fig4 shows a two - stage amplifier 400 including an exemplary arrangement of amplifier stages according to the embodiments of the present invention . in this embodiment , the pre - distortion biasing scheme used for the fets of the two stages 10 , 20 shown in fig1 is reversed . in other words , in two - stage amplifier 400 of this figure , a driver amplifier 70 is formed by a class a or class ab amplifier and is followed by a power or final stage amplifier 80 formed by a class b or class c amplifier . this arrangement presents a post - distortion approach to the linearity problem . class c amplifiers , despite their high efficiency , are rarely used in the field because their output suffers from a high degree of distortion and needs filtering . at the same time , gain expansion or gain enhancement , such as that shown in fig2 , mostly occurs in the high efficiency class c mode . combining gain expansion with class c amplifier operation offers a novel approach for compensating for the effects of gain compression , typical in power amplifiers , while including the higher efficiency attributes of the class c amplifier stage . class c bias for the output stage 80 yields a higher possible efficiency , ( 78 %) than an output stage biased with class ab or class b . for the driver stage 70 , a class a or nearly class a bias is used to provide the necessary phase and amplitude distortion ( am - am , am - pm ) to compensate for the distortion in the output stage 80 . by reversing the order of biasing , a much smaller device can be used as the driver 70 thus improving overall efficiency . one of the features of the present invention is that it broadens the sweet spot 60 region shown in fig3 over a greater range of input power levels p in . another feature positions this sweet spot 60 for minimum im3 at different input or output power levels . with these two features , the range of input power levels or amplifier dynamic range is increased while simultaneously the im3 50 level is kept low and the efficiency is kept high . varying the drain to source bias voltage v ds levels of the driver amplifier and the final amplifier , in a two - stage amplifier , helps achieve these simultaneous benefits . in other words , at low input power p in levels , v ds1 and v ds2 are set low and at high input power levels p in , v ds1 and v ds2 are set high . fig5 , 6 , 7 , and 8 show simulated amplitude and phase responses of the driver 70 and output 80 stages from their respective simulated large signal s - parameters . these four figures all include the input power p in on the horizontal axis . fig5 and 7 show the s - parameter s ( 2 , 1 ) on the vertical axis and fig6 and 8 show the s - parameter s ( 4 , 3 ) on the vertical axis . amplitudes or magnitudes of the s - parameters appearing in fig5 and 6 are expressed in units of db . the phases of the s - parameters shown in fig7 and 8 are in units of degrees . the numerical values of the four figures are derived using four parallel fets each having a 40 μm channel length in the first stage 70 and six parallel fets each having a 100 μm channel length in the second stage 80 . fig5 and 7 show the compression in the amplitude 90 and the positive increase in the phase 120 of the first stage 70 that corresponds to the characteristics of a class a biased device . the biasing parameters used to obtain the curves of these figures include a v ds1 of 10 volts and a v gs1 of − 0 . 4 volts . fig6 and 8 show the amplitude 110 expansion and negative phase response of the phase 130 of the second stage 80 that are typical for a class c biased device . the biasing parameters used to obtain the curves of these figures include a v ds2 of 20 volts and a v gs2 of − 3 . 6 volts . as is apparent in fig5 and 6 , the amplitudes 90 , 110 of the first stage 70 and the second stage 80 amplifiers compensate each other . for example , at p in value of 20 dbm , the amplitude 90 of the first stage 70 , shown in fig5 , is at a low point while the amplitude 110 of the second stage 80 , shown in fig6 , is at a high point . referring to fig7 and 8 , the phases 120 , 130 of the first and second stages 70 , 80 similarly compensate each other . fig9 a and 9 b show the simulated improved im3 140 , 170 and drain efficiency 150 , 180 of the amplifier of fig4 given a first and a second set of gate to source voltages . these figures again pertain to the post - distortion scheme for the two - stage power amplifier 400 using gan double heterostructure field effect transistors ( dhfets ). a low im3 level 140 , 170 and high drain efficiency 150 , 180 are achieved over a wider range of input power levels 160 , 190 than what is available in the prior art . in both simulations , whose results appear in fig9 a and 9 b , the center frequency was f = 10 ghz and the frequency range was δf = 2 mhz . in the first simulation , whose results appear in fig9 a , the gate to source voltage of the first stage 70 transistor is v sg1 =− 0 . 48 volts and that of the second stage 80 transistor is v gs2 =− 3 . 6 volts . in the second simulation , whose results appear in fig9 b , the gate to source voltage of the first stage 70 transistor is v sg1 =− 0 . 49 volts and that of the second stage 80 transistor is v gs2 =− 3 . 45 volts . v ds1 and v ds2 are the drain to source voltages of the transistors used in the first 70 and second 80 stages that are output by each stage of the two - stage amplifier 400 . in the first simulation v ds1 = 10 volts and v ds2 = 30 volts . in the second simulation v ds1 = 25 volts and v ds2 = 10 volts . in fig9 a , the upper and lower horizontal axes show the input power p in in units of dbm and the left and right vertical axes show the im3 140 in dbc and the drain efficiency 150 as a percentage . the results of the first simulation indicate an improved im3 of less than − 32 dbc with drain efficiency of greater than 38 % with an input power level p in between 17 and 18 dbm in the sweet spot 160 valley of this im3 curve . in fig9 b , similarly , the upper and lower horizontal axes show the input power p in and the left and right vertical axes show the im3 170 and the drain efficiency 180 . the results of the second simulation indicate an improved im3 of less than − 32 dbc with drain efficiency of greater than 35 % with an input power level p in of 15 . 5 dbm in the sweet spot 190 valley of this im3 curve 170 . fig1 shows a block diagram of a system incorporating an embodiment of the invention . fig1 shows transfer characteristics of a generic gan schottky diode used in the system of fig1 . fig1 shows a conventional buck - boost converter used in the system of fig1 . an embodiment of the invention that includes an on - chip system 1000 using dynamic biasing for rf power amplification is shown in fig1 . this system 1000 includes feedback provided to the amplifier 400 in order to correct for nonlinearity of the amplifier 400 through back - end processing . the rf output power from the two - stage power amplifier 400 is sampled on - chip using a directional coupler 220 . the output of the directional coupler 220 is the output of the overall dynamically biased feedback system 1000 . the directional coupler 220 can be a simple quarter wave ( λ / 4 ) coupled line . two analog - to - digital ( a / d ) converters 241 , 242 are used to sample and quantize the forward and reflected traveling waves using two schottky diodes 231 , 232 . the combination of the schottky diodes 231 , 232 and the a / d converters 241 , 242 behave as a digitizer for the forward and reflected traveling waves that are sampled by the directional coupler 220 . this system allows the power in the output envelope of the amplifier 400 to be tracked . these diodes 231 , 232 are easily realized using gan transistors . the outputs of the a / d converters 241 , 242 are fed into a processor 250 . the processor 250 monitors the output of the amplifier 400 received through the coupler 220 and evaluates the harmonics of the output . the processor 250 then provides a signal that changes the drain voltages of the transistors used in the amplifier 400 in order to minimize the harmonics in the amplifier 400 output . in other words , the processor 250 provides a negative feedback loop in order to minimize the harmonics output by the amplifier 400 . the output of the processor 250 is provided to a modulated dc power supply 260 which provides the drain to source voltages v ds1 , v ds 2 of the transistors used in the amplifier 400 . a dc bias is provided to both the processor 250 and the modulated dc power supply 260 . this system 1000 corrects the non - linearity of the amplifier 400 output by back - end processing rather than dynamic gate biasing . in one exemplary embodiment of the invention , the schottky diodes 231 , 232 of the system 1000 may be implemented with the transfer characteristics shown in fig1 . fig1 shows the measured conversion characteristics 1100 for a gan schottky diode with a channel length of 150 μm operating at 15 ghz . the horizontal axis shows the input power p in in dbm and the vertical axis shows the output voltage of the diode in volts . as the figure shows , for an input power range of 4 to 20 dbm the resulting output voltage varies from 0 to − 4 volts . these voltage levels can be sampled on - chip 1000 using the a / d converters 241 , 242 . in one embodiment , 3 - bit a / d converters ( not shown ) have been realized in gaas using 56 metal semiconductor field effect transistors ( mesfets ). this scheme provides the ability to track the input / output power levels and vary the v ds to obtain low im3 and high efficiency . in various embodiments , the sampled forward and reflected waves at the output of the power amplifier 400 can go to an on - chip processor 250 or an off - chip processor ( not shown ). the processor then determines the necessary bias voltage for the power amplifier 400 to control the appropriate output power , efficiency , and im3 level . in one embodiment , the power supply 260 being modulated by the processor 250 can be a buck - boost converter 1200 shown in fig1 to vary the level of v ds1 and v ds2 . at low input power levels , v ds1 and v ds2 are set low and at high input power levels , v ds1 and v ds2 are set high . this allows the low im3 sweet spot 160 , 190 to vary for a wide range of input or output power levels . the buck - boost converter 1200 of fig1 includes a dc bias source or a battery 310 , an inductor 320 , a capacitor 330 , a resistor or a load 340 , a transistor 350 , and a diode 360 coupled together . the capacitor 330 and the load 340 are coupled in parallel to form an rc circuit . the inductor 320 and the battery 310 are each coupled to one node of the rc circuit . the other node of the inductor 320 is coupled to the other node of the rc circuit through the diode 360 that allows the current to flow only from the rc circuit to the inductor 320 and not back . the node of the inductor 320 coupled to the diode 360 is also coupled to the transistor 350 . the transistor 350 is coupled between the battery 310 and the inductor 320 . in the buck - boost converter 1200 of fig1 , when the transistor 350 is on , energy is transferred to the inductor 320 . when the transistor 350 turns off , inductor current is forced through the diode 360 . energy stored in the inductor 320 is transferred to the capacitor 330 and the load 340 . the switching frequency is determined using an on chip oscillator . also , the main energy storage elements , the inductor 320 and the capacitor 340 , can be located off - chip . although the present invention has been described with reference to certain exemplary embodiments , it is understood that a variety of modifications and variations may be made to the present invention without departing from the spirit or scope of the invention defined in the appended claims , and their equivalents .