Abstract:
A radio frequency power amplifier for a battery powered handset unit of a wireless communications system having a low power signal amplification path and a high power signal amplification path. Logic and biasing means within the handset select between the low power signal path and the high power signal depending upon the handset being within or outside a prescribed distance from a base station. In this way, the signals received at the base station from the handset are at the required power level.

Description:
TECHNICAL FIELD 
     The present invention relates, in general, radio frequency power amplifiers and, in particular, to a radio frequency power amplifier having an improved efficiency and adapted for use in a battery powered handset unit of a wireless communications system. 
     BACKGROUND OF THE INVENTION 
     The efficiency of the radio frequency power amplifier used in a battery powered handset unit of a wireless communications system is a major consideration because this power amplifier is the largest power load on the battery. The higher the efficiency of the power amplifier, the longer time use of the battery powered handset without a recharge. 
     For a J-CDMA power amplifier, the unit should meet the accepted performance standards for power amplifier efficiency, gain and adjacent channel power ratio (ACPR) at full power (i.e., +27 dBm), while providing exceptionally good efficiency at lower power (i.e., +13 dBm). 
     Typical prior art implementations of 27 dBm to 28 dBm power amplifiers provide gains on the order of 28 dB, power amplifier efficiency on the order of 32% for GaAs MOSFET (Anadigics). It is believed that exotic and costly technologies like GaAs PHMET could achieve 45% to 50% power amplifier efficiency in such prior art configuration at full power. 
     SUMMARY OF THE INVENTION 
     It is an objective of the present invention to provide a new and improved radio frequency power amplifier for a battery powered handset unit of a wireless communications system. 
     It is another objective of the present invention to provide a radio frequency power amplifier for a battery powered handset unit of a wireless communications system that overcomes deficiencies of prior art radio frequency power amplifiers of this type. 
     To achieve these and other objectives, a radio frequency power amplifier for a battery powered handset unit of a wireless communications system, constructed in accordance with the present invention, includes means for supplying a radio frequency signal, a low power signal path having: a first radio frequency amplifier for amplifying the radio frequency signal and a first control switch for controlling passage of the radio frequency signal through the low power signal path to a first output; and a high power signal path having a second radio frequency amplifier for amplifying the radio frequency signal and a second control switch for controlling passage of the radio frequency signal through the high power signal path to a second output. This radio frequency power amplifier also includes bias and logic means for: (a) controlling the first control switch to permit passage of the radio frequency signal through the low power signal path when the handset is located within a prescribed distance from a base station and prevent passage of the radio frequency signal through the low power signal path when said handset is located outside the prescribed distance from the base station, (b) biasing the first radio frequency amplifier to amplify the radio frequency signal when the handset is located within the prescribed distance from the base station and prevent amplification of the radio frequency signal by the first radio frequency amplifier when the handset is located outside the prescribed distance from the base station, (c) controlling the second control switch to permit passage of the radio frequency signal through the high power signal path when the handset is located outside the prescribed distance from the base station and prevent passage of the radio frequency signal through the high power signal path when the handset is located within the prescribed distance from the base station, and (d) biasing the second radio frequency amplifier to amplify the radio frequency signal when the handset is located outside the prescribed distance from the base station and prevent amplification of the radio frequency signal by the second radio frequency amplifier when the handset is located within the prescribed distance from the base station. This radio frequency power amplifier further includes means for coupling together the first output and the second output. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is best understood from the following detailed description when read in conjunction with the accompanying drawings. 
     FIG. 1 is a block diagram of a radio frequency power amplifier for a battery powered handset unit of a wireless communications system constructed in accordance with the present invention. 
     FIG. 2 is a schematic diagram of the amplifier and switch portions of a radio frequency power amplifier for a battery powered handset unit of a wireless communications system constructed in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, a radio frequency power amplifier for a battery powered handset unit of a wireless communications system, constructed in accordance with the present invention, includes means for supplying a radio frequency signal. Such means include a terminal  10  to which a radio frequency signal is conducted from an audio source and modulator that are not shown in FIG.  1 . 
     A radio frequency amplifier, constructed in accordance with the present invention, further includes a low power signal path  12  having a first radio frequency power amplifier  14  of conventional design for amplifying the radio frequency signal and a first control switch  16  of conventional design for controlling passage of the radio frequency signal through the low power signal path to a first output; namely the output of first radio frequency power amplifier  14 . For the embodiment of the present invention shown in FIG. 1, control switch  16  is a series switch. 
     A radio frequency amplifier, constructed in accordance with the present invention, further includes a high power signal path  18  having a second radio frequency power amplifier  20  of conventional design for amplifying the radio frequency signal and a second control switch  22  of conventional design for controlling passage of the radio frequency signal through the high power signal path to a second output; namely the output of second radio frequency power amplifier  20 . For the embodiment of the present invention shown in FIG. 1, control switch  22  is a shunt switch. 
     For the embodiment of the invention illustrated in FIG. 1, high power signal path  18  also includes a drive radio frequency amplifier  24  of conventional design upstream of second radio frequency power amplifier  20 . Drive radio frequency amplifier  24  is included in high power signal path  18  to support the higher gain required by providing an amplified radio frequency signal to second radio frequency amplifier  20 . Second control switch  22  in high power signal path  18  is disposed between second radio frequency power amplifier  20  and drive radio frequency amplifier  24 . Thus, high power signal path  18  has two stages of amplification. In contrast, low power signal path  12  has only one stage of amplification. 
     A radio frequency amplifier, constructed in accordance with the present invention, also includes bias and logic means  26  for controlling first control switch  16 , biasing first radio frequency power amplifier  14 , controlling second control switch  22 , and biasing second radio frequency power amplifier  20 . Specifically, bias and logic means  26  are arranged, by employing conventional design techniques, to: 
     (1) control first control switch  16  to either permit passage of the radio frequency signal through low power signal path  12  or prevent passage of the radio frequency signal through low power signal path  12 , 
     (2) bias first radio frequency power amplifier  14  to either amplify the radio frequency signal or prevent first radio frequency power amplifier  14  from amplifying the radio frequency signal, 
     (3) control second control switch  22  to either permit passage of the radio frequency signal through high power signal path  18  or prevent passage of the radio frequency signal through high power signal path  18 , and 
     (4) bias second radio frequency power amplifier  20  to either amplify the radio frequency signal or prevent second radio frequency power amplifier  20  from amplifying the radio frequency signal. 
     CDMA operation calls for the same power level of signals received at the base station from handsets within the cell. In accordance with the present invention, bias and logic means  26  control selecting between low power signal path  12  and high power signal path  18  to assure that the signals received at the base station from the handset are at the required power level. This selection is accomplished by the handset sensing by the reception of a signal from the base station whether the base station is receiving signals from the handset at the required power level and by bias and logic means  26  switching, in response to a control signal applied at a terminal  28 , between low power signal path  12  and high power signal path  16 , as needed, to transmit signals from the hand set to the base station at the required power level. 
     To run low power signal path  12 , first control switch  16 , a series switch in the low power signal path, is turned on by bias and logic means  26  to permit the radio frequency signal to pass to first radio frequency power amplifier  14 . At the same time, bias and logic means  26  applies a bias to first radio frequency power amplifier  14  to permit the first radio frequency power amplifier to amplify the radio frequency signal. Meanwhile, to not run high power signal path  18  and conserve power by second radio frequency power amplifier  20  not drawing power, second control switch  22 , a shunt switch in the high power signal path  18 , is turned on by bias and logic means  26  to connect the second radio frequency power amplifier to ground and prevent the radio frequency signal from passing to the second radio frequency power amplifier. At the same time, bias and logic means  26  disconnect a bias from second radio frequency power amplifier  20  and drive radio frequency amplifier  24  to prevent the second radio frequency power amplifier from amplifying the radio frequency signal and the drive radio frequency amplifier from amplifying the radio frequency signal. 
     To run high power signal path  18 , second control switch  22 , a shunt switch in the high power signal path, is turned off by bias and logic means  26  to disconnect second radio frequency power amplifier  20  from ground and permit the radio frequency signal to pass to the second radio frequency amplifier. At the same time, bias and logic means  26  connects a bias to second radio frequency power amplifier  20  and drive radio frequency amplifier  24  to permit the second radio frequency power amplifier to amplify the radio frequency signal and the drive radio frequency amplifier to amplify the radio frequency signal. Meanwhile, to not run low power signal path  12  and conserve power by first radio frequency power amplifier  14  not drawing power, first control switch  16 , a series switch in the low power signal path, is turned off by bias and logic means  26  to prevent the radio frequency signal from passing to the first radio frequency power amplifier. At the same time, bias and logic means  26  disconnects a bias from first radio frequency power amplifier  14  to prevent the first radio frequency power amplifier from amplifying the radio frequency signal. 
     A radio frequency power amplifier, constructed in accordance with the present invention, further includes means for coupling together the first output, namely the output of first radio frequency power amplifier  14 , and the second output, namely the output of second radio frequency power amplifier  20 . The outputs of the two radio frequency power amplifiers  14  and  20  are connected together at a terminal  30  from which the signals from the radio frequency power amplifiers are conducted to an antenna that is not shown in FIG.  1 . In CDMA applications, to protect the radio frequency power amplifier against reflected power due to impedance mismatches at the antenna, a circulator or isolator, neither of which are shown in FIG. 1, are typically positioned between the radio frequency power amplifier and the antenna. 
     Modifications in the implementation of the radio frequency amplifier, constructed in accordance with the FIG. 1 block diagram can impact the tradeoffs between performance at 27 dBm and performance at 13 dBm. Referring to the schematic diagram of FIG. 2, the upper half, identified as “MODE 1,” corresponds to high power signal path  18  of the FIG. 1 radio frequency amplifier and the lower half, identified as “MODE 2,” corresponds to low power signal path  12  of the FIG. 1 radio frequency amplifier. Biasing is applied to three radio frequency amplification transistors Q 1 , Q 2 , and Q 3  (corresponding reference numerals  24 ,  20  and  14 , respectively, in FIG. 1) through three integrated inductors  32 ,  34 , and  36 , respectively. 
     For MODE 1 (high power signal path  18 ) operation, RC shunt feedback by resistor  38  and capacitor  40  is utilized to ensure stability. Series C and shunt L elements, namely capacitor  42  and inductor  34 , are used to provide interstage matching. 
     For MODE 2 (low power signal path  12 ) operation, a series NFET control switch  44 , corresponding to switch  16  of FIG. 1, couples the input radio frequency signal to a DC blocking capacitor  46 . Additional tuning means, composed of a parasitic inductor  48 , a capacitor  50  and inductor  36 , serve to establish impedance matching in the low power signal path. 
     If lower power performance is favored, as shown shunt capacitors  52  and  54  and a series inductor  56  are added between the low power signal path and the high power signal path. This ensures a proper impedance transformation from the low power signal path reference plane to the reference plane of the high power signal path that it is matched to on the board while the high power signal path is in operation. However, higher performance in the high power signal path can be achieved if this interstage match is eliminated and the OFF power signal path is directly coupled. 
     Putting the lower power signal path control switch  16  (reference numeral  44  in FIG. 2) on the input side of first radio frequency power amplifier  14  instead of on the output side of first radio frequency power amplifier  14  is important because the voltage tolerance, typically of 0.5 μm CMOS (3.6V), is otherwise problematic with voltage swings on the order of, for example, 8 volts or so (under maximum power supply and highest VSWR buffered by a circulator). Likewise, putting the higher power signal path control switch  22  (reference numeral  58  in FIG. 2) on the input side of second radio frequency power amplifier  20  instead of the output side of the second radio frequency power amplifier  20  avoids similar problems and provides similar benefits. 
     Furthermore, total power loss in the switch is significantly reduced when the switch is operating at a lower power level instead of at a higher power level. By separating the two signal paths, it is possible, and preferred, to use a transistor with a higher cutoff frequency (Ft) and a lower breakdown voltage in the high power signal path input stage. This allows better efficiency verses linearity for that mode without having voltage tolerance problems. However, this requires a separate 3 volt supply because 4.5 volts on a battery would damage that device. 
     Finally, it should be noted that when the second radio frequency power amplifier  20  is not in use and is shorted to ground, the linearity of the transistor in use (i.e., the transistor in the first radio frequency power signal path) is improved because a clean short is presented instead of a non-linear capacitive load. 
     Although illustrated and described herein with reference to certain exemplary embodiments, the present invention, nevertheless, is not intended to be limited to the details shown and described. Rather, various modifications may be made to those exemplary embodiments within the scope and range of equivalents of the claims without departing from the invention.