Patent Publication Number: US-9887671-B2

Title: Reconfigurable load modulation amplifier

Description:
RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 14/501,453, filed Sep. 30, 2014, now U.S. Pat. No. 9,484,865, which claims priority to U.S. provisional patent application No. 61/884,571, filed Sep. 30, 2013, and U.S. provisional patent application No. 61/892,683, filed Oct. 18, 2013. 
     This application is related to concurrently filed U.S. patent application Ser. No. 15/278,270, entitled “RECONFIGURABLE LOAD MODULATION AMPLIFIER,” which is a continuation in part of U.S. patent application Ser. No. 14/501,453, filed Sep. 30, 2014, now U.S. Pat. No. 9,484,865, which claims priority to U.S. provisional patent application No. 61/884,571, filed Sep. 30, 2013, and U.S. provisional patent application No. 61/892,683, filed Oct. 18, 2013. 
     All of the applications listed above are hereby incorporated herein by reference in their entireties. 
    
    
     FIELD OF THE DISCLOSURE 
     The present disclosure pertains to amplifiers and in particular to load modulation amplifiers having a carrier amplifier and a peak amplifier that are coupled in parallel. 
     BACKGROUND 
     The quest for high efficiency over a large peak to average power ratio (PAPR) and the ability to cover a wide bandwidth (BW) of operation (&gt;160 MHz and typically &gt;5% of carrier frequency) is desired for future high data rate wireless transmissions. Envelope Tracking (ET) has been demonstrated to provide high efficiency across &gt;10 dB of PAPR for 20 MHz long term evolution (LTE) mobile applications. However, this approach is BW challenged and is strongly dependent on fundamental semiconductor technology trades. Bandwidths of up to 30 MHz have been demonstrated for mobile applications. Wireless Fidelity (Wi-Fi) BW requirements for 160 MHz 802.11 ac are challenging without costly semiconductor technology disruptions. 
     Doherty amplifiers can be configured to provide high efficiency over a 6 dB OPBO (output power back off or PAPR range). In particular, Doherty amplifiers may be configured to achieve larger PAPRs of 9 dB and 12 dB or more using N-way and/or asymmetric Doherty approaches. However, these are typically limited to operating BW&lt;5% due to the narrow-band quarter wave impedance inverters used. Doherty amplifiers also rely on Digital Pre-Distortion (DPD) to correct for phase and amplitude distortion resulting from the Doherty operation. For base stations, DPD has shown operation up to 60 MHz with capability in the hundreds of MHz and exceeding 1 GHz for future point to point radio systems. However, Doherty amplifiers are not able to keep up with the BW capability of DPD. 
     Thus, what is needed is a new amplifier paradigm that is reconfigurable to provide the advantageous characteristics of Doherty amplifier operation while providing the BW capability of DPD. Moreover, an associated need is a load modulation type amplifier that is configured for improved OPBO efficiency by reducing impedance at lower power levels. It is important to note that this need is contrary to conventional load modulation amplifier approaches. 
     SUMMARY 
     A reconfigurable load modulation amplifier having a carrier amplifier and a peak amplifier that are coupled in parallel is disclosed. The peak amplifier provides additional power amplification when the carrier amplifier is driven into saturation. A quadrature coupler that is coupled between the carrier amplifier and the peak amplifier is configured to combine power from both the carrier amplifier and the peak amplifier for output through an output load terminal. 
     In at least some of the embodiments, the carrier amplifier is coupled to the output load terminal through zero phase shift ports of the quadrature coupler. Moreover, at least some embodiments include control circuitry at an isolation port of the quadrature coupler. The control circuitry is configured to provide adjustable impedance at the isolation port of the quadrature coupler. Further still, in at least some embodiments, the reconfigurable load modulation amplifier includes voltage standing wave ratio (VSWR) detection circuitry configured to detect a VSWR mismatch condition associated with the output load terminal. The VSWR detection circuitry is usable by the control circuitry to set an isolation impedance of the quadrature coupler to a termination impedance range relative to the nominal output load termination impedance to increase isolation between the carrier and peak amplifiers for improving load insensitive load modulation operation. 
     Those skilled in the art will appreciate the scope of the disclosure and realize additional aspects thereof after reading the following detailed description in association with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings 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  is a schematic diagram of a conventional Doherty amplifier that is related art. 
         FIG. 2  is a schematic diagram of a related art broadband reconfigurable Doherty power amplifier with tunable coupler. 
         FIGS. 3A and 3B  are schematic diagrams of a related art multi-band Doherty power amplifier. 
         FIG. 4  is a schematic of a related art inverted Doherty amplifier that exchanges phases between a carrier amplifier and a peak amplifier. 
         FIG. 5A  is a schematic diagram of a related art reconfigurable tuned Doherty amplifier. 
         FIG. 5B  is a graph depicting ideal carrier amplifier transistor impedance and peak amplifier current profiles for various peak-to-average power ratio (PAPR) type Doherty amplifiers. 
         FIG. 6A  is a graph of drain efficiency versus normalized frequency for a conventional Doherty amplifier of the related art. 
         FIG. 6B  is a graph of drain efficiency versus output power back off (OPBO) for a conventional Doherty amplifier of the related art. 
         FIG. 7  is a schematic diagram of a first embodiment that in accordance with the present disclosure is a reconfigurable wideband load modulated power amplifier that is closed loop tunable. 
         FIG. 8  is a schematic diagram of a second embodiment that in accordance with the present disclosure is a reconfigurable wideband load modulated power amplifier that is open loop tunable. 
         FIG. 9  is a schematic diagram of a third embodiment that in accordance with the present disclosure is a reconfigurable wideband inverted phase load modulated power amplifier that is closed loop tunable. 
         FIG. 10  is a schematic diagram of a fourth embodiment that in accordance with the present disclosure is a reconfigurable wideband inverted phase load modulated power amplifier that is open loop tunable. 
         FIG. 11  is a schematic diagram of a fifth embodiment that in accordance with the present disclosure is a reconfigurable wideband inverted phase load modulated power amplifier that is open loop tunable. 
         FIG. 12  is a graph depicting 4-port quadrature coupler impedance modulation in accordance to the present disclosure. 
         FIG. 13  is a graph depicting 4-port quadrature coupler scattering parameter S 12  isolation between the carrier amplifier and the peak amplifier during operation with output impedance of 50Ω and a voltage standing wave ratio (VSWR) of 1:1. 
         FIG. 14  is a graph depicting 4-port quadrature coupler scattering parameter S 12  isolation between the carrier amplifier and the peak amplifier during operation with an output impedance of 5Ω and a VSWR of 10:1. 
         FIG. 15  is a graph depicting 4-port quadrature coupler scattering parameter S 12  isolation between the carrier amplifier and the peak amplifier during operation with an output impedance of 500Ω and a VSWR of 10:1. 
         FIG. 16  is a graph depicting gain versus power for a Lange-coupled modulated amplifier during operation at isolation termination impedances of 50Ω, 100Ω, 500Ω, 1000Ω, and 5000Ω. 
         FIG. 17  is a graph depicting gain versus power for a Lange-coupled load modulated amplifier during operation at isolation termination impedances of 0Ω, 5Ω, 10Ω, 20Ω, 30Ω, 40Ω and 50Ω. 
         FIG. 18  is a graph depicting gain versus power for a Lange-coupled inverted load modulated amplifier during operation at isolation termination impedances of 50Ω, 100Ω, 500Ω, 1000Ω, and 5000Ω. 
         FIG. 19  is a graph depicting gain versus power for a Lange-coupled inverted load modulated amplifier during operation at isolation termination impedances of 0Ω, 5Ω, 10Ω, 20Ω, 30Ω, 40Ω and 50Ω. 
         FIG. 20  is a graph depicting gain versus power for a quadrature coupled-line load modulated amplifier during operation at isolation termination impedances of 50Ω, 100Ω, 500Ω, 1000Ω, and 5000Ω. 
         FIG. 21  is a graph depicting gain versus power for a quadrature coupled-line load modulated amplifier during operation at isolation termination impedances of 0Ω, 5Ω, 10Ω, 20Ω, 30Ω, 40Ω and 50Ω. 
         FIG. 22  is a graph depicting gain versus power for a quadrature coupled-line inverted load modulated amplifier during operation at isolation termination impedances of 50Ω, 100Ω, 500Ω, 1000Ω, and 5000Ω. 
         FIG. 23  is a graph depicting gain versus power for a quadrature coupled-line inverted load modulated amplifier during operation at isolation termination impedances of 0Ω, 5Ω, 10Ω, 20Ω, 30Ω, 40Ω and 50Ω. 
         FIG. 24  is a graph depicting gain versus power for a Lange-coupled inverted load modulated amplifier during operation at output frequencies of 1.8 GHz, 2.15 GHz, and 2.5 GHz, ±15% bandwidth (BW). 
         FIG. 25  is a graph depicting gain versus power for a conventional Doherty amplifier of the related art during operation at output frequencies of 1.8 GHz, 2.15 GHz, and 2.5 GHz, ±15% BW. 
         FIG. 26  is a graph depicting wideband output power and power added efficiency (PAE) of an inverted load modulation embodiment of the present disclosure versus a conventional Doherty amplifier with both operating at 6 dB output power back off (OPBO). 
         FIG. 27  is a schematic diagram of a quadrature combined load modulated power amplifier. 
         FIG. 28  is a schematic diagram of an inverted phase quadrature combined load modulated power amplifier. 
         FIG. 29  is a graph depicting measured quadrature phase load modulated gain and efficiency versus output power. 
         FIG. 30  is a graph depicting measured quadrature inverted phase load modulated gain and efficiency versus output power. 
         FIG. 31  is a graph depicting conventional 50Ω quadrature operation for class A, class B, and class C amplifiers with no load modulation. 
         FIG. 32  is an IV curve graph with loadlines depicting conventional class A (Rload) load line operation, low impedance (RLoad/2) load modulation under lower power backed off signal, and low impedance (Rload/2) load modulation under low power backed off signal and lower supply voltage Vq_m, illustrating more efficient load line impedance at lower supply voltage with higher current swing (than 2*Rload conventional Doherty OPBO load-line configuration). 
         FIG. 33  is a graph of load modulation combined with a Doherty amplifier configured for envelope tracking (ET). 
         FIG. 34  is a graph of measured total gain and collector efficiency versus output power for a quadrature phase load modulated amplifier of the present disclosure illustrating improved backed off efficiency through the combination of load modulation (to lower Zc impedance˜RL/2) and lower supply modulated operating voltage. 
     
    
    
     DETAILED DESCRIPTION 
     The embodiments set forth below represent the necessary information to enable those skilled in the art to practice the disclosure and illustrate the best mode of practicing the disclosure. Upon reading the following description in light of the accompanying drawings, those skilled in the art will understand the concepts of the disclosure and will recognize applications of these concepts not particularly addressed herein. It should be understood that these concepts and applications fall within the scope of the disclosure and the accompanying claims. 
     In this disclosure, a new type of load modulated amplifier is described. This new type of load modulated amplifier operates with a large departure from fundamental Doherty amplifier operation in three major aspects. In particular, the phase of a carrier amplifier and peak amplifier making up the disclosed load modulated amplifier have been inverted, while at the same time, the impedance modulation versus power may be inverted for the carrier and peak amplifiers resulting in better efficiency and peak to average power ratio (PAPR) while providing improved bandwidth (BW) performance over conventional Doherty amplifiers, and the modulated load impedance seen by either carrier or peak amplifier can achieve a range that is as low as half of the nominal normalized load impedance of the quadrature coupler, and as high as twice the nominal normalized load impedance of the quadrature coupler. It should be appreciated that conventional Doherty operation reconfigures the carrier amplifier load impedance to be as high as twice the nominal normalized load impedance of the quadrature amplifier, but does not reconfigure the carrier amplifier below the nominal normalized load impedance. 
     Because of the departure from Doherty operation of the impedance versus power profile, where the carrier impedance is now decreasing with reduced input power level, and can produce a modulation load to either the peak or carrier amplifier that is lower than the nominal normalized load impedance of the quadrature coupler, and as low as half of the nominal normalized load impedance of the quadrature amplifier, while maintaining good output power back off (OPBO) efficiency, the amplifier embodiments of the present disclosure are not a type of Doherty amplifier. In effect, the embodiments of the presently disclosed amplifier are novel and merit a new classification—an amplifier sui generis. 
     In addition, the embodiments of the present disclosure enable a method of preserving port-to-port (i.e., carrier to peak) amplifier isolation over a wide frequency bandwidth, which involves detecting a voltage standing wave ratio (VSWR) and phase at the output, and then adjusting an isolation impedance on the coupler isolation port to provide improved load insensitive and high efficiency operation, which is a fundamental challenge in Doherty amplifiers. Further still, this disclosure also focuses on a reduction to practice of the novel amplifier embodiments disclosed herein. The disclosed reduction to practice is a large departure from fundamental Doherty amplifier load operation in at least two major aspects. The impedance modulation versus power is the opposite of what is characteristic for a Doherty amplifier in which a carrier amplifier is introduced to relatively lower impedance. Another aspect is that the impedance modulation that is presented to one or the other amplifiers is lower than the nominal normalized impedance of the quadrature coupler. In particular, embodiments of the present disclosure see half of the load impedance whereas a Doherty amplifier experiences twice the load impedance during OPBO operation. As a result, an improved small signal efficiency and PAPR is achieved while providing BW improvements over traditional Doherty amplifiers. Moreover, since the present embodiments see half the load impedance, improvements in applications having drain-voltage modulation techniques such as envelope tracking (ET), and average power tracking (APT) can be realized. The present embodiments can also be used in place of traditional Doherty amplifiers for enhanced operation of applications that include traditional Doherty amplifiers. 
       FIG. 1  is a schematic of a related art Doherty amplifier  10  that is configured conventionally in a 2-way symmetrical design. One difference between the related art Doherty amplifier  10  and embodiments of the present disclosure is that a carrier amplifier  12  of the Doherty amplifier  10  is communicatively coupled to an output terminal  14  through a 90° phase shifter  16 . A peak amplifier  18  is coupled to an input terminal  20  through another 90° phase shifter  22 . A termination impedance  24  is coupled between the peak amplifier  18  and the output terminal  14 . Another difference is that the impedance presented to a carrier amplifier at ˜6 dB OPBO is twice the load impedance (i.e., 2*Rload). In the present disclosure, at least one of the embodiments has a carrier amplifier that is coupled to the output through a zero degree coupler. Moreover, at least one other embodiment of the present disclosure has an impedance presented to the carrier amplifier that is approximately Rload/2 at roughly 6 dB OPBO or greater. This lower impedance results in higher carrier amplifier power efficiency when the amplifier is backed off from saturation. Note, that the load impedance Rload (not shown) is provided by a load that is usually coupled to the output terminal  14 . 
     In particular, the embodiments of the present disclosure depart from conventional Doherty amplifiers in at least the following ways. In at least some of the disclosed embodiments, the carrier amplifier is directly coupled to the zero phase terminal through the ports of a 4-port coupler, as opposed to coupling through a 90° phase shift to a summing node. Other embodiments are configured such that a termination impedance of the coupler is adjustable. Due to the termination impedance adjustability, the carrier amplifier load impedance decreases as power is backed off instead of increasing as occurs in a conventional Doherty amplifier operation. Moreover, at least some of the disclosed embodiments are configured to detect a VSWR and automatically adjust the isolation termination impedance (Z ISO ) of the coupler to provide maximum power amplifier efficiency with good antenna VSWR load immunity. Further still, at least some embodiments are reconfigurable for load modulated amplification using an open loop signal that adjusts termination impedance for maximizing characteristics such as power, efficiency, linearity, bandwidth, load insensitivity or a combination thereof. 
       FIG. 2  is a schematic of a related art reconfigurable broadband Doherty amplifier  26  employing a 4-port quadrature coupler combiner  28  at an output load terminal  30  where load modulation of the carrier amplifier is achieved by changing a termination impedance  32  of a termination port  34  to create Doherty amplifier operation. The termination impedance  32  may be optimized for a given Pout, frequency or temperature, and may involve continuous or discrete control from a control signal generated by a detector  36 . Note the coupled port polarity and impedance change with power is inverted for the embodiments of the present disclosure. A second coupler  38  is coupled between an input terminal  40  and a peak amplifier  42  and a carrier amplifier  44 . A second termination impedance  46  is coupled to a second termination port  48 . 
       FIGS. 3A and 3B  are schematics of a related art low band amplifier  50  and a high band amplifier  52 , respectively, that each illustrates the broadband nature of the use of Lange couplers  54 ,  56 ,  58 , and  60 . More specifically, the related art amplifiers  50  and  52  are configured for operation at an operational frequency fo and 3fo and rely on the periodic nature of the Lange quadrature couplers  54 ,  56 ,  58 , and  60 . Phasing during operation is such that the Lange couplers  54  and  56  each provide 90° quadrature characteristics at 1 st  and roughly 3 rd  harmonics with low loss. Thus, reconfigurability can be achieved in multiple bands. Note that a direct coupled output  62  at 0° is coupled to the peak amplifier  42  as in a conventional Doherty amplifier which departs from embodiments of the present disclosure which are not limited to Doherty amplifier operation. Also, a carrier impedance Zcarrier and a peak impedance Zpeak are &gt;Zload for a load (not show) that is typically coupled to the output load terminal  30 , whereas embodiments of this disclosure may utilize Zcarrier and Zpeak&lt;Zload for achieving both efficiency and load insensitivity. 
       FIG. 4  is a schematic of a related art inverted Doherty power amplifier  64 . The inverted Doherty power amplifier  64  saves area and thus cost for mobile applications by eliminating matching circuitry. Basically, the inverted Doherty power amplifier  64  exchanges the phases between a carrier amplifier  66  and a peak amplifier  68 . Note that a wider bandwidth can be obtained by eliminating output impedance transformer  70 , which in this case is a narrow-band impedance λ/4 transformer. This inherent feature is also preserved in embodiments of the present disclosure, since a 4-port quadrature coupler produces a wider and broader band impedance transformation from the 50Ω output impedance. As a result, there is no need for an output impedance transformer such as output impedance transformer  70  for the present embodiments, regardless of the output phase configuration. The inverted Doherty power amplifier  64  also includes a power splitter  72  coupled between an input IN and the carrier amplifier  66  and the peak amplifier  68 . Also, a narrow-band λ/4 impedance inverter  74  that is communicatively coupled between outputs of the carrier amplifier  66  and the peak amplifier  68  through coupling capacitors C 1  and C 2 . 
       FIG. 5A  is a schematic of a related art reconfigurable Doherty amplifier  76  that in effect is tunable. The objective of this type of Doherty amplifier is to increase the efficiency of the power amplifier over a larger PAPR by augmenting the load modulation of the reconfigurable Doherty amplifier  76  using matching networks  78  and  80  made up of micro-electro-mechanical systems (MEMs) tuning networks at the outputs of a carrier amplifier  82  and a peak amplifier  84 . However, it should be noted that this is not a continuous operation. Although the range of impedances presented to the carrier amplifier  82  is much larger than a conventional Doherty amplifier, the reconfigurable Doherty amplifier  76  still utilizes an increase of carrier load impedance as output power is backed off as opposed to a reduction of carrier load impedance taught for operation of the embodiments of the present disclosure. Furthermore, as shown in  FIG. 5B , the impedance modulation range practiced for the reconfigurable Doherty amplifier  76  shows impedances larger than R L  (50Ω), whereas the present embodiments may be configured to operate at impedance modulations below and above R L . The reconfigurable Doherty amplifier  76  also includes a power divider  86  coupled between a power input P IN  and the carrier amplifier  82  and the peak amplifier  84 . A combining network  88  is coupled between the matching networks  78  and  80 , and the load R L . 
       FIGS. 6A and 6B  are graphs for an enhanced related art Doherty amplifier using drain modulation. In these examples, the supply or drain voltage of the carrier amplifier is under output power backed-off conditions in order to further improve the output power efficiency. As shown, a larger OPBO efficiency range is one frequency bandwidth of operation that is also improved by eliminating the output impedance transformer. However, a traditional quarter wave impedance inverter remains included and is designed to have broadband at only the peak efficiency backed-off power level. A region shown inside a dashed box of  FIG. 6B  illustrates the improvement in OPBO when carrier V DS1  supply is reduced compared to peak V DS2 . 
       FIGS. 7-11  are schematics of embodiments of the present disclosure. Embodiments of the present disclosure differ from the related art in at least the following significant ways. For one, the carrier amplifier of this disclosure in  FIGS. 7-9  may be directly coupled to the zero phase transmission ports of a 4-port coupler, as opposed to coupling through a 90° phase shift to a summing node. 
     Moreover, the termination port impedance Z ISO  of the coupler is adjustable such that the carrier amplifier load impedance decreases as power is backed off instead of increasing as in related art and the related art discussed above. Further still, the present embodiments are configured to adjust the termination port impedance Z ISO  over a wider range such that the carrier amplifier load impedance port of the coupler may see a range from Zload/2 to greater than 2*Zload, where Zload is the nominal output port impedance of the coupler. This extends the reconfigurable OPBO efficiency region of the load modulated amplifier. Even further, the embodiments of the present disclosure provide enhanced load insensitivity by detecting the output VSWR and adjusting the coupler termination impedance above the nominal Zload impedance when the detected VSWR mismatch is due to a lower than Zload mismatch impedance, and vice versa (for a non-inverting load modulation configuration). This flexibility improves port to port isolation between the carrier amplifier and the peak amplifier, which is a drawback of conventional Doherty operation. Moreover, the Z ISO  tuning range mode may be selected from at least the choices of 0-Zload, or Zload-2*Zload for improved load insensitive performance. Even further, load modulated amplification is reconfigurable using an open loop signal that adjusts termination impedance for maximizing power, efficiency, linearity, bandwidth, load insensitivity or a combination thereof. 
       FIG. 7  is a schematic diagram of a first embodiment, that in accordance with the present disclosure, is a reconfigurable load modulated power amplifier  90  that is wideband and closed loop tunable. A carrier amplifier  92  is biased for class A operation and a peak amplifier  94  is biased for class C operation. The carrier amplifier  92  and the peak amplifier  94  are coupled in parallel using a first quadrature coupler  96  at an input  98  and a second quadrature coupler  100  at an output load terminal  102 . It should be noted that the bias of the peak amplifier  94  may be set for other classes such as A or AB in order to configure the amplifier for a desired response. An input impedance tuning network  104  is coupled between the first quadrature coupler  96  and ground. An output impedance tuning network  106  is coupled between the second quadrature coupler  100  and ground. A detector  108  is a controller that generates signals that tune the impedances of the input impedance tuning network  104  and the output impedance tuning network  106  under continuous control. 
       FIG. 8  is a schematic diagram of a second embodiment that in accordance with the present disclosure is a reconfigurable wideband load modulated power amplifier  110  that is open loop tunable. In particular, the reconfigurable wideband load modulated power amplifier  110  includes an open loop baseband signal look up table (LUT)  112  that is a controller that outputs baseband signals that include a first control signal S 1  and a second control signal S 2  that each are responsive to power, frequency f0, and temperature T or any combination thereof. 
     In the present case the desired goal is to achieve high efficiency at OPBO by use of amplifier load modulation. The quadrature couplers may be implemented as a Lange, a branch-line, or a coupled-line coupler construction or any other structure that provides a 90° phase split. In the embodiments shown in  FIGS. 7 and 8 , the polarity of the input couplers is such that the carrier amplifier  92  receives a signal that is in-phase with the input signal. The peak amplifier  94  receives a signal that has a 90° phase shift with respect to the input signal. To maintain proper power combining at the output, the carrier amplified output signal undergoes a 90° phase shift before combining at a common output node (Z 3 ), while the peak amplified signal is coupled to the output with zero or no phase shift. A termination port (Z 4 ) is presented with a variable isolation termination impedance, Z ISO , which may be adjusted continuously or discretely to modulate a carrier amplifier load impedance (Z 2 ) seen by the carrier amplifier  92  in order to improve efficiency as the output power is backed off of compression. This variable tuning impedance means may be realized by a silicon-on-insulator (SOI) linear varistor for continuous or closed loop operation. Alternatively, the variable tuning impedance may also be realized by a switched impedance network of inductors, resistors, and capacitors using MEMs or SOI switch technologies. The input impedance tuning network  104  at a 4 th  (isolation) port of the input quadrature 4-port coupler may be tuned in conjunction with the output impedance tuning network  106  in order to adjust the amplitude and phase combining of the carrier and peak amplifiers  92 ,  94  for best power performance. The impedance tuning networks  104  and  106  are controlled by the baseband signals that are generated by the detector  108  ( FIG. 7 ), or by baseband generated control signals S 1  and S 2  generated by the open loop baseband signal LUT  112  ( FIG. 8 ) that may be in response to output detected power, input detector power, DC current characteristics of the peak amplifier and the carrier amplifier, or output VSWR mismatch information. 
     Unlike a Doherty amplifier or the other related art discussed above, the embodiments of  FIGS. 7 and 8  operate where Z ISO  is tuned such that Z 2  presented to the carrier amplifier  92  may be lower than the nominal load impedance, Zload (which is typically 50Ω), in order to obtain high OPBO efficiency. This is counter to the 2*Zload, in this case ˜100Ω, typically presented to the carrier amplifier at 6 dB OPBO for Doherty operation. Secondly, the embodiments of  FIG. 7  and  FIG. 8  practice a range of Z 2  impedances (˜Zload/2 to ˜2*Zload that are presented to the carrier amplifier  92  that is twice that of a conventional Doherty amplifier. Thirdly, it is demonstrated in this disclosure that the isolation characteristics of the coupler between the carrier amplifier Z 2  and peak amplifier Z 1  may be improved under output VSWR mismatch by adjusting the Z ISO  impedance to be &gt;Zload (50Ω) when the VSWR mismatch is due to an output impedance &lt;Zload (50Ω) or vice versa. 
     A significant application is realized by a wide range of carrier and peak amplifier load impedances that is used as a general load modulation means for improving power efficiency. For example, in conjunction with envelope tracking (ET) or supply modulation techniques, where the supply is reduced at lower power levels for the carrier amplifier to improve power efficiency at OPBO, Z ISO  may be tuned such that the carrier amplifier load impedance Z 2  is lower than Zload (50Ω) to provide an efficient load at the lower supply condition. The Z 2  carrier load tuning provides an extra degree of freedom to appropriately set the power and efficiency load line on the I-V curves of a power transistor of the carrier amplifier  92 . Moreover, the same technique may be applied to a power amplifier system where drain voltage and bias current are both throttled down at lower powers to improve efficiency. The ability to adjust the load line can enable linear operation in addition to efficiency operation. This capability is very useful for improving the power efficiency of gallium nitride-based devices and PAs whose high (˜5V) I-V knee voltage limits the efficiency performance when operating at lower supply and drain voltages. The extra degree of freedom to modulate the load impedance of the amplifier enables improvement in efficiency when the minimum practical drain voltage is reached in an envelope tracking system. 
       FIGS. 9-11  show the 3 rd , 4 th  and 5th embodiments of the present disclosure. These embodiments are fundamentally different from the 1 st  and 2 nd  embodiments in the phase combining of the carrier amplifier  92  and the peak amplifier  94  and depart from the phase combining of the Doherty amplifier operation in general. Beginning with  FIG. 9 , a reconfigurable wideband inverted phase load modulated power amplifier  114  that is closed loop tunable is shown. In particular, the carrier amplifier  92  biased in class A and the peak amplifier  94  biased in class C are in a combined parallel connection using a first quadrature coupler  116  at the input and a second quadrature coupler  118  at the output load terminal  102 . It should be noted that the bias of the peak amplifier  94  may be set for other classes such as A or AB in order to configure the amplifier for a desired response. In the present case the desired goal is to achieve high efficiency at OPBO by use of amplifier load modulation. The quadrature couplers  116  and  118  may be implemented as a Lange, a branch-line, or a coupled-line coupler construction or any other structure that provides a 90 degree phase split. In these 3 rd , 4 th  and 5 th  embodiments, the polarity of the input couplers are different than in the 1 st  and 2 nd  embodiments, and are such that the carrier amplifier  92  receives a signal that is phase shifted 90 degrees with respect to the input signal. The peak amplifier  94  receives a signal that is in-phase (0 degree phase shifted) with respect to the input signal. 
     To maintain proper power combining at the output, the carrier amplified output signal undergoes zero or no phase shift before combining at the common output node (Z 3 ), while the peak amplified signal is coupled to the output with 90 degree phase shift. Unlike the 1 st  and 2 nd  embodiments and the related art discussed above, the termination port (Z 4 ) is presented with a variable isolation termination impedance, Z ISO , which may be adjusted continuously or discretely to modulate the carrier amplifier load impedance (Z 1 ) seen by the carrier amplifier  92  in order to improve efficiency as the output power is backed off of compression. This variable tuning impedance means may be realized by an SOI linear varistor for continuous or closed loop operation. The variable tuning impedance may also be realized by a switched impedance network of inductors, resistors, and capacitors using micro-mechanical-systems (MEMs) switch networks or SOI switch technologies. The input impedance tuning network  104  at the 4 th  (isolation) port of the input quadrature 4-port coupler may be tuned in conjunction with the output impedance tuning network  106  in order to adjust the amplitude and phase combining of the carrier and peak amplifiers  92 ,  94  for best power performance. The impedance tuning networks  104 ,  106  may be controlled with a control signal that is generated by the detector  108 , a look up table (LUT) or baseband generated signals (4 th  embodiment) that may be in response to output detected power, input detected power, DC current characteristics of the peak amplifier  94  and the carrier amplifier  92 , or output VSWR mismatch information (5 th  embodiment). For example,  FIG. 10  is a schematic diagram of the 4 th  embodiment that in accordance with the present disclosure is a reconfigurable wideband inverted phase load modulated power amplifier  120  that is open loop tunable by way of the open loop baseband signal LUT  112 . 
     Unlike a Doherty amplifier, the related art discussed above, or the 1 st  and 2 nd  embodiments, the 3 rd , 4 th  and 5 th  embodiments operate wherein firstly Z ISO  is tuned such that Z 1  presented to the carrier amplifier  92  may be lower than the nominal load impedance, Zload (which is typically 50Ω), in order to obtain high OPBO efficiency characteristics. Presenting Z 1  lower to the carrier amplifier  92  is counter to the ˜2*Zload (˜100Ω) typically presented to the carrier amplifier at 6 dB OPBO for Doherty operation. Secondly, the 3 rd , 4 th  and 5 th  embodiments are practicing a range of Z 1  impedances (˜Zload/2 to ˜2*Zload,  FIG. 12 ) presented to the carrier amplifier or peak amplifier that is twice that of a conventional Doherty amplifier. Thirdly, it is demonstrated in this disclosure that the isolation characteristics of the coupler between the carrier amplifier Z 1  and peak amplifier Z 2  may be improved under output VSWR mismatch by adjusting the Z ISO  impedance to be &gt;Zload (50Ω) when the VSWR mismatch is due to an output impedance &lt;Zload (50Ω) or vice versa. 
     A significant application of the embodiments of the present disclosure is that a wide range of the carrier amplifier and the peak amplifier load impedances may be used as a general load modulation means for improving power efficiency. For example, in conjunction with envelope tracking or supply modulation techniques where the supply is reduced at lower power levels for the carrier amplifier to improve power efficiency at OPBO, Z ISO  may be tuned such that the carrier amplifier load impedance Z 1  is lower than Zload (50Ω) to provide an optimized load at the lower supply condition. This Z 1  carrier load tuning provides an extra degree of freedom to tune the power and efficiency load line on the I-V curve of the power transistor of the carrier amplifier  92  for a desired performance. Moreover, the same technique may be applied to a power amplifier system where drain voltage and bias current are both throttled down at lower powers to improve efficiency. The ability to adjust the load line can enable linear operation in addition to efficiency operation. This capability is very useful for improving the power efficiency of gallium nitride-based devices and power amplifiers whose high (˜5V) I-V knee voltage limits the efficiency performance when operating at lower supply and drain voltages. The extra degree of freedom to modulate the load impedance of the amplifier enables improvement in efficiency when the minimum practical drain voltage is reached in an envelope tracking system. 
       FIG. 11  is a schematic diagram of a 5 th  embodiment that in accordance with the present disclosure is a reconfigurable wideband inverted phase load modulated power amplifier  124  that is open loop tunable. This 5 th  embodiment shows an application of the 3 rd  and 4 th  embodiments, but is also applicable to the 1 st  and 2 nd  embodiments for improving the load-insensitivity of the load modulated power amplifier under high VSWR conditions such as when there is a mismatch with an antenna coupled to the output of the load modulated power amplifier. This 5 th  embodiment shows a system where the output VSWR is detected through VSWR detector  126  that includes circuitry that determines the mismatched load impedance. For example, the VSWR detection circuitry can detect real and imaginary or S-parameter magnitude and angle or other VSWR related parameters. This information is usable to determine whether the VSWR is due to a mismatched load impedance (Zload) that is &gt;50Ω (nominal design) or &lt;50Ω in its simplest practice. 
     The characteristic nature of a quadrature coupler which is illustrated in  FIGS. 13, 14, and 15 , indicate that when the output is perfectly matched to 50Ω and Z ISO  is 50Ω, the isolation between the coupler impedance ports Z 1  and Z 2 , corresponding to the isolation between the carrier amplifier  92  and the peak amplifier  94 , is excellent, as illustrated in  FIG. 13 . When the output port (Z 3 ) is mismatched with a 5Ω output load corresponding to a 10:1 VSWR mismatched condition, then the isolation degrades when Z ISO  is 50Ω or less. However, if Z ISO  is increased to a value &gt;50Ω, the isolation between ports Z 1  and Z 2  improves dramatically. When the output port (Z 3 ) is mismatched with a 500Ω output load corresponding to a 10:1 VSWR mismatched condition, then the isolation degrades when Z ISO  is 50Ω or greater. However, if Z ISO  is decreased to a value &lt;50Ω, the isolation between ports Z 1  and Z 2  improves dramatically. It is illustrated in  FIGS. 16-23  that improved OPBO efficiency may be achieved in either region where Z ISO  is &gt;50Ω or Z ISO &lt;50Ω. Therefore, a means for providing load insensitivity and high OPBO efficiency may be achieved by detecting the VSWR mismatch condition and tuning Z ISO  in the appropriate region for maximum efficiency:
         1) When VSWR detector measures Zload&gt;50Ω, Z ISO  is optimized for &lt;50 Ω   2) When VSWR detector measures Zload&lt;50Ω, Z ISO  is optimized for &gt;50 Ω
 
This may be implemented through a pre-calibrated LUT that contains optimum efficiency solutions for Z ISO  as a function of (Pin, frequency, VSWR mismatch information) as an example.
       

     An exemplary embodiment of the present disclosure was modeled and simulated in a gallium nitride (GaN) high electron mobility transistor (HEMT) technology with a transition frequency fT˜90 GHz and off-state breakdown voltage of ˜50V. The knee voltage of the I-V characteristics of the technology is ˜5V while the nominal supply voltage is no greater than 12V at an approximate current density of 500 mA/mm of gate periphery. Embodiments of this disclosure were designed using AWR Corporation&#39;s microwave office (MWO) simulation tools with a foundry process design kit (PDK). 
       FIG. 12  shows the 4-port quadrature coupler impedance modulation of the carrier amplifier  92  (Z 2 , zero degree) and peak amplifier  94  (Z 1 , 90 degree) amplifier load impedance ports as a function of the isolation termination impedance Z ISO . It should be recognized that Z ISO  at Z 4  may also be complex impedance. For simplicity Z 2  and Z 1  load impedance characteristics are shown for swept real values of Z ISO  for various quadrature couplers, 1) Surface Mount Branch-line, 2) Coupled line, and 3) Lange. The Lange coupler has the widest frequency bandwidth capability and also shows the widest range of the carrier amplifier (Z 2 ) and the peak amplifier (Z 1 ) port impedance tuning range. The impedance tuning range for Z 2  may vary from Zload/2 to greater than 2*Zload where Zload is the normalized 50Ω coupler characteristic impedance. This range is 2 times larger than a conventional symmetric 2-way Doherty impedance inverter which typically ranges from Zload up to 2*Zload at 6 dB back off power. Therefore, the embodiments of the present disclosure provide both two times greater load modulation as well as the ability to modulate the impedance below Zload, unlike the conventional Doherty impedance inverter operation. Moreover, the present disclosure demonstrates that high efficiency may be achieved by inverting the direction of the impedance modulation where the carrier amplifier load Z 2  is tuned for lower impedance than Zload at output power back off (OPBO) (Z ISO &lt;Zload) while the peak amplifier  94  is tuned for high impedance Z 1  at OPBO which is the opposite of the Doherty operation. Therefore, embodiments of this disclosure can be assigned to a more general load modulated amplifier class. 
       FIG. 13  is a graph illustrating that when the output is matched, the best carrier-peak amplifier isolation occurs when Z ISO  is equal to Zload=Zout=50Ω. At the extreme Z ISO  impedances of 0.1Ω and 1 KΩ, the isolation is still reasonably better than 7 dB across a wide band but is not ideal. However, it is still better than the isolation a quarter-wave impedance inverter would be able to achieve across this 2 octave frequency band or even a more modest 2-2.7 GHz cellular application band. Under extreme output VSWR mismatches it will be shown that Z ISO  may be chosen to improve the 2 octave broadband isolation by choosing one of two ranges of Z ISO : 1) Z ISO &gt;Zload or 2) Z ISO &lt;Zload (50Ω). 
       FIG. 14  is a graph showing that when the output has a VSWR of 10:1 due to a Zout impedance of 5Ω (Zout&lt;50Ω), a Z ISO  in the range of &gt;50Ω may be chosen for improved isolation and optimum efficiency performance. Later in the disclosure it will be shown that a Z ISO  may be chosen in either Z ISO &gt;50Ω or Z ISO &lt;50Ω ranges to achieve the enhanced OPBO efficiency, counterintuitive to Doherty amplifier operation, and therefore this disclosure is referred to more generally as a load modulation amplifier. 
       FIG. 15  is a graph illustrating that when the output has a VSWR of 10:1 due to a Zout impedance of 500Ω (Zout&gt;50Ω), a Z ISO  in the range of &lt;50Ω) may be chosen for improved isolation and optimum efficiency performance. Later in the disclosure it will be shown that a Z ISO  may be chosen in either Z ISO &gt;50Ω or Z ISO &lt;50Ω to achieve the enhanced OPBO efficiency, counterintuitive to Doherty operation and conventional wisdom. 
       FIG. 16  is a graph depicting gain and power added efficiency (PAE) versus Pout for the Lange-coupled load modulated amplifier of the 1 st  and 2 nd  embodiments where the isolation impedance Z ISO  is swept from 50Ω to 5KΩ. Also shown as a reference comparison is the gain and PAE versus Pout for a conventional 2-way Doherty Amplifier. As Z ISO  is increased the Z 2  load presented by the coupler to the carrier amplifier also increases and results in a higher increase in PAE as the output power is backed off. Note that the gain is flat, exhibiting reasonably low AM-AM distortion. The AM-PM distortion (not shown) is also well behaved and is less than a couple of degrees over the applicable output power range. Also note that these amplifiers were designed with linear operation as a focus. 
       FIG. 17  is graph illustrating gain and PAE versus Pout for the Lange-coupled load modulated amplifier of the 1 st  and 2 nd  embodiments ( FIGS. 7 and 8 ), where the isolation impedance Z ISO  is swept from 50Ω to 0Ω. Also shown as a reference comparison is the gain and PAE versus Pout for a conventional 2-way Doherty amplifier. As Z ISO  is decreased, the Z 2  load presented by the coupler to the carrier amplifier also decreases, and results in a higher increase in PAE as the output power is backed off. This mode of operation departs from the traditional Doherty operation where the carrier amplifier is presented a lower impedance (˜Zload/2) at OPBO and actually resulted in slightly better OPBO efficiency than when the carrier amplifier was presented a large impedance (˜2*Zload) under OPBO operation (Doherty type amplifier). This is believed to be due to the complex characteristics of the quadrature coupler combined with the carrier amplifier and peak amplifier bias settings where a more favorable complex load line is provided. Note that the gain is still flat, exhibiting reasonably low AM-AM distortion. The AM-PM distortion (not shown) is also well behaved and is less than a couple of degrees over the applicable output power range. Also note that these amplifiers were designed with linear operation as a focus. 
       FIG. 18  is a graph depicting gain and PAE versus Pout for the Lange-coupled inverted load modulated amplifier of the 3 rd  through 5 th  embodiments ( FIGS. 9-11 ) where the isolation impedance Z ISO  is swept from 50Ω to 5KΩ. Also shown as a reference comparison is the gain and PAE versus Pout for a conventional 2-way Doherty amplifier. As Z ISO  is increased, the Z 1  load presented by the coupler  118  to the carrier amplifier  92  decreases and results in a higher increase in PAE as the output power is backed off. This mode of operation departs from the traditional Doherty amplifier operation where the carrier amplifier is presented a lower impedance (˜Zload/2) at OPBO and actually resulted in slightly better OPBO efficiency than when the carrier amplifier  92  was presented a large impedance (˜2*Zload) under OPBO operation (Doherty type amplifier) as shown in  FIG. 19 . This is believed to be due to the complex characteristics of the quadrature coupler combined with the carrier amplifier and peak amplifier bias settings where a more optimum complex load line is provided. Note that the gain is still flat, exhibiting reasonably low AM-AM distortion. The AM-PM distortion (not shown) is also well behaved and is less than a couple of degrees over the applicable output power range. Also note that the amplifiers  114 ,  120 , and  124  were designed with linear operation as a focus. 
       FIG. 19  is a graph illustrating gain and PAE versus Pout for the Lange-coupled inverted load modulated amplifier of the 3 rd  through 5 th  embodiments ( FIGS. 9-11 ), where the isolation impedance Z ISO  is swept from 0Ω to 50Ω. Also shown as a reference comparison is the gain and PAE versus Pout for a conventional 2-way Doherty amplifier. As Z ISO  is decreased, the Z 1  load presented by the coupler  118  to the carrier amplifier  92  increases and results in a higher increase in PAE as the output power is backed off. Note that the gain is flat, exhibiting reasonably low AM-AM distortion. The AM-PM distortion (not shown) is also well behaved and is less than a couple of degrees over the applicable output power range. Also note that the amplifiers  114 ,  120 , and  124  were designed with linear operation as a focus. 
       FIG. 20  is a graph showing gain and PAE versus Pout for the quadrature coupled line load modulated amplifier of the 1 st  and 2 nd  embodiments ( FIGS. 7 and 8 ) where the isolation impedance Z ISO  is swept from 50Ω to 5KΩ. Also shown as a reference comparison is the gain and PAE versus Pout for a conventional 2-way Doherty. As Z ISO  is increased the Z 2  load presented by the coupler to the carrier amplifier also increases and results in a higher increase in PAE as the output power is backed off. Note that the gain is flat exhibiting reasonably low AM-AM distortion. The AM-PM distortion (not shown) is also well behaved and is less than a couple of degrees over the applicable output power range. Also note that the amplifiers  90  and  110  were designed with linear operation as a focus. 
       FIG. 21  is a graph illustrating gain and PAE versus Pout for the quadrature coupled line load modulated amplifier of the 1 st  and 2 nd  embodiments ( FIGS. 7 and 8 ) where the isolation impedance Z ISO  is swept from 50Ω to 0Ω. Also shown as a reference comparison is the gain and PAE versus Pout for a conventional 2-way Doherty amplifier. As Z ISO  is decreased, the Z 2  load presented by the coupler to the carrier amplifier also decreases and results in a higher increase in PAE as the output power is backed off. This mode of operation departs from the traditional Doherty operation where the carrier amplifier is presented a lower impedance (˜Zload/2) at OPBO and actually resulted in slightly better OPBO efficiency than when the carrier amplifier was presented a large impedance (˜2*Zload) under OPBO operation (Doherty type). This is believed to be due to the complex characteristics of the quadrature coupler combined with the carrier amplifier and peak amplifier bias settings where a more optimum complex load line is provided. Note that the gain is still flat exhibiting reasonably low AM-AM distortion. The AM-PM distortion (not shown) is also well behaved and is less than a couple of degrees over the applicable output power range. Also note that the amplifiers  90  and  110  were designed with linear operation as a focus. 
       FIG. 22  illustrates gain and PAE versus Pout for the quadrature coupled-line inverted load modulated amplifier of the 3 rd  through 5 th  embodiments ( FIGS. 9-11 ) where the isolation impedance Z ISO  is swept from 50Ω to 5KΩ. Also shown as a reference comparison is the gain and PAE versus Pout for a conventional 2-way Doherty amplifier. As Z ISO  is increased, the Z 1  load presented by the coupler to the carrier amplifier decreases and results in a higher increase in PAE as the output power is backed off. This mode of operation departs from the traditional Doherty amplifier operation where the carrier amplifier is presented a lower impedance (˜Zload/2) at OPBO and actually resulted in slightly better OPBO efficiency than when the carrier amplifier was presented a large impedance (˜2*Zload) under OPBO operation (Doherty type) as shown in the upcoming  FIG. 29 . This is believed to be due to the complex characteristics of the quadrature coupler combined with the carrier amplifier and peak amplifier bias settings where a more optimum complex load line is provided. Note that the gain is still flat, exhibiting reasonably low AM-AM distortion. The AM-PM distortion (not shown) is also well behaved and is less than a couple of degrees over the applicable output power range. Also note that the amplifiers  114 ,  120 , 1nd  124  were designed with linear operation as a focus. 
       FIG. 23  illustrates gain and PAE versus Pout for the quadrature coupled line inverted load modulated amplifier of the 3 rd  through 5 th  embodiments ( FIGS. 9-11 ) where the isolation impedance Z ISO  is swept from 0Ω to 50Ω. Also shown as a reference comparison is the gain and PAE versus Pout for a conventional 2-way Doherty amplifier. As Z ISO  is decreased, the Z 1  load presented by the coupler to the carrier amplifier increases and results in a higher increase in PAE as the output power is backed off. Note that the gain is flat, exhibiting reasonably low AM-PM distortion. The AM-PM distortion (not shown) is also well behaved and is less than a couple of degrees over the applicable output power range. Also note that the amplifiers  114 ,  120 , and  124  were designed with linear operation as a focus. 
       FIG. 24  is a graph illustrating Lange-coupled inverted load modulated amplifier efficiency and gain response at 1.8, 2.15, and 2.5 GHz. The graph traces are for the isolation impedance Z ISO =5K Ω when the carrier amplifier  92  sees a small load impedance ˜Zload/2. Over the frequency range of +/−15% and a back off power range of 6 dB, the inverted load modulated amplifier achieves roughly better than 39% efficiency across the band. As expected, the efficiency is poorest at the upper band frequency of 2.5 GHz. The upper band efficiency may be improved by adding inductive impedance to Z ISO  in series with a resistive load impedance Rload. 
       FIG. 25  is a graph illustrating the Doherty amplifier efficiency and gain response at 1.8, 2.15, and 2.5 GHz. Over the frequency range of +/−15% and a back off power of 6 dB, the conventional Doherty amplifier achieves no better than 38% efficiency at the lower band of 1.8 GHz and only 29% at the upper band frequency of 2.5 GHz. This is roughly a 10% degradation over the inverted load modulated power amplifiers of this disclosure over the +/−15% bandwidth. 
       FIG. 26  is a graph showing the wideband response of the inverted load modulated amplifier versus the conventional Doherty amplifier over a +/−15% bandwidth at an OPBO of ˜6 dB for each amplifier respectively. In particular,  FIG. 26  illustrates the improved 6 dB OPBO PAE of the inverted load modulated amplifier embodiment which maintains &gt;39% PAE over a wide band with 1 dB more 6-dB OPBO Pout while the conventional Doherty amplifier has as much as 14% and on average &gt;10% poorer 6-dB OPBO PAE over the frequency range. 
       FIG. 27  is a schematic diagram of a quadrature combined load modulated power amplifier  128 . The terminating impedance of the quadrature coupler  100  coupled to the output load terminal  102  may be dynamically changed (continuous or discrete steps) to present a desired load impedance to either the carrier amplifier  92  or the peak amplifier  94  for maximizing power, efficiency or linearity at a given power level, frequency, or temperature. The input termination port impedance may also be tuned simultaneously to obtain optimum RF performance. Note that the phase relationship of the carrier amplifier  92  in relation to the output is 90 degrees offset, which is similar to the Doherty amplifier. What is different is that in this particular embodiment, the carrier amplifier  92  will see a load impedance which is lower than Zload at some OPBO level below the overall amplifier saturation point, and this is achieved by tuning Z ISO &lt;Zload. This is contrary to the operating principles of the Doherty amplifier whose carrier load impedance increases from Rload to 2*Rload at OPBO. Moreover, the carrier amplifier  92  may see an impedance that is Zload/2 at OPBO below the saturated power level of the carrier amplifier  92 . It should be noted that the Doherty amplifier cannot provide a carrier load impedance below Rload and therefore is restricted in load modulation range. The result of low carrier load impedance at OPBO is illustrated in upcoming  FIG. 29 . 
       FIG. 28  is a schematic diagram of an inverted phase quadrature combined load modulated power amplifier  130 . This is different from the previous embodiment of  FIG. 27 , in that the output of the carrier amplifier  92  is coupled in-phase at a combined amplifier output. The terminating impedance of the output coupler may be dynamically changed (continuous or discrete steps) to present a desired load impedance to either the carrier amplifier  92  or the peak amplifier  94  for optimizing power, efficiency, or linearity at a given power level, frequency, or temperature. The input termination port impedance may also be tuned simultaneously to obtain optimum RF performance. Note that the phase relationship of the carrier amplifier  92  in relation the output is now 0 degrees offset, unlike the Doherty amplifier. Like the previous embodiment, the carrier amplifier  92  will see a load impedance which is lower than Zload at some OPBO level below the overall amplifier&#39;s saturation point, and this is achieved by tuning Z ISO &gt;Zload. This is contrary to the operating principles of the Doherty amplifier whose carrier load impedance increases from Rload to 2*Rload at OPBO. Moreover, the carrier amplifier  92  may see an impedance that is Zload/2 at OPBO below the saturated power level of the carrier amplifier  92 . It should be noted that the Doherty amplifier cannot provide a carrier load impedance below Rload and therefore is restricted in load modulation range. The result of low carrier load impedance at OPBO is illustrated in upcoming  FIG. 30 . 
       FIG. 29  is a graph of measured gain and collector efficiency vs. Pout for the quadrature phase load modulated power amplifier  128  of  FIG. 27 . The carrier amplifier  92  is biased in class A and the peak amplifier is biased in class C. These characteristics were measured for Z ISO =50Ω and Z ISO =0Ω to illustrate the range of load modulation between these Z ISO  impedances, but should be noted that a continuous or discrete range of impedances may be employed. These impedances correspond to a carrier amplifier load impedance of 50Ω and 25Ω respectively. For the lower carrier impedance of 25Ω, a dramatic improvement in OPBO efficiency is obtained compared to the 50Ω carrier impedance setting. This operating principle is contrary to the Doherty amplifier, which modulates the carrier load to higher impedances at ˜6 dB OPBO to increase the efficiency. The fundamental difference is that the Doherty amplifier is setting the OPBO load for maximum saturated class A operation, whereas the present disclosure places the operation of the carrier amplifier  92  in the linear region. 
       FIG. 30  is a graph of measured gain and collector efficiency versus Pout for the inverted quadrature phase load modulated power amplifier  130  of  FIG. 28 . The carrier amplifier  92  is biased in class A and the peak amplifier  94  is biased in class C. These characteristics were measured for Z ISO =50Ω and Z ISO =5KΩ but should be noted that a continuous or discrete range of impedances may be employed. These impedances correspond to a carrier amplifier load impedance of 50Ω and 25Ω respectively. For the lower carrier impedance of 25Ω, a dramatic improvement in OPBO efficiency is obtained compared to the 50Ω carrier impedance setting. This operating principle is contrary to the Doherty amplifier, which modulates the carrier load to higher impedances at ˜6 dB OPBO to increase the efficiency. The fundamental difference is that the Doherty amplifier is setting the OPBO load for maximum saturated class A operation, whereas the present disclosure places the amplifier in the linear region. 
     As a reference comparison,  FIG. 31  is a graph that shows the measured conventional 50Ω quadrature operation of the quadrature combined load modulated power amplifier  128  where both the carrier amplifier  92  and the peak amplifier  94  are operated at the same bias and Z ISO =50Ω. The corresponding efficiency and gain curves for class A, B, and C are illustrated and show the expected characteristic efficiency ‘drop-off’ response as output power is backed off. The plots show the characteristic gain expansion of the class C case and its associated higher efficiency than the class B or C cases as power ramps up. At ˜5-6 dB OPBO, or an output power of ˜36 dBm, the efficiency is at best 30% for the class C case and 25% for the class A case, as compared to 38 and 40% for the quadrature phase and inverted phase load modulated amplifiers at the lower load impedance. The lower impedance capability of this new load modulation amplifier has far reaching implications to high linear efficiency optimization at lower voltages. It may be combined with envelope tracking or drain mode biasing such as class G in order to optimize power efficiency further. And this would come with other inherent benefits such as wider bandwidth and real time reconfigurability. 
       FIG. 32  is a graph illustrating current-voltage I-V load line operation of the carrier amplifier  92  when the amplifier is backed off of its saturation. In particular, the IV curve graph includes load-lines depicting conventional class A (Rload) load line operation, low impedance (RLoad/2) load modulation under lower power backed off signal, and low impedance (Rload/2) load modulation under low power backed off signal and lower supply voltage Vq_m, illustrating more efficient load line impedance at lower supply voltage with higher current swing (than 2*Rload). The voltage and current excursion for a conventional load modulation amplifier such as a Doherty amplifier is constricted by the 2*Rload impedance and is limiting in power efficiency for smaller than saturated power operation. In order to improve the linear efficiency of the carrier amplifier  92  at backed off powers below saturation for the carrier amplifier, the load impedance may be reduced as we are suggesting in the present disclosure. In the embodiments described in this disclosure, higher linear power can be obtained using a load impedance of Rload/2 resulting in a root mean squared current Irms having a larger excursion for the same DC power. This results in improved small signal power efficiency as compared to the larger load. In addition, drain voltage could be reduced via APT or ET (as examples) in conjunction with lowering the load impedance in order to obtain even higher small signal power efficiency illustrated in thick dashed line. One approach is to reduce the Rp load towards ˜Rload/2 to improve the RF Ids swing for a given RF Vout swing. This will increase the Pav=Vrms*Irms by a large factor due to the larger AC current swing (assuming that gain is large). The factor of four reduction in Rload will result in a factor of four increase in RF Ids for the given RF Vout swing. This will require a larger RF Vin swing, however, if there is sufficient amplifier gain, then this should be negligible and linear efficiency will be improved dramatically. Since the RF Vout voltage is not clipped by the knee, further improvement in drain efficiency may be achieved by reducing the quiescent Vdrain or voltage applied to the amplifier (via average power tracking (APT) or ET for example). The potential improvement is conceptually illustrated for increasing the efficiency of a related art Doherty amplifier designed for envelope tracking (ET). 
       FIG. 33  is a graph that illustrates how load modulation may improve the efficiency of a Doherty amplifier designed for ET. It is previously known that ET can be used to improve the &gt;6 dB backed off efficiency of a conventional Doherty amplifier. At 6 dB OPBO, the peak amplifier is off and the carrier amplifier is presented with a 2*Ropt class A load and is saturated. As output power is backed off even further, the carrier amplifier becomes uncompressed and its efficiency starts to decrease. ET or APT may be applied to recover the efficiency by lowering the drain voltage of the carrier amplifier as conceptually illustrated by the efficiency envelope of  FIG. 33  shown in thick black line. To further improve the efficiency, the load of the carrier may be reduced in order to allow more Irms excursion and achieve better Pout for a given drain voltage bias point depicted as a bold dotted line. The present load modulation amplifier embodiments disclosed herein can enable this type of operation while providing reconfigurability over wide modulation bandwidths, power ranges, and carrier frequency of operation. 
       FIG. 34  is a graph of measured total gain and collector efficiency versus output power for a quadrature phase load modulated amplifier of the present disclosure illustration improved backed off efficiency through the combination of load modulation (to lower Zc impedance˜RL/2) and lower supply modulated operating voltage. Moreover,  FIG. 34  depicts preliminary data indicating the benefits that can be achieved by combining the load modulation amplifier embodiments of this disclosure with drain-voltage modulation at low powers in order to further improve the OPBO efficiency. It should be noted that this is a breadboard demonstration using available amplifier cells, and that a printed circuit board (PCB) used for bread-boarding and the breadboard tuning was not designed for maximum efficiency or power. It is also believed that the inverted phase load modulation amplifier, such as those shown in  FIGS. 9-11 , will achieve less amplitude modulation-amplitude modulation (AM-AM) distortion than will the reconfigurable load modulated power amplifier  90  of  FIG. 7 . The measured data of  FIG. 34  is very encouraging in light of this and demonstrates the fundamental linear efficiency improvement that can be obtained from combining low impedance load modulation with low drain voltage operation. 
     Those skilled in the art will recognize improvements and modifications to the 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.