fig1 illustrates communications system 100 embodying the principles of the invention . in this illustrative embodiment , system 100 is a portable cellular telephone for wireless communications over designated frequency bands pursuant to the advanced mobile phone system ( amps ) cellular standard . fig2 illustrates the designated cellular bands comprising transmit ( xmt ) band 201 and receive ( rcv ) band 203 in the frequency spectrum . specifically , band 201 ranging from 824 mhz to 849 mhz is assigned for transmission by system 100 of cellular communication signals to a remote base station ( not shown ), where the signals are processed and directed to an intended receiver in a conventional manner . to realize duplex wireless communications , band 203 ranging from 869 mhz to 894 mhz is assigned for reception by system 100 of cellular communication signals from the remote base station . the received communications signals came from an originating transmitter ( not shown ) and have been processed by the remote base station in a conventional manner . referring back to fig1 system 100 comprises , inter alia , transmitter 101 in accordance with the invention and receiver 151 of conventional design . a digital signal representing information to be transmitted is fed to modulating circuitry 103 in transmitter 101 . in response to the digital signal , circuitry 103 generates a digitally modulated signal in accordance with a standard code division multiple access ( cdma ) scheme . the modulated signal is forwarded to intermediate frequency ( if ) filter 105 which is a conventional band - pass filter . illustratively , the center frequency of filter 105 is 85 mhz and it has a passband of ± 600 khz from the center frequency . filter 105 is used to eliminate from the modulated signal unwanted signal components outside the passband . the filtered signal is then provided to voltage variable amplifier 108 of conventional design . it should be pointed out at this juncture that in accordance with a standard wireless communications service , system 100 uses control circuit 112 to listen to pilot signals sent from the remote base station during an idle state . each pilot signal includes paging information . by processing such information using control circuit 112 , system 100 knows , for example , when it is about to receive a message , and how much rf power is required for transmitting a message . the amount of required transmission power varies directly with the current distance between system 100 and the remote base station . control circuit 112 sends a power level control signal indicative of the required transmission power amount to voltage variable amplifier 108 . in response to the control signal , amplifier 108 imparts a selected gain to the filtered signal to drive power amplifier 130 to deliver the required transmission power . in other words , amplifier 108 controls the drive level of the ultimate input signal to power amplifier 130 , under the influence of the control signal . the structure of power amplifier 130 is fully described hereinbelow . in accordance with the invention , power amplifier 130 is dynamically biased to vary its dc operating point to provide linear amplification over a broad transmission power range . at the same time , it attains a high power efficiency especially when power amplifier 130 operates at low output power levels ( i . e ., during backoffs from the maximum required output ). advantageously , with the dynamic biasing , the rf signal to be transmitted by system 100 is minimally distorted . in addition , system 100 is powered by a battery having a limited capacity , and thus affords a long transmission time due to the high power efficiency . to provide the dynamic biasing to power amplifier 130 in accordance with the invention , the power level control signal indicative of the required transmission power amount is also fed to dc level scaler 114 . the latter is pre - calibrated to provide a dc voltage input to power amplifier 130 in response to the control signal . the dc voltage level varies with the required transmission power amount to dynamically bias amplifier 130 . accordingly , a dc operating point for amplifier 130 is defined for each specified transmission power amount . the dc voltage level provided by scaler 114 ranges between a first dc level and a second dc level . the first dc level corresponds to the maximum required transmission power output by amplifier 130 . when the dc voltage input to amplifier 130 is at the first dc level , amplifier 130 is biased as a class - a amplifier . with the class a biasing , amplifier 130 would not cut off any part of the anticipated large input signal excursions ( corresponding to a high transmission power output ) to either side of the dc operating point or become saturated , thus avoiding distortions in the amplifier output . the power efficiency afforded by amplifier 130 is high in this case as the amplifier output power is extremely high despite high dc input power . the second dc level corresponds to the minimum required transmission power output by amplifier 130 . when the dc voltage input to amplifier 130 is at the second dc level , amplifier 130 is biased as a class b amplifier . with the class b biasing , although the dc operating point is placed close to the cutoff region of the output characteristics of amplifier 130 , because of the small input signal excursions to either side of the operating point , amplifier 130 minimally cuts off part of the signal . accordingly , the distortion in the amplifier output is minimal . however , the power efficiency remains high in this case because of the requirement of a low dc input power attributable to the second dc level input . thus , by varying the dc level in scaler 114 in response to the power level control signal , in delivering relatively high transmission power , power amplifier 130 is biased as a class a amplifier in anticipation of a relatively high drive level input signal . in delivering medium to relatively low transmission power , power amplifier 130 is biased as a class a - b or even class b amplifier in anticipation of a relatively low drive level input signal . advantageously , the inventive dynamic biasing approach affords both high output linearity and power efficiency throughout the continuum from low to high transmission power delivery . the amplified signal from voltage variable amplifier 108 is provided to if filter 116 similar to filter 105 . filter 116 is used to eliminate from the amplified signal any distributed noise outside the passband of 84 . 4 mhz to 85 . 6 mhz . the filtered signal is then fed to mixer 118 where it is frequency - translated into cellular xmt band 201 . the frequency - translated signal is forwarded to image rejection filter 120 . filter 120 is a band - pass filter having a passband of 824 mhz to 849 mhz . it eliminates from the signal unwanted signal components outside cellular xmt band 201 . the filtered signal is fed to amplifier 122 which is a low power class a amplifier . amplifier 122 imparts a predetermined gain to the filtered signal to adjust the drive level of the input signal to power amplifier 130 . before the amplified signal is introduced to power amplifier 130 , the signal is filtered by receive noise rejection filter 126 similar to filter 120 . filter 126 eliminates from the amplified signal any distributed noise outside cellular xmt band 201 . the filtered signal is then provided to power amplifier 130 . in accordance with the inventive dynamic biasing approach described above , power amplifier 130 delivers a cellular transmit signal having the required transmission power through band duplexer 132 . the latter is a standard common junction separating cellular transmit signals from cellular receive signals . duplexer 132 passes the transmit signal from amplifier 130 to antenna 134 , and thence onto cellular xmt band 201 . in duplex wireless communications , band duplexer 132 receives a cellular receive signal from cellular rcv band 203 through antenna 134 . the received signal is then passed to receiver 151 . in a well - known manner , receiver 151 recovers the information represented by the received signal . the structure of power amplifier 130 will now be described . fig3 illustrates amplifier structure 300 which may be assumed by amplifier 130 . central to structure 300 is field effect transistor ( fet ) 301 . the source terminal of transistor 301 is connected to ground . its gate terminal is connected to input matching network 310 coupled to gate bias circuit 330 . the drain terminal of the transistor is connected to drain bias circuit 340 and also to output matching network 350 . capacitor 313 in network 310 at one end is connected to ground , and at the other end to an input to structure 300 and transmission line 315 . transmission line 315 connects capacitor 313 to capacitor 317 . capacitor 317 at one end is connected to ground , and at the other end to transmission line 319 and circuit 330 , in addition to transmission line 315 . transmission line 319 connects both capacitor 317 and circuit 330 to the gate terminal of transistor 301 . gate bias circuit 330 comprises capacitor 333 and inductive choke 335 . capacitor 333 is connected at one end to ground , and at the other end to inductive choke 335 and a dc voltage provided by scaler 114 for dynamically biasing amplifier 130 in accordance with the invention . inductive choke 335 is also connected to capacitor 317 and transmission line 319 in network 310 . similar to gate bias circuit 330 , drain bias circuit 340 comprises capacitor 343 and inductive choke 345 . capacitor 343 at one end is connected to ground , and at the other end to inductive choke 345 and a power source ( e . g . a battery ) providing a predetermined voltage + v . inductive choke 345 is also connected to the drain terminal of transistor 301 and output matching network 350 . transmission line 351 in network 350 connects the drain terminal of transistor 301 to capacitor 353 . capacitor 353 at one end is grounded and at the other end connected to transmission line 357 , in addition to transmission line 351 . transmission line 357 is connected , at an output of amplifier structure 300 , to capacitor 359 which is grounded at its other end . power amplifier 130 pursuant to structure 300 will now be defined . in power amplifier 130 , output matching network 350 is synthesized such that the load impedance ( z l ) of band duplexer 132 of fig1 presented to transistor 301 over xmt band 201 is transformed according to the following expression : ## equ1 ## where v dd represents the drain supply voltage , and i qa represents the quiescent drain current of class - a operation . in a well - known manner , the values of the individual components within network 350 can be readily determined to realize such a transformation . bias circuit 340 in amplifier 130 is synthesized such that the impedance presented to transistor 301 over band 201 is much greater than the load impedance z l . input matching network 310 in amplifier 130 is synthesized such that the source impedance ( z s ) presented by receive noise rejection filter 126 over band 201 is transformed according to the following expression : where z * input represents the complex conjugate of the input impedance of transistor 301 when presented with output matching network 350 . again , in a well - known manner , the values of the individual components within network 310 can be readily determined to realize such a transformation . bias circuit 330 in amplifier 130 is synthesized such that the impedance presented to transistor 301 over band 201 is much greater than the source impedance z s . it should be pointed out that the gain afforded by amplifier structure 300 can be increased by repeating the structure in concatenation . thus , power amplifier 130 may assume general amplifier structure 400 in fig4 having n stages in cascade each comprising structure 300 , where n & gt ; 1 . it can be shown that the gain afforded by structure 400 is n times that afforded by structure 300 . fig5 is a graph for comparing the power efficiencies of amplifier 130 when biased dynamically in accordance with the invention , strictly as a class - a amplifier , strictly as a class a - b amplifier , and strictly as a class - b amplifier . as shown in fig5 line 501 is a plot of output rf power by amplifier 130 when biased dynamically in accordance with the invention against the input dc power . line 502 is a plot of output rf power by amplifier 130 when biased strictly as a class b amplifier against the input dc power . line 503 is a plot of output rf power by amplifier 130 when biased strictly as a class a - b amplifier against the input dc power . line 504 is a plot of output rf power by amplifier 130 when biased strictly as a class a amplifier against the input dc power . in this illustrative embodiment , the maximum required rf power by amplifier 130 is 30 dbm . however , in practice , amplifier 130 usually operates at a power backoff of 10 db or more ( i . e ., at 20 dbm or lower ). by comparing the input dc power incurred by the various biasing techniques in the 15 - 20 dbm output rf power region , one realizes that an approximately 2 to 1 power consumption reduction is desirably achieved by the inventive dynamic biasing , with respect to the class a - b biasing ; and an approximately 3 to 1 power consumption reduction , with respect to the class a biasing . the foregoing merely illustrates the principles of the invention . it will thus be appreciated that a person skilled in the art will be able to devise numerous communications systems which , although not explicitly shown or described herein , embody the principles of the invention and are thus within its spirit and scope . for example , the disclosed system is a digital communications system , more specifically , a cellular telephone in accordance with the cdma digital standard . however , the inventive dynamic biasing approach is also applicable to other digital standards such as north america digital cellular ( nadc ) standard , personal handy - phone system ( phs ) standard , personal handy phone ( php ) standard , etc . it is equally applicable to an analog communications system and , in fact , any system requiring an output power control . in addition , in the disclosed embodiment , communications system 100 is operative to transmit and receive signals over the cellular communication bands . these communications bands are illustrative and are not intended to be limiting in terms of their number or frequencies . use of two or more communications bands including , for example , the personal communications system ( pcs ) frequency bands and industrial , scientific , medical ( ism ) frequency bands , in accord with the invention suggests itself to those skilled in the art . finally , the amplifying element in the disclosed power amplifier illustratively comprises one or more fets . it will be appreciated that a person skilled in the art may have the amplifying element comprise one or more bipolar transistors , integrated circuit ( ic ) amplifiers and such .