that the second port  28  (i.e., the isolation port of the first quadrature coupler  22 ) has a quadrature phase shift with respect to the fourth port  32 . While the peaking RF amplifier  20  is deactivated, the input impedance of the peaking RF amplifier  20  essentially appears like an open circuit. As such, in this case, the RF signal  16  is not split, but is passed to the third port  30  and an input terminal  36  of the main RF amplifier  18 . Accordingly, the first quadrature coupler  22  is configured to provide the RF signal  16  such that there is a quadrature phase difference between the RF signal  16  at the first port  26  and the third port  30  while the peaking RF amplifier  20  is deactivated. The main RF amplifier  18  is then configured to amplify the RF signal  16  and output the RF signal  16  from the output terminal  48 . In this manner, the RF signal  16  is provided to the second quadrature coupler  24  at the fifth port  40 . 
     The second quadrature coupler  24  is configured to receive the RF signal  16  at the fifth port  40  while the peaking RF amplifier  20  is deactivated. 
     The second quadrature coupler  24  is configured such that the fifth port  40  is phase-aligned with the seventh port  44  (i.e., the output port of the Doherty amplification circuit  12 ) and such that the sixth port  42  is phase-aligned with the eighth port  46 . In addition, the second quadrature coupler  24  is configured such that the fifth port  40  has a quadrature phase shift with respect to the eighth port  46  (i.e., the isolation port of the second quadrature coupler  24 ) and the sixth port  42  has a quadrature phase shift with respect to the seventh port  44 . While the peaking RF amplifier  20  is deactivated, the output impedance of the peaking RF amplifier  20  essentially appears like an open circuit. However, since the fifth port  40  has the quadrature phase shift with respect to the eighth port  46  (i.e., the isolation port of the second quadrature coupler  24 ) and since the first quadrature coupler  22  provided the quadrature phase shift to the RF signal  16  at the third port  30 , the tunable impedance appears very high (ideally, infinite) at the eighth port  46 . The RF signal  16  is again not split. Instead, the second quadrature coupler  24  is configured to pass the RF signal  16  to the seventh port  44  while the peaking RF amplifier  20  is deactivated. As such, the Doherty amplification circuit  12  is configured to output the RF signal  16  from the seventh port  44  to downstream RF circuitry (not shown) once the main RF amplifier  18  has amplified the RF signal  16 . Consequently, the total amplification gain of the Doherty amplification circuit  12  is set entirely by the amplification gain of the main RF amplifier  18  while the peaking RF amplifier  20  is deactivated. 
     The peaking RF amplifier  20  is activated when the signal level of the RF signal  16  is reaches or is above the threshold voltage at the first port  26  (i.e., the input port of the Doherty amplification circuit  12 ). While the peaking RF amplifier  20  is activated, the input impedance of the peaking RF amplifier  20  decreases inversely with respect to the RF signal level of the RF signal  16 . As such, in this case, the RF signal  16  is split, but is passed to the third port  30  and the input terminal  36  of the main RF amplifier  18 . Accordingly, the first quadrature coupler  22  is configured to provide the RF signal  16  such that there is a quadrature phase difference between the RF signal  16  at the first port  26  and the third port  30  while the peaking RF amplifier  20  is deactivated. The main RF amplifier  18  is then configured to amplify the RF signal  16  and output the RF signal  16  from the output terminal  48 . In this manner, the RF signal  16  is provided to the second quadrature coupler  24  at the fifth port  40 . 
     The second quadrature coupler  24  is configured to receive the RF signal  16  at the fifth port  40  while the peaking RF amplifier  20  is deactivated. The second quadrature coupler  24  is configured such that the fifth port  40  is phase-aligned with the seventh port  44  (i.e., the output port of the Doherty amplification circuit  12 ) and such that the sixth port  42  is phase-aligned with the eighth port  46 . In addition, the second quadrature coupler  24  is configured such that the fifth port  40  has a quadrature phase shift with respect to the eighth port  46  (i.e., the isolation port of the second quadrature coupler  24 ) and the sixth port  42  has a quadrature phase shift with respect to the seventh port  44 . 
     While the peaking RF amplifier  20  is deactivated, the output impedance of the peaking RF amplifier  20  essentially appears like an open circuit. However, since the fifth port  40  has the quadrature phase shift with respect to the eighth port  46  (i.e., the isolation port of the second quadrature coupler  24 ) and since the first quadrature coupler  22  provided the quadrature phase shift to the RF signal  16  at the third port  30 , the tunable impedance appears very high (ideally, infinite) at the eighth port  46 . The RF signal  16  is again not split. Instead, the second quadrature coupler  24  is configured to pass the RF signal  16  to the seventh port  44  while the peaking RF amplifier  20  is deactivated. As such, the Doherty amplification circuit  12  is configured to output the RF signal  16  from the seventh port  44  to downstream RF circuitry (not shown) once the main RF amplifier  18  has amplified the RF signal  16 . Consequently, the total amplification gain of the Doherty amplification circuit  12  from the first port  26  (i.e., the input port of the Doherty amplification circuit  12 ) to the seventh port  44  (i.e., the output port of the Doherty amplification circuit  12 ) is set entirely by the amplification gain of the main RF amplifier while the peaking RF amplifier  20  is deactivated. 
     The Doherty amplification circuit  12  is configured such that the peaking RF amplifier  20  is activated when the RF signal level of the RF signal  16  is at or above the threshold level. While the main RF amplifier  18  is activated and the peaking RF amplifier  20  is activated, the first quadrature coupler  22  is configured to split the RF signal  16  into a first RF split signal  62  and a second RF split signal  64 . As explained above, the first quadrature coupler  22  is configured to receive the RF signal  16  at the first port  26 . The first quadrature coupler  22  provides the quadrature phase shift from the first port  26  to the fourth port  32 , and thus, the first RF split signal  62  is received by the main RF amplifier  18  at the input terminal  36  with a quadrature phase shift while both the main RF amplifier  18  and the peaking RF amplifier  20  are activated. The first port  26  of the first quadrature coupler  22  is also phase-aligned with the fourth port  32 . As such, the second RF split signal  64  is phase-aligned with the RF signal  16  at the first port  26 . Consequently, the first RF split signal  62  at the third port  30  and the second RF split signal  64  at the fourth port  32  have a quadrature phase difference with respect to one another. 
     In this case, the main RF amplifier  18  is (or is nearly) saturated, and thus the input impedance of the main RF amplifier  18  increases as the RF signal level of the RF signal  16  increases. On the other hand, the input impedance of the peaking RF amplifier  20  decreases as the RF signal level of the RF signal  16  increases above the threshold level. As such, a proportion of an amount of power of the RF signal  16  in the second RF split signal  64  relative to an amount of power of the RF signal  16  in the first RF split signal  62  increases as the RF signal level of the RF signal  16  at the first port  26  increases relative to the threshold level. The inverse of this relationship is also true, and therefore the proportion decreases as the RF signal level of the RF signal  16  decreases relative to the threshold level at the first port  26  while both the main RF amplifier  18  and the peaking RF amplifier  20  are activated. 
     Additionally, while both the main RF amplifier  18  and the peaking RF amplifier  20  are activated, the main RF amplifier  18  is configured to amplify the first RF split signal  62  in accordance with the amplification gain of the main RF amplifier  18  and output the first RF split signal  62  from the output terminal  48 . The second quadrature coupler  24  is configured to receive the first RF split signal  62  from the main RF amplifier  18  at the fifth port  40 . The peaking RF amplifier  20  is configured to receive the second RF split signal  64  at an input terminal  38 . While both the main RF amplifier  18  and the peaking RF amplifier  20  are activated, the peaking RF amplifier  20  is configured to amplify the second RF split signal  64  in accordance with the amplification gain of the peaking RF amplifier  20  and output the second RF split signal  64  from the output terminal  50 . The second quadrature coupler  24  is configured to receive the second RF split signal  64  from the peaking RF amplifier  20  at the sixth port  42 . 
     The second quadrature coupler  24  is configured to combine the first RF split signal  62  and the second RF split signal  64  back into the RF signal  16  after the first RF split signal  62  and the second RF split signal  64  are amplified by the main RF amplifier  18  and the peaking RF amplifier  20 , respectively. More specifically, the first quadrature coupler  22  is configured such that the first RF split signal  62  passes to the seventh port  44  just like the RF signal  16  when the peaking RF amplifier  20  is deactivated. 
     Similarly since the sixth port  42  has a quadrature phase shift with respect to the seventh port  44  (i.e., the isolation port of the second quadrature coupler  24 ) and since the first quadrature coupler  22  provides no phase shift to the second RF split signal  64  at the fourth port  32  with respect to the first port  26 , the second RF split signal  64  thus passes to the seventh port  44 . Furthermore, the first RF split signal  62  and the second RF split signal  64  become phase-aligned at the seventh port  44  because the second quadrature coupler  24  is configured to provide the quadrature phase shift to the second RF split signal  64  between the sixth port  42  and the seventh port  44 . As such, the second quadrature coupler  24  combines the first RF split signal  62  and the second RF split signal  64  into the RF signal  16  at the seventh port  44  once the Doherty amplification circuit  12  has amplified the RF signal  16 . Consequently, the total amplification gain of the Doherty amplification circuit  12  from the first port  26  (i.e., the input port of the Doherty amplification circuit  12 ) to the seventh port  44  (i.e., the output port of the Doherty amplification circuit  12 ) is set in accordance with the amplification gain of the main RF amplifier  18 , the amplification gain of the peaking RF amplifier  20 , and the proportion of the RF signal  16  provided in the first RF split signal  62  at the third port  30  relative to the second RF split signal  64  at the fourth port  32  while both the main RF amplifier  18  and the peaking RF amplifier  20  are activated. 
     It should be noted that while the tunable impedance load  54  is coupled to the eighth port  46  in the embodiment shown in  FIG. 1 , this may or may not be the case in other embodiments. For example, in a first alternative embodiment, the tunable impedance load  54  is coupled to the second port  28  of the first quadrature coupler  22 , while the impedance load  34  is coupled to the eighth port  46  of the second quadrature coupler  24 . In this case, the control circuit  14  would thus control the tunable impedance of the tunable impedance load  54  in the same manner described above, except that with regard to tuning the tunable impedance of the tunable impedance load  54 , the load impedance should be switched with the output impedance of the upstream RF circuitry at the first port  26  (i.e., the other exogenous connection port). In a second alternative embodiment, the tunable impedance load  54  is still coupled to the eighth port  46 , but another tunable impedance load that is similar to the tunable impedance load  54  is coupled to the second port  28 . In this case, the control circuit  14  would simultaneously tune the tunable impedance load  54 , as described above for the RF amplification device  10  shown in  FIG. 1 , and tune a tunable impedance of the other tunable impedance load at the second port  28  in the same manner as the tunable impedance load  54  in the first alternative embodiment. 
     Referring again to the RF amplification device  10  shown in  FIG. 1 , the RF amplification device  10  is formed as an integrated circuit (IC) on a semiconductor substrate  66 . The semiconductor substrate  66  has a substrate body  68  formed from a wafer and/or doped layers of a suitable semiconductor material. For example, the semiconductor material may be Silicon (Si), Silicon Germanium (SiGe), Gallium Arsenide (GaAs), Indium Phosphorus (InP), and/or the like. Typical dopants that may be utilized to dope the semiconductor layers are Gallium (Ga), Arsenic (As), Silicon (Si), Tellurium (Te), Zinc (Zn), Sulfur (S), Boron (B), Phosphorus (P), Aluminum Gallium Arsenide (AlGaAs), Indium Gallium Arsenide (InGaAs), and/or the like. Furthermore, metallic layers may be formed on a top, within, and/or on a bottom of the substrate body  68  to provide terminals, traces, contact pads, coils, connections, passive impedance elements, active semiconductor components, and/or the like. Also, any type of suitable semiconductor technology may be used to provide the topology of the semiconductor substrate  66 . For example, the semiconductor technology of the semiconductor substrate  66  may be Complementary Metal-On-Oxide Semiconductor (CMOS) technology, BiComplementary Metal-On-Oxide Semiconductor (BiCMOS) technology, Silicon-On-Insulator (SOI) technology, and/or the like. In this embodiment, the semiconductor technology is SOI, and thus the semiconductor material of the substrate body  68  is Si. 
       FIG. 2  illustrates an exemplary RF amplification device  10 ( 1 ), which is one embodiment of the RF amplification device  10  shown in  FIG. 1 . The RF amplification device  10 ( 1 ) includes a Doherty amplification circuit  12 ( 1 ) and a control circuit  14 ( 1 ). The Doherty amplification circuit  12 ( 1 ) is one embodiment of the Doherty amplification circuit  12  shown in  FIG. 1 . Thus, the Doherty amplification circuit  12 ( 1 ) includes a main RF amplifier  18 ( 1 ), a peaking RF amplifier  20 ( 1 ), a first quadrature coupler  22 ( 1 ), and a second quadrature coupler  24 ( 1 ). 
     The main RF amplifier  18 ( 1 ) is one embodiment of the main RF amplifier  18  described above with respect to  FIG. 1 , and thus operates in the same manner as the main RF amplifier  18  described above. The main RF amplifier  18 ( 1 ) thus includes the input terminal  36  and the output terminal  48 . In this embodiment, the main RF amplifier  18 ( 1 ) is a Class A amplifier built using one or more field effect transistors (FETs). The main RF amplifier  18 ( 1 ) is configured to receive a supply voltage VS in order to power amplification by the main RF amplifier  18 ( 1 ). As such, a supply current ID 1  is generated from the supply voltage VS within the main RF amplifier  18 ( 1 ) while the main RF amplifier  18 ( 1 ) is activated. 
     The peaking RF amplifier  20 ( 1 ) is one embodiment of the peaking RF amplifier  20  described above with respect to  FIG. 1 , and thus operates in the same manner as the peaking RF amplifier  20  described above. The peaking RF amplifier  20 ( 1 ) thus includes the input terminal  38  and the output terminal  50 . In this embodiment, the peaking RF amplifier  20 ( 1 ) is a Class C amplifier built using one or more FETs. The peaking RF amplifier  20 ( 1 ) is also configured to receive the supply voltage VS in order to power amplification by the peaking RF amplifier  20 ( 1 ). As such, a supply current ID 2  is generated by the supply voltage VS within the peaking RF amplifier  20 ( 1 ), where a supply current level of the supply current ID 2  depends on harmonic characteristics of the Doherty amplification circuit  12 ( 1 ), both while the peaking RF amplifier  20 ( 1 ) is activated and while the peaking RF amplifier  20 ( 1 ) is deactivated. 
     The first quadrature coupler  22 ( 1 ) is one embodiment of the first quadrature coupler  22  described above with respect to  FIG. 1 , and thus operates in the same manner as the first quadrature coupler  22  described above. The first quadrature coupler  22 ( 1 ) thus includes the first port  26 , the second port  28 , the third port  30 , and the fourth port  32 . Also, the RF signal  16  is received at the first port  26 , the impedance load  34  is coupled to the second port  28 , the input terminal  36  is coupled to the third port  30 , and the input terminal  38  is coupled to the fourth port  32 . In this embodiment, the first quadrature coupler  22 ( 1 ) is a hybrid coupler. 
     The second quadrature coupler  24 ( 1 ) is one embodiment of the second quadrature coupler  24  described above with respect to  FIG. 1 , and thus operates in the same manner as the second quadrature coupler  24  described above. The second quadrature coupler  24 ( 1 ) thus includes the fifth port  40 , the sixth port  42 , the seventh port  44 , and the eighth port  46 . Also, the output terminal  48  is coupled to the fifth port  40 , the output terminal  50  is coupled to the sixth port  42 , the RF signal  16  is output from the seventh port  44 , and the tunable impedance load  54  is coupled to the eighth port  46 . In this embodiment, the second quadrature coupler  24 ( 1 ) is also a hybrid coupler. 
     The control circuit  14 ( 1 ) is one embodiment of the control circuit  14  described above with respect to  FIG. 1 , and thus operates in the same manner as the control circuit  14  described above. The control circuit  14 ( 1 ) is configured to generate the impedance control output  58  in the same manner described above to tune the tunable impedance of the tunable impedance load  54  as the function of the RF power detected in the Doherty amplification circuit  12 ( 1 ). However, in this embodiment, the control circuit  14 ( 1 ) is coupled to the seventh port  44  (i.e., the output port of the Doherty amplification circuit  12 ( 1 )) to detect RF power in the Doherty amplification circuit  12 ( 1 ). More specifically, the control circuit  14 ( 1 ) is coupled to the seventh port  44  in order to receive a feedback signal  56 ( 1 ). 
     The feedback signal  56 ( 1 ) is one embodiment of the feedback input  56  described above with respect to  FIG. 1 , and the feedback signal  56 ( 1 ) has a feedback signal level that indicates the RF power of the Doherty amplification circuit  12 ( 1 ). More specifically, the feedback signal level (e.g., feedback voltage level, feedback current level, etc.) of the feedback signal  56 ( 1 ) is set in accordance with the RF signal level (e.g., RF voltage level, RF current level, etc.) of the RF signal  16  after the Doherty amplification circuit  12 ( 1 ) has amplified the RF signal  16 . The RF signal level (e.g., RF voltage level, RF current level, etc.) of the RF signal  16  is related to the RF power by the Doherty amplification circuit  12 ( 1 ), since the RF signal level of the RF signal  16  at the seventh port  44  depends on an amount of power provided by the Doherty amplification circuit  12 ( 1 ). Since the feedback signal level of the feedback signal  56 ( 1 ) is set in accordance with the RF signal level of the RF signal  16  at the seventh port  44 , the feedback signal level of the feedback signal  56 ( 1 ) indicates the RF power of the Doherty amplification circuit  12 ( 1 ). By detecting the feedback signal level of the feedback signal  56 ( 1 ), the control circuit  14 ( 1 ) is configured to detect the RF power of the Doherty amplification circuit  12 ( 1 ). The control circuit  14 ( 1 ) is also configured to receive the control input  60  described above with respect to  FIG. 1 . The function implemented by the control circuit  14 ( 1 ) thus maps the feedback signal level of the feedback signal  56 ( 1 ) and the operational frequency characteristic(s) indicated by the control input  60  to permutations of the impedance control output  58 . The control circuit  14 ( 1 ) is thus configured to dynamically tune the tunable impedance of the tunable impedance load  54  with the impedance control output  58  as described above with respect to the control circuit  14  shown in  FIG. 1 . 
       FIG. 3  illustrates another exemplary RF amplification device  10 ( 2 ), which is another embodiment of the RF amplification device  10  shown in  FIG. 1 . The RF amplification device  10 ( 2 ) includes the Doherty amplification circuit  12 ( 1 ) described above with respect to  FIG. 2 . However, the RF amplification device  10 ( 2 ) includes another embodiment of a control circuit  14 ( 2 ). The control circuit  14 ( 2 ) is another embodiment of the control circuit  14  described above with respect to  FIG. 1 , and thus operates in the same manner as the control circuit  14  described above. The control circuit  14 ( 2 ) is configured to generate the impedance control output  58  in the same manner described above to tune the tunable impedance of the tunable impedance load  54  as the function of the RF power detected in the Doherty amplification circuit  12 ( 1 ). However, in this embodiment, the control circuit  14 ( 2 ) is coupled to the first port  26  (i.e., the input port of the Doherty amplification circuit  12 ( 1 )) to detect the RF power in the Doherty amplification circuit  12 ( 1 ). More specifically, the control circuit  14 ( 2 ) is coupled to the first port  26  in order to receive a feedback signal  56 ( 2 ). 
     The feedback signal  56 ( 2 ) is another embodiment of the feedback input  56  described above with respect to  FIG. 1 , and the feedback signal  56 ( 2 ) has a feedback signal level that indicates the RF power of the Doherty amplification circuit  12 ( 1 ). More specifically, the feedback signal level (e.g., feedback voltage level, feedback current level, etc.) of the feedback signal  56 ( 2 ) is set in accordance with the RF signal level (e.g., RF voltage level, RF current level, etc.) of the RF signal  16  before the Doherty amplification circuit  12 ( 1 ) has amplified the RF signal  16 . The RF signal level (e.g., RF voltage level, RF current level, etc.) of the RF signal  16  is related to the RF power by the Doherty amplification circuit  12 ( 1 ), since the RF signal level of the RF signal  16  at the first port  26  indicates an amount of power that the Doherty amplification circuit  12 ( 1 ) will use to amplify the RF signal  16 . Since the feedback signal level of the feedback signal  56 ( 2 ) is set in accordance with the RF signal level of the RF signal  16  at the first port  26 , the feedback signal level of the feedback signal  56 ( 2 ) indicates the RF power of the Doherty amplification circuit  12 ( 1 ). By detecting the feedback signal level of the feedback signal  56 ( 2 ), the control circuit  14 ( 2 ) is configured to detect the RF power of the Doherty amplification circuit  12 ( 1 ). The control circuit  14 ( 2 ) is also configured to receive the control input  60  described above with respect to  FIG. 1 . The function implemented by the control circuit  14 ( 2 ) thus maps the feedback signal level of the feedback signal  56 ( 2 ) and the operational frequency characteristic(s) indicated by the control input  60  to permutations of the impedance control output  58 . The control circuit  14 ( 2 ) is thus configured to dynamically tune the tunable impedance of the tunable impedance load  54  with the impedance control output  58  as described above with respect to the control circuit  14  shown in  FIG. 1 . 
       FIG. 4  illustrates another exemplary RF amplification device  10 ( 3 ), which is another embodiment of the RF amplification device  10  shown in  FIG. 1 . The RF amplification device  10 ( 3 ) also includes the Doherty amplification circuit  12 ( 1 ) described above with respect to  FIG. 2 . However, the RF amplification device  10 ( 3 ) includes another embodiment of a control circuit  14 ( 3 ). The control circuit  14 ( 3 ) is another embodiment of the control circuit  14  described above with respect to  FIG. 1 , and thus operates in the same manner as the control circuit  14  described above. The control circuit  14 ( 3 ) is configured to generate the impedance control output  58  in the same manner described above to tune the tunable impedance of the tunable impedance load  54  as the function of the RF power detected in the Doherty amplification circuit  12 ( 1 ). However, in this embodiment, the control circuit  14 ( 3 ) is coupled to an internal node (not explicitly shown) of the peaking RF amplifier  20 ( 1 ). More specifically, the control circuit  14 ( 3 ) is coupled to the internal node in order to receive a feedback signal  56 ( 3 ). 
     The feedback signal  56 ( 3 ) is another embodiment of the feedback input  56  described above with respect to  FIG. 1 , and the feedback signal  56 ( 3 ) has a feedback signal level that indicates the RF power of the Doherty amplification circuit  12 ( 1 ). More specifically, the feedback signal level (e.g., feedback voltage level, feedback current level, etc.) of the feedback signal  56 ( 3 ) is set in accordance with the supply current ID 2  provided to the peaking RF amplifier  20 ( 1 ). Since the peaking RF amplifier  20 ( 1 ) is a Class C amplifier, a supply current level of the supply current ID 2  is related to the RF power of the Doherty amplification circuit  12 ( 1 ). The control circuit  14 ( 3 ) is coupled to the peaking RF amplifier  20 ( 1 ) such that a feedback signal level of the feedback signal  56 ( 3 ) is set in accordance with the supply current level of the supply current ID 2 . The feedback signal level of the feedback signal  56 ( 3 ) therefore indicates the RF power of the Doherty amplification circuit  12 ( 1 ). By detecting the feedback signal level of the feedback signal  56 ( 3 ), the control circuit  14 ( 3 ) is configured to detect the RF power of the Doherty amplification circuit  12 ( 1 ). The control circuit  14 ( 3 ) is also configured to receive the control input  60  described above with respect to  FIG. 1 . The function implemented by the control circuit  14 ( 3 ) thus maps the feedback signal level of the feedback signal  56 ( 3 ) and the operational frequency characteristic(s) indicated by the control input  60  to permutations of the impedance control output  58 . The control circuit  14 ( 3 ) is thus configured to dynamically tune the tunable impedance of the tunable impedance load  54  with the impedance control output  58  as described above with respect to the control circuit  14  shown in  FIG. 1 . 
       FIG. 5  illustrates an exemplary tunable impedance load  54 ( 1 ). The tunable impedance load  54 ( 1 ) is one embodiment of the tunable impedance load  54  described above with respect to  FIG. 1 . The tunable impedance load  54 ( 1 ) shown in  FIG. 5  includes a plurality of selectable impedance branches (referred to generically as elements  72  and specifically as elements  72 ( 1 )- 72 ( 6 )). The selectable impedance branches  72  are each coupled in parallel with respect to one another. An input terminal  74  of the tunable impedance load  54 ( 1 ) may be coupled to the eighth port  46  (shown in  FIGS. 1-4 ). 
     The selectable impedance branches include resistors (referred to generically as elements  76  and specifically as elements  76 ( 1 )- 76 ( 6 )) and switches (referred to generically as elements  78  and specifically as elements  78 ( 1 )- 78 ( 6 )). More specifically, the selectable impedance branch  72 ( 1 ) includes a resistor  76 ( 1 ) coupled in series with a switch  78 ( 1 ). The selectable impedance branch  72 ( 2 ) includes a resistor  76 ( 2 ) coupled in series with a switch  78 ( 2 ). Additionally, the selectable impedance branch  72 ( 3 ) includes a resistor  76 ( 3 ) coupled in series with a switch  78 ( 3 ). Furthermore, the selectable impedance branch  72 ( 4 ) includes a resistor  76 ( 4 ) coupled in series with a switch  78 ( 4 ). Also, the selectable impedance branch  72 ( 5 ) includes a resistor  76 ( 5 ) coupled in series with a switch  78 ( 5 ). Finally, the selectable impedance branch  72 ( 6 ) includes a resistor  76 ( 6 ) coupled in series with a switch  78 ( 6 ). 
     For each of the selectable impedance branches  72 , the selectable impedance branch  72  is configured to be selected when the switch  78  in the selectable impedance branch  72  is closed, and to be deselected when the switch  78  in the selectable impedance branch  72  is open. Each of the resistors  76  may have a different resistance. The resistance of the resistor  76  of the selectable impedance branch  72  is presented at the input terminal  74  (and thus at the eighth port  46  shown in  FIGS. 1-4 ) when the switch  78  in the selectable impedance branch  72  is closed. Otherwise, when the switch  78  in the selectable impedance branch  72  is open, the selectable impedance branch  72  appears as an open circuit. Each of the switches  78  shown in  FIG. 5  may be any type of suitable switch. For example, the switches  78  may be provided as FETs and/or as microelectromechanical switches (MEMSs). 
     The switches  78  are opened and closed in response to an impedance control output  58 ( 1 ), which is one embodiment of the impedance control output  58  described above with respect to  FIGS. 1-4 . In this embodiment, the impedance control output  58 ( 1 ) includes a plurality of switch control signals (referred to generically as elements  80  and specifically as elements  80 ( 1 )- 80 ( 6 )). Each of the switch control signals  80  is received by a corresponding one of the switches  78 . For each of the switch control signals  80 , the switch control signal  80  received by the switch  78 . The switch  78  is activated when the switch control signal  80  is in a switch activation state and the switch  78  is deactivated when the switch control signal  80  is in a switch deactivation state. A switch control signal  80 ( 1 ) provided in the impedance control output  58 ( 1 ) is received by the switch  78 ( 1 ). A switch control signal  80 ( 2 ) provided in the impedance control output  58 ( 1 ) is received by the switch  78 ( 2 ). A switch control signal  80 ( 3 ) provided in the impedance control output  58 ( 1 ) is received by the switch  78 ( 3 ). A switch control signal  80 ( 4 ) provided in the impedance control output  58 ( 1 ) is received by the switch  78 ( 4 ). A switch control signal  80 ( 5 ) provided in the impedance control output  58 ( 1 ) is received by the switch  78 ( 5 ). A switch control signal  80 ( 6 ) provided in the impedance control output  58 ( 1 ) is received by the switch  78 ( 6 ). Permutations of the impedance control output  58 ( 1 ) thus refer to particular combinations of the switch control signals  80  that are in the switch activation state and in the switch deactivation state. 
     Since the switches  78  are used to vary the tunable impedance presented at the input terminal  74 , the tunable impedance load  54 ( 1 ) discretely varies the tunable impedance. Furthermore, in this embodiment, the tunable impedance is purely resistive because each of the selectable impedance branches  72  only includes the resistors  76  and no reactive components. 
     Referring now to  FIG. 2 ,  FIG. 5 , and  FIG. 6 ,  FIG. 6  is a graph illustrating power curves  84 ,  86 ,  88 , and  90  that describe power added efficiency (PAE) as a function of output power in the RF amplification device  10 ( 1 ) shown in  FIG. 2 . Furthermore, to obtain the power curves  84 ,  86 ,  88 , and  90 , the tunable impedance load  54  (shown in  FIG. 2 ) is provided as the tunable impedance load  54 ( 1 ) shown in  FIG. 5 , and the impedance control output  58  (shown in  FIG. 2 ) is provided as the impedance control output  58 ( 1 ) (shown in  FIG. 5 ). 
     The power curve  84  is provided by the RF amplification device  10 ( 1 ). More specifically, the power curve  84  is provided by the RF amplification device  10 ( 1 ) when a carrier frequency of the RF signal  16  (shown in  FIG. 2 ) is approximately 680 Megahertz (MHz) and the tunable impedance of the tunable impedance load  54 ( 1 ) is set to approximately equal to Znorm×10 by the impedance control output  58 ( 1 ) shown in  FIG. 5 . 
     The power curve  86  is provided by the RF amplification device  10 ( 1 ). More specifically, the power curve  86  is provided by the RF amplification device  10 ( 1 ) when a carrier frequency of the RF signal  16  (shown in  FIG. 2 ) is approximately 850 MHz and the tunable impedance of the tunable impedance load  54 ( 1 ) is set to approximately equal Znorm×10 by the impedance control output  58 ( 1 ) shown in  FIG. 5 . Again, Znorm is the normalized (conventional characteristic impedance, Zo) termination impedance of the second quadrature coupler  24 ( 1 ) (shown in  FIG. 2 ) at the eighth port  46  (shown in  FIG. 2 ). 
     The power curve  88  is provided by the RF amplification device  10 ( 1 ). More specifically, the power curve  88  is provided by the RF amplification device  10 ( 1 ) when a carrier frequency of the RF signal  16  (shown in  FIG. 2 ) is approximately 1020 MHz. Furthermore, the tunable impedance of the tunable impedance load  54 ( 1 ) is set to approximately equal to Znorm×10 by the impedance control output  58 ( 1 ) shown in  FIG. 5 . 
     The power curve  90  is provided by the RF amplification device  10 ( 1 ). More specifically, the power curve  90  is provided by the RF amplification device  10 ( 1 ) when a carrier frequency of the RF signal  16  (shown in  FIG. 2 ) is approximately 850 MHz and the tunable impedance of the tunable impedance load  54 ( 1 ) is set to approximately equal to Znorm by the impedance control output  58 ( 1 ) shown in  FIG. 5 . 
     The tunable impedance of the tunable impedance load  54 ( 1 ) is entirely resistive. The PAE is fifty-five percent (55%) when output power is at two (2) watts, and a PAE improvement of eight percent (8%) is provided at backed-off levels compared to conventional Doherty amplification circuits. As mentioned above, the power curve  90  is provided when the carrier frequency of the RF signal is at 850 MHz and when the tunable impedance of the tunable impedance load  54 ( 1 ) is set to the characteristic impedance Zo, which is equal to Znorm. As such, the power curve  90  shows the Doherty amplification circuit  12 ( 1 ) operating in a non-Doherty fashion to provide amplification. Therefore, a configuration indicated by power curve  90  is often employed in amplification device designs that are load-insensitive given particular cellular applications. The performance of the RF amplification device  10 ( 1 ) decreases frequency variation when the tunable impedance of the tunable impedance load  54 ( 1 ) is set to Znorm×10. As such, the RF amplification device  10 ( 1 ) demonstrates better overall bandwidth-efficiency. 
       FIG. 7  illustrates another exemplary tunable impedance load  54 ( 2 ). The tunable impedance load  54 ( 2 ) is another embodiment of the tunable impedance load  54  described above with respect to  FIG. 1 . Like the tunable impedance load  54 ( 1 ) shown in  FIG. 5 , the tunable impedance load  54 ( 2 ) includes a plurality of selectable impedance branches (referred to generically as elements  92  and specifically as elements  92 ( 1 )- 92 ( 6 )). The selectable impedance branches  92  are each coupled in parallel with respect to one another. An input terminal  94  of the tunable impedance load  54 ( 2 ) may be coupled to the eighth port  46  (shown in  FIGS. 1-4 ). 
     The selectable impedance branches include resistors (referred to generically as elements  96  and specifically as elements  96 ( 1 )- 96 ( 6 )), switches (referred to generically as elements  98  and specifically as elements  98 ( 1 )- 98 ( 6 )), and inductors (referred to generically as elements  100  and specifically as elements  100 ( 1 )- 100 ( 6 )). More specifically, the selectable impedance branch  92 ( 1 ) includes a resistor  96 ( 1 ), a switch  98 ( 1 ), and an inductor  100 ( 1 ) coupled in series. The selectable impedance branch  92 ( 2 ) includes a resistor  96 ( 2 ), a switch  98 ( 2 ), and an inductor  100 ( 2 ) coupled in series. Additionally, the selectable impedance branch  92 ( 3 ) includes a resistor  96 ( 3 ), a switch  98 ( 3 ), and an inductor  100 ( 3 ) coupled in series. Furthermore, the selectable impedance branch  92 ( 4 ) includes a resistor  96 ( 4 ), a switch  98 ( 4 ), and an inductor  100 ( 4 ) coupled in series. Also, the selectable impedance branch  92 ( 5 ) includes a resistor  96 ( 5 ), a switch  98 ( 5 ), and an inductor  100 ( 5 ) coupled in series. Finally, the selectable impedance branch  92 ( 6 ) includes a resistor  96 ( 6 ), a switch  98 ( 6 ), and an inductor  100 ( 6 ) coupled in series. 
     For each of the selectable impedance branches  92 , the selectable impedance branch  92  is configured to be selected when the switch  98  in the selectable impedance branch  92  is closed, and to be deselected when the switch  98  in the selectable impedance branch  92  is open. Each of the resistors  96  may have a different resistance and each of the inductors  100  may have a different inductance. The resistance of the resistor  96  and the inductance of the inductor  100  of the selectable impedance branch  92  is presented at the input terminal  94  (and thus at the eighth port  46  shown in  FIGS. 1-4 ) when the switch  98  in the selectable impedance branch  92  is closed. Otherwise, when the switch  98  in the selectable impedance branch  92  is open, the selectable impedance branch  92  appears as an open circuit. Each of the switches  98  shown in  FIG. 5  may be any type of suitable switch. For example, the switches  98  may be provided by as FETs and/or MEMs. 
     The switches  98  are opened and closed by an impedance control output  58 ( 2 ). The impedance control output  58 ( 2 ) is one embodiment of the impedance control output  58  described above with regard to  FIGS. 1-4 . In this embodiment, the impedance control output  58 ( 2 ) includes a plurality of switch control signals (referred to generically as elements  102  and specifically as elements  102 ( 1 )- 102 ( 6 )). Each of the switch control signals  102  is received by a corresponding one of the switches  98 . For each of the switch control signals  102 , the switch control signal  102  is received by the corresponding switch  98 . The switch  78  is activated when the switch control signal  102  is in a switch activation state and the switch  78  is deactivated when the switch control signal  102  is in a switch deactivation state. A switch control signal  102 ( 1 ) provided in the impedance control output  58 ( 2 ) is received by the switch  98 ( 1 ). A switch control signal  102 ( 2 ) provided in the impedance control output  58 ( 2 ) is received by the switch  98 ( 2 ). A switch control signal  102 ( 3 ) provided in the impedance control output  58 ( 2 ) is received by the switch  98 ( 3 ). A switch control signal  102 ( 4 ) provided in the impedance control output  58 ( 2 ) is received by the switch  98 ( 4 ). A switch control signal  102 ( 5 ) provided in the impedance control output  58 ( 2 ) is received by the switch  98 ( 5 ). A switch control signal  102 ( 6 ) provided in the impedance control output  58 ( 2 ) is received by the switch  98 ( 6 ). Thus, in this embodiment, permutations of the impedance control output  58 ( 2 ) refer to particular combinations of the switch control signals  102  that are in the switch activation state and in the switch deactivation state. 
     Since the switches  98  are used to vary the tunable impedance presented at the input terminal  94 , the tunable impedance load  54 ( 2 ) discretely varies the tunable impedance. Furthermore, in this embodiment, the tunable impedance is both resistive and reactive, since each of the selectable impedance branches  92  includes one of the resistors  96  and one of the inductors  100 . 
     Referring now to  FIG. 2 ,  FIG. 7 , and  FIG. 8 ,  FIG. 8  is a graph illustrating power curves  104 ,  106 ,  108 , and  110  that describe PAE as a function of output power from the RF amplification device  10 ( 1 ) shown in  FIG. 2 . Furthermore, to obtain the power curves  104 ,  106 ,  108 , and  110 , the tunable impedance load  54  (shown in  FIG. 2 ) is provided as the tunable impedance load  54 ( 2 ) shown in  FIG. 7  and the impedance control output  58  (shown in  FIG. 2 ) is provided as the impedance control output  58 ( 2 ) (shown in  FIG. 7 ). 
     The power curve  104  is provided by the RF amplification device  10 ( 1 ). More specifically, the power curve  104  is provided by the RF amplification device  10 ( 1 ) when a carrier frequency of the RF signal  16  (shown in  FIG. 2 ) is approximately 680 MHz and the tunable impedance of the tunable impedance load  54 ( 2 ) is set to approximately equal Znorm×10 by the impedance control output  58 ( 2 ) shown in  FIG. 7 . The power curve  106  is also provided by the RF amplification device  10 ( 1 ). More specifically, the power curve  106  is provided by the RF amplification device  10 ( 1 ) when a carrier frequency of the RF signal  16  (shown in  FIG. 2 ) is approximately 850 MHz and the tunable impedance of the tunable impedance load  54 ( 2 ) is set to approximately equal Znorm×10 by the impedance control output  58 ( 2 ) shown in  FIG. 7 . Again, Znorm is the normalized (conventional characteristic impedance, Zo) termination impedance of the second quadrature coupler  24 ( 1 ) (shown in  FIG. 2 ) at the eighth port  46  (shown in  FIG. 2 ). 
     Additionally, the power curve  108  is provided by the RF amplification device  10 ( 1 ). More specifically, the power curve  108  is provided by the RF amplification device  10 ( 1 ) when a carrier frequency of the RF signal  16  (shown in  FIG. 2 ) is approximately 1020 MHz. Furthermore, the tunable impedance of the tunable impedance load  54 ( 2 ) is set to approximately equal Znorm×10 by the impedance control output  58 ( 2 ) shown in  FIG. 7 . 
     Finally, the power curve  110  is provided by the RF amplification device  10 ( 1 ). More specifically, the power curve  110  is provided by the RF amplification device  10 ( 1 ) when a carrier frequency of the RF signal  16  (shown in  FIG. 2 ) is approximately 850 MHz and the tunable impedance of the tunable impedance load  54 ( 2 ) is set to approximately equal Znorm by the impedance control output  58 ( 2 ) shown in  FIG. 7 . 
     In this embodiment, the tunable impedance of the tunable impedance load  54 ( 2 ) is both resistive and reactive (e.g., inductive). The power curves  104 - 110  thus show a frequency sweep of the RF amplification device  10 ( 1 ) described above with respect to  FIG. 2  when the tunable impedance load  54 ( 2 ) described above with respect to  FIG. 7  is used by the RF amplification device  10 ( 1 ). The PAE is 58% when the output power of the RF amplification device  10 ( 1 ) is at 2 watts. This indicates an 11% improvement in PAE at the backed-off power level. Also, since the tunable impedance of the tunable impedance load  54 ( 2 ) provides a variable reactance (i.e., the inductances of the inductors  100 ), the tunable impedance load  54 ( 2 ) in  FIG. 7  is configured to improve the PAE than the tunable impedance load  54 ( 1 ) shown in  FIG. 5 . 
       FIG. 9  is a graph illustrating a power curve  112  and a power curve  114  that describe PAE as a function of output power. The power curve  112  and the power curve  114  are both produced by the RF amplification device  10 ( 1 ) shown in  FIG. 2  when the carrier frequency of the RF signal  16  is at 680 MHz and the tunable impedance is approximately equal to Znorm×10. However, the tunable impedance load  54 ( 1 ) shown in  FIG. 5  is provided as the tunable impedance load  54  shown in  FIG. 2  to produce the power curve  112 . Furthermore, the tunable impedance load  54 ( 2 ) shown in  FIG. 7  is provided as the tunable impedance load  54  to produce the power curve  114 .  FIG. 9  thus illustrates that the tunable impedance load  54 ( 2 ) shown in  FIG. 7  has slightly better PAE at 680 MHz than the tunable impedance load  54 ( 1 ) shown in  FIG. 5 , which is purely resistive. 
       FIG. 10  is a graph illustrating a power curve  116  and a power curve  118  that describe PAE as a function of output power. The power curve  116  and the power curve  118  are both produced by the RF amplification device  10 ( 1 ) shown in  FIG. 2  when the carrier frequency of the RF signal  16  is at 1020 MHz and the tunable impedance is approximately equal to Znorm×10. However, the tunable impedance load  54 ( 1 ) shown in  FIG. 5  is provided as the tunable impedance load  54  shown in  FIG. 2  to produce the power curve  116 . Furthermore, the tunable impedance load  54 ( 2 ) shown in  FIG. 7  is provided as the tunable impedance load  54  to produce the power curve  118 . Because the tunable impedance load  54 ( 2 ) has an imaginary impedance that is inductive, the tunable impedance of the tunable impedance load  54 ( 2 ) increases at higher frequencies. As a result, the tunable impedance load  54 ( 2 ) improves the upper-band power-efficiency performance of the Doherty amplification circuit  12 ( 1 ) and therefore can extend its bandwidth capability. Since the tunable impedance of the tunable impedance load  54 ( 2 ) is complex, the control circuit  14 ( 1 ) can tune the tunable impedance of the tunable impedance load  54 ( 2 ) so that the Doherty amplification circuit  12 ( 1 ) operates optimally in multi-band applications. 
       FIG. 11  is a graph illustrating a power curve  120 , a power curve  122 , and a power curve  124  that describe PAE as a function of output power. The power curve  120 , the power curve  122 , and the power curve  124  are each produced by the RF amplification device  10 ( 1 ) shown in  FIG. 2  when the carrier frequency of the RF signal  16  is at 1020 MHz. However, the tunable impedance load  54 ( 1 ) shown in  FIG. 5  is provided as the tunable impedance load  54  shown in  FIG. 2  to produce the power curve  120  and the power curve  122 . In contrast, the tunable impedance load  54 ( 2 ) shown in  FIG. 7  is provided as the tunable impedance load  54  shown in  FIG. 2  to produce the power curve  124 . 
     With regard to the power curve  120 , the tunable impedance of the tunable impedance load  54 ( 1 ) has been set to approximately equal Znorm×10. The tunable impedance load  54 ( 1 ) has been set to approximately equal Znorm×40 to produce the power curve  124 . As such, the tunable impedance of the tunable impedance load  54 ( 1 ) can be optimized by adjusting the tunable impedance of the tunable impedance load  54 ( 1 ) to approximately equal Znorm×40 at 1020 MHz. For example, if the required frequency of operation is shifted to 1020 MHz, the control circuit  14 ( 1 ) shown in  FIG. 2  dynamically tunes the tunable impedance of the tunable impedance load  54 ( 1 ) so that the tunable impedance approximately equals Znorm×40. 
     However, as shown in  FIG. 11 , the tunable impedance load  54 ( 2 ) can be used to achieve even greater power efficiency at 1020 MHz. In this example, the tunable impedance of the tunable impedance load  54 ( 2 ) has been set to Znorm×10. Thus, the tenability of the reactive impedance of the tunable impedance provided by the tunable impedance load  54 ( 2 ) allows for increased power efficiency. However, in some applications, the inductors  100  shown in  FIG. 7  may be large in size with respect to a size of the resistors  96 . Thus, the tunable impedance load  54 ( 1 ) may be implemented in an application in order to conserve space. In different embodiments, different combination of inductors (e.g., the inductors  100  in  FIG. 7 ) and resistors (e.g., the resistors  96  in  FIG. 7 ) may be employed to provide the tunable impedance load  54  shown in  FIG. 2 . It should be noted at this point that the present disclosure is not limited to any particular arrangement for the tunable impedance load  54 . For example, any combination of passive or active IC impedance components may be used to provide the tunable impedance. All such variations are considered to be within the scope of this disclosure. 
       FIG. 12  is a graph illustrating a power curve  126 , a power curve  128 , and a power curve  130 . In this embodiment, the power curve  126 , the power curve  128 , and the power curve  130  are all produced by the RF amplification device  10 ( 1 ) shown in  FIG. 2 , where the tunable impedance load  54  is provided as the tunable impedance load  54 ( 1 ) shown in  FIG. 5 .  FIG. 12  demonstrates the advantage provided to the Doherty amplification circuit  12 ( 1 ) as a result of the dynamic tunability provided by the control circuit  14 ( 1 ). Both the power curve  126  and the power curve  128  are produced with the tunable impedance load  54 ( 1 ) is set to Znorm×10. Both the power curve  128  and the power curve  130  are produced with the carrier frequency of the RF signal  16  set to 1020 MHz. The power curve  126  is produced with the carrier frequency of the RF signal  16  set to 850 MHz. The power curve  130  is produced with the tunable impedance load  54 ( 1 ) is set to Znorm×30. 
     A comparison of the power curve  126  with the power curve  128  demonstrates that PAE is degraded when the tunable impedance of the tunable impedance load  54 ( 1 ) is set to Znorm×10 if the carrier frequency of the RF signal  16  is adjusted from 850 MHz to 1020 MHz. The power curve  130  demonstrates that increased power efficiency can be achieved by tuning the tunable impedance of the tunable impedance load  54 ( 1 ) to Znorm×30. The control circuit  14 ( 1 ) is configured to tune the tunable impedance of the tunable impedance load  54 ( 1 ) to Znorm×30 when the carrier frequency is adjusted from 850 MHz to 1020 MHz. In this manner, the Doherty amplification circuit  12 ( 1 ) can be tuned so as to optimize power performance. 
     As mentioned above, any combination of passive or active IC impedance components may be used to provide the tunable impedance.  FIGS. 13 and 14  illustrate additional embodiments of the tunable impedance load  54  shown in  FIGS. 1-4 . 
     In this regard,  FIG. 13  illustrates another embodiment of a tunable impedance load  54 ( 3 ). The tunable impedance load  54 ( 3 ) is another embodiment of the tunable impedance load  54  described above with respect to  FIG. 1 . The tunable impedance load  54 ( 3 ) is similar to the tunable impedance load  54 ( 2 ) shown in  FIG. 7 . Like the tunable impedance load  54 ( 2 ) shown in  FIG. 7 , the tunable impedance load  54 ( 3 ) includes a plurality of selectable impedance branches (referred to generically as elements  132  and specifically as elements  132 ( 1 )- 132 ( 6 )). The selectable impedance branches  132  are each coupled in parallel with respect to one another. An input terminal  134  of the tunable impedance load  54 ( 3 ) may be coupled to the eighth port  46  (shown in  FIGS. 1-4 ). 
     The selectable impedance branches include resistors (referred to generically as elements  136  and specifically as elements  136 ( 1 )- 136 ( 6 )), switches (referred to generically as elements  138  and specifically as elements  138 ( 1 )- 138 ( 6 )), and inductors (referred to generically as elements  140  and specifically as elements  140 ( 1 )- 140 ( 6 )). More specifically, the selectable impedance branch  132 ( 1 ) includes a resistor  136 ( 1 ), a switch  138 ( 1 ), and an inductor  140 ( 1 ) coupled in series. The selectable impedance branch  132 ( 2 ) includes a resistor  136 ( 2 ), a switch  138 ( 2 ), and an inductor  140 ( 2 ) coupled in series. Additionally, the selectable impedance branch  132 ( 3 ) includes a resistor  136 ( 3 ), a switch  138 ( 3 ), and an inductor  140 ( 3 ) coupled in series. Furthermore, the selectable impedance branch  132 ( 4 ) includes a resistor  136 ( 4 ), a switch  138 ( 4 ), and an inductor  140 ( 4 ) coupled in series. Also, the selectable impedance branch  132 ( 5 ) includes a resistor  136 ( 5 ), a switch  138 ( 5 ), and an inductor  140 ( 5 ) coupled in series. Finally, the selectable impedance branch  132 ( 6 ) includes a resistor  136 ( 6 ), a switch  138 ( 6 ), and an inductor  140 ( 6 ) coupled in series. 
     For each of the selectable impedance branches  132 , the selectable impedance branch  132  is configured to be selected when the switch  138  in the selectable impedance branch  132  is closed, and to be deselected when the switch  138  in the selectable impedance branch  132  is open. Each of the resistors  136  may have a different resistance and each of the inductors  140  may have a different inductance. The resistance of the resistor  136  and the inductance of the inductor  140  of the selectable impedance branch  132  is presented at the input terminal  134  (and thus at the eighth port  46  shown in  FIGS. 1-4 ) when the switch  138  in the selectable impedance branch  132  is closed. Otherwise, when the switch  138  in the selectable impedance branch  132  is open, the selectable impedance branch  132  appears as an open circuit. Each of the switches  138  shown in  FIG. 5  may be any type of suitable switch. For example, the switches  138  may be provided by as FETs and/or MEMs. 
     The switches  138  are opened and closed by an impedance control output  58 ( 3 ). The impedance control output  58 ( 3 ) is one embodiment of the impedance control output  58  described above with regard to  FIGS. 1-4 . In this embodiment, the impedance control output  58 ( 3 ) includes a plurality of switch control signals (referred to generically as elements  142  and specifically as elements  142 ( 1 )- 142 ( 6 )). Each of the switch control signals  142  is received by a corresponding one of the switches  138 . For each of the switch control signals  142 , the corresponding switch control signal  142  received by the switch  138  is activated when the switch control signal  142  is in a switch activation state, and is deactivated when the switch control signal  142  is in a switch deactivation state. A switch control signal  142 ( 1 ) provided in the impedance control output  58 ( 3 ) is received by the switch  138 ( 1 ). A switch control signal  142 ( 2 ) provided in the impedance control output  58 ( 3 ) is received by the switch  138 ( 2 ). A switch control signal  142 ( 3 ) provided in the impedance control output  58 ( 3 ) is received by the switch  138 ( 3 ). A switch control signal  142 ( 4 ) provided in the impedance control output  58 ( 3 ) is received by the switch  138 ( 4 ). A switch control signal  142 ( 5 ) provided in the impedance control output  58 ( 3 ) is received by the switch  138 ( 5 ). A switch control signal  142 ( 6 ) provided in the impedance control output  58 ( 3 ) is received by the switch  138 ( 6 ). Thus, in this embodiment, permutations of the impedance control output  58 ( 3 ) refer to particular combinations of the switch control signals  142  that are in the switch activation state and in the switch deactivation state. 
     Since the switches  138  are used to vary the tunable impedance presented at the input terminal  134 , the tunable impedance load  54 ( 2 ) discretely varies the tunable impedance. Furthermore, in this embodiment, the tunable impedance is both resistive and reactive, since each of the selectable impedance branches  132  includes one of the resistors  136  and one of the inductors  140 . However, unlike the tunable impedance load  54 ( 2 ) shown in  FIG. 7 , the inductors  140 ( 4 ),  140 ( 5 ), and  140 ( 6 ) each have a much higher inductance than an inductance of each of the inductors  140 ( 1 ),  140 ( 2 ), and  140 ( 3 ). Accordingly, the inductors  140 ( 4 ),  140 ( 5 ), and  140 ( 6 ) may physically be significantly larger than the inductors  140 ( 1 ),  140 ( 2 ), and  140 ( 3 ). This thus allows for the tunable impedance load  54 ( 3 ) to be used in multi-band applications when certain RF communication specifications have stringent quality (Q) factor requirements. 
     With regard to  FIG. 14 ,  FIG. 14  illustrates another embodiment of a tunable impedance load  54 ( 4 ). The tunable impedance load  54 ( 4 ) is another embodiment of the tunable impedance load  54  described above with respect to  FIG. 1 . The tunable impedance load  54 ( 4 ) includes a plurality of selectable impedance branches (referred to generically as elements  144  and specifically as elements  144 ( 1 )- 144 ( 6 )). The selectable impedance branches  144  are each coupled in parallel with respect to one another. An input terminal  146  of the tunable impedance load  54 ( 4 ) may be coupled to the eighth port  46  (shown in  FIGS. 1-4 ). 
     In this embodiment, the selectable impedance branches  144 ( 1 ),  144 ( 3 ), and  144 ( 5 ) (referred to collectively as elements  144 CAP) include capacitors (referred to generically as elements  148  and specifically as elements  148 ( 1 )- 148 ( 3 )). In contrast, the selectable impedance branches  144 ( 2 ),  144 ( 4 ), and  144 ( 6 ) (referred to collectively as elements  144 RL) include resistors (referred to generically as elements  150  and specifically as elements  150 ( 1 )- 150 ( 3 )) and inductors (referred to generically as elements  152  and specifically as elements  152 ( 1 )- 152 ( 3 )). The selectable impedance branches  144  also include switches (referred to generically as elements  154  and specifically as elements  154 ( 1 )- 154 ( 6 )). 
     More specifically, the selectable impedance branch  144 ( 1 ) includes a capacitor  148 ( 1 ) and a switch  154 ( 1 ) coupled in series. The selectable impedance branch  144 ( 2 ) includes a resistor  150 ( 1 ), an inductor  152 ( 1 ), and a switch  154 ( 2 ) coupled in series. Additionally, the selectable impedance branch  144 ( 3 ) includes a capacitor  148 ( 2 ) and a switch  154 ( 3 ) coupled in series. Furthermore, the selectable impedance branch  144 ( 4 ) includes a resistor  150 ( 2 ), an inductor  152 ( 2 ), and a switch  154 ( 4 ) coupled in series. Also, the selectable impedance branch  144 ( 5 ) includes a capacitor  148 ( 3 ) and a switch  154 ( 5 ) coupled in series. Finally, the selectable impedance branch  144 ( 6 ) includes a resistor  150 ( 3 ), an inductor  152 ( 3 ), and a switch  154 ( 6 ) coupled in series. 
     For each of the selectable impedance branches  144 , the selectable impedance branch  144  is configured to be selected when the switch  154  in the selectable impedance branch  144  is closed, and to be deselected when the switch  154  in the selectable impedance branch  144  is open. Each of the capacitors  148  may have a different capacitance, each of the resistors  150  may have a different resistance, and each of the inductors  152  may have a different inductance. The capacitance of the capacitor  148  of each of the selectable impedance branches  144 CAP is presented at the input terminal  146  when the switch  154  in the selectable impedance branches  144 CAP is closed. The resistance of the resistor  150  and the inductance of the inductor  152  of each of the selectable impedance branches  144 RL is presented at the input terminal  146  (and thus the eighth port  46  shown in  FIGS. 1-4 ) when the switch  154  in the selectable impedance branches  144 RL is closed. Otherwise, when the switch  154  in the selectable impedance branches  144  is open, the selectable impedance branch  144  appears as an open circuit. 
     In this embodiment, different resonant impedance tanks may be presented at the input terminal  146  by selecting different combinations of the capacitors  148 , the resistors  150 , and the inductors  152 . More specifically, different resonant impedance tanks may be presented at the input terminal  146  by selecting one or more of the selectable impedance branches  144 CAP and by selecting one or more of the selectable impedance branches  144 RL. In this manner, the tunable impedance of the tunable impedance load  54 ( 4 ) can be provided with a high Q factor, which is advantageous in high frequency applications. 
     The switches  154  are opened and closed by an impedance control output  58 ( 4 ). The impedance control output  58 ( 4 ) is one embodiment of the impedance control output  58  described above with regard to  FIGS. 1-4 . In this embodiment, the impedance control output  58 ( 4 ) includes a plurality of switch control signals (referred to generically as elements  156  and specifically as elements  156 ( 1 )- 156 ( 6 )). Each of the switch control signals  156  is received by a corresponding one of the switches  154 . For each of the switch control signals  156 , the corresponding switch control signal  156  received by the switch  154  is activated when the switch control signal  156  is in a switch activation state and is deactivated when the switch control signal  156  is in a switch deactivation state. A switch control signal  156 ( 1 ) provided in the impedance control output  58 ( 4 ) is received by the switch  154 ( 1 ). A switch control signal  156 ( 2 ) provided in the impedance control output  58 ( 4 ) is received by the switch  154 ( 2 ). A switch control signal  156 ( 3 ) provided in the impedance control output  58 ( 4 ) is received by the switch  154 ( 3 ). A switch control signal  156 ( 4 ) provided in the impedance control output  58 ( 4 ) is received by the switch  154 ( 4 ). A switch control signal  156 ( 5 ) provided in the impedance control output  58 ( 4 ) is received by the switch  154 ( 5 ). A switch control signal  156 ( 6 ) provided in the impedance control output  58 ( 4 ) is received by the switch  154 ( 6 ). Thus, in this embodiment, permutations of the impedance control output  58 ( 4 ) thus refer to particular combinations of the switch control signals  156  that are in the switch activation state and in the switch deactivation state. 
     Since the switches  156  are used to vary the tunable impedance presented at the input terminal  146 , the tunable impedance load  54 ( 4 ) discretely varies the tunable impedance. Furthermore, in this embodiment, the tunable impedance is both resistive and reactive since each of the selectable impedance branches  144 RL include one of the resistors  150  and one of the inductors  152 , and since each of the selectable impedance branches  144 CAP includes one of the capacitors  148 . In this manner, the tunable impedance of the tunable impedance load  54 ( 4 ) is a complex impedance and thus includes a real impedance and an imaginary impedance. 
     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.