Power amplifier and power amplification method

Abstract: In a dual-mode radio telephone transmitter operative in both an analog modulation mode based on the FDMA system and a digital modulation mode based on the TDMA system, a DC bias voltage Vdd to be applied to a power amplification circuit is varied in accordance with the type of modulation, so that the input/output characteristics of the power amplification circuit can be optimized. To this end, a micro processor controls a switch interposed between the power amplification circuit and a positive DC power unit in such a manner that the DC bias voltage Vdd is 4.8 V for the FDMA mode, while the DC bias voltage Vdd is 6.0 V for the TDMA mode. The value of another DC bias voltage Vgg in the power amplification circuit is fixed. The value of the DC bias voltage Vdd is set higher in the TDMA mode than in the FDMA mode; thus, it becomes possible to increase the power added efficiency in the FDMA mode, without deteriorating high linearity and high efficiency in the TDMA mode.

Description: BACKGROUND OF THE INVENTION The present invention relates to a power amplifier used for a mobile radio communication system, including cellular telephones, operative in both an analog modulation mode and a digital modulation mode. Recent advancement toward digitization is magnificent in the field of mobile radio communication system represented by portable telephones and automotive vehicle telephones. For this reason, well-known conventional analog modulation systems are gradually being replaced by digital modulation systems, or by dual-mode modulation systems operative in both an analog modulation mode and a digital modulation mode. The dual-mode cellular telephones are advantageous in practical use in view of the fact that the service area available to the digital modulation system is still limited within a specific area. Of components required for the radio transmitter used in such a mobile radio communication, a power amplifier amplifying RF signals is definitely necessary for the power supply operation to an antenna. In the dual-mode transmitter, the power amplifier is generally required to have high efficiency in the analog modulation mode, while high linearity is required in addition to high efficiency in the digital modulation mode. From patent abstracts of Japan volume 17, No. 466 (E-1421), 25 August, 1993 and JP-A-05,110 348 of 30 April, 1993, a power amplifier is known which deals with the requirement of the linearity needed in the case of amplifying a digital modulation wave without causing a reduction in a power conversion efficiency of an analog modulation wave. The amplifier is provided with a power amplifier including an amplifier transistor the base of which is supplied with a biased voltage which is changed in dependency of the kind of the modulation wave. This change is effected by a biased change over switch having two switching positions. In one of said switching positions the first bias is effective at the base of the amplifier transistor causing same to operate as a class C power amplifier. In the other position of the bias change over switch a bias voltage is effective at the amplifier transistor which causes same to operate as a class A power amplifier The change of the operation point of the amplifier transistor results in a change of the input impedance of said transistor. A reduction of power consumption in one of the switching positions of the biased changeover switch with respect to the other switching position is not possible. In the conventional power amplifiers for transmitters, also known is a technology of switching two power amplification circuits in accordance with the modulation type. Among them, the most simplest arrangement is provision of selectable two power amplification circuits, one power amplification circuit dedicated to the analog modulation system and the other power amplification circuit dedicated to the digital modulation system. Unexamined Japanese patent application No. HEI 5-199127/1993 discloses such a power amplification circuit for a transmitter which includes both a non-linear power amplification circuit and a linear power amplification circuit combined with each other through switches. Important thing in the analog modulation mode is to realize high efficiency; therefore, a non-linear operation region (i.e. the vicinity of a saturation region) of the non-linear power amplification circuit is preferably utilized. On the other hand, the digital modulation mode is likely to cause distortions in the modulation. To reduce such distortions, a linear operation region of the non-linear power amplification circuit is preferably utilized in a relatively lower output range. Meanwhile, the digital modulation mode, when it is operated in a relatively high power output range, requires us to selectively use the non-linear power amplification circuit and the linear power amplification circuit in order to realize both high efficiency and high linearity. However, the conventional power amplifier utilizing the above-described switching technology is disadvantageous in cost since it requires two different types of power amplification circuits. In addition, it is not satisfactory that a significant amount of power loss is recognized when each RF signal passes through the switches. Hence, it is earnestly expected at present that a single power amplification circuit will be developed so as to possess capability of being operative in both the analog modulation mode and the digital modulation mode. When a single power amplification circuit is operated in both the analog modulation mode and the digital modulation mode, a problem will arise in the power added efficiency, as will be explained in detail below. The power added efficiency η of the power amplifier is generally defined as follows: η = (AC Output Power - AC Input Power) / DC Input Power where AC Input Power represents a power of the input RF signal, AC Output Power represents a power of the output RF signal, and DC Input Power represents a power supplied from a DC power unit to the power amplifier. That is, the power added efficiency should be understood as a conversion efficiency from DC power to AC power. In the case of a cellular telephone adopting a battery as DC power source, it is sincerely desired that the power added efficiency is Sufficiently high to suppress exhaustion of battery. Radio (wireless) telephone transmitters, used in Japan, must comply with the NTT (Nippon Telegraph and Telephone Corporation) standard when they are operative in the analog modulation mode, or comply with the RCR (Research and Development Center for Radio System) standard when they are operative in the digital modulation mode. The NTT standard stipulates the Frequency Division Multiple Access (FDMA) system. The RCR standard, under the requirements of Article STD-27B, stipulates the Time Division Multiple Access (TDMA) system. Both the FDMA and TDMA systems require a power amplifier having the same AC output power of 1.5 W. Fig. 6 is a graph showing a typical example of input/output characteristics and power added efficiency in accordance with a conventional power amplifier of a dual-mode transmitter produced for use in Japan. This power amplifier includes a single power amplification circuit to which a DC bias voltage Vdd of 6 V is supplied. The power added efficiency in a 1.5 W output condition, as shown in Fig. 6, is approximately 40% in each mode of FDMA (i.e. analog modulation) and TDMA (i.e. digital modulation). In short, this conventional technology causes a problem in forcing the power amplifier to victimize the power added efficiency in the FDMA mode in order to realize high linearity in the TDMA mode. On the other hand, in the United States, there is the TIA (Telecommunications Industry Association) standard applied to the dual-mode radio telephone transmitters. The TIA standard, under the requirements of Article IS-95, stipulates the FDMA system as an access system for the analog modulation mode and the Code Division Multiple Access (CDMA) system as an access system for the digital modulation mode. The FDMA system requires an AC output power of 1.5 W, while the CDMA system requires an AC output power of only 0.5 W. Fig. 7 is a graph showing an example of input/output characteristics and power added efficiency in accordance with a conventional power amplifier of a dual-mode transmitter produced for use in the Unites States. This power amplifier includes a single power amplification circuit to which a DC bias voltage Vdd of 4.8 V is supplied. The power added efficiency in the FDMA mode is approximately 60% in a 1.5 W output condition, while the power added efficiency in the CDMA mode is approximately 30% in a 0.5 W output condition. In short, this conventional technology causes a problem in forcing the power amplifier to victimize the power added efficiency in the CDMA mode in order to realize high efficiency in the FDMA mode. As explained in the foregoing description, the conventional single-power-amplification-circuit type power amplifier for a transmitter is subjected to the problem that, if it is produced for use in Japan, the power added efficiency is worsened in the analog modulation (FDMA) mode or the other problem that, if it is produced for use in the United States, the power added efficiency is worsened in the digital modulation (CDMA) mode. In short, there was an essential problem that the power added efficiency is worsen in either of the analog and digital modulation modes. SUMMARY OF THE INVENTION Accordingly, in view of above-described problems encountered in the prior art, a principal object of the present invention is to provide a power amplifier for a transmitter including an excellent single power amplification circuit operative in both analog and digital modulation modes and capable of gaining high power added efficiency in each of these modulation modes. In order to accomplish this and other related objects, the present invention provides a power amplifier for a radio transmitter operative in both the analog and digital modulation modes, wherein there are produced two kinds of DC voltages to be applied to a single power amplification circuit, and either of these two kinds of DC voltages is selectively supplied to an output terminal of a power amplifying element comprised in the power amplification circuit in accordance with the type of mode, thereby controlling input/output characteristics of the power amplification circuit. More specifically, in the FDMA/TDMA dual-mode system, the AC output power required is constant irrespective of the difference of mode between the analog modulation mode and the digital modulation mode. It is desirable that the input/output characteristics has a higher saturation output in the digital modulation (TDMA) mode than in the analog modulation (FDMA) mode. To realize this, the DC voltage applied to the power amplification circuit in the TDMA mode is set higher than the corresponding voltage in the FDMA mode. On the other hand, in the case of the FDMA/CDMA dual-mode system, the AC output power required in the digital modulation (CDMA) mode is fairly lower than that required in the analog modulation (FDMA) mode. It is thus desirable that the input/output characteristics has a lower saturation output in the digital modulation (CDMA) mode than in the analog modulation (FDMA) mode. To realize this, the DC voltage applied to the power amplification circuit in the CDMA mode is set lower than the corresponding voltage in the FDMA mode. Accordingly, for the FDMA/TDMA dual-mode system, the present invention sets the input/output characteristics (i.e. AC input power v.s. AC output power) of the power amplification circuit in such a manner that the saturation output power in the TDMA mode becomes higher than that in the FDMA mode. Hence, it becomes possible to gain high efficiency in the FDMA mode without victimizing both the high linearity and high efficiency in the TDMA mode. Meanwhile, for the FDMA/CDMA dual-mode system, the present invention sets the input/output characteristics of the power amplification circuit in such a manner that the saturation output power in the CDMA mode becomes lower than that in the FDMA mode. Hence, it becomes possible to gain both high linearity and high efficiency in the CDMA mode without victimizing the high efficiency in the FDMA mode. As described above, the present invention selectively changes the DC voltage applied to a single power amplification circuit in accordance with the difference of mode between the analog modulation mode and the digital modulation mode, so that the input/output characteristics of the power amplification circuit can be always optimized. Thus, the present invention provides an excellent power amplifier for a transmitter capable of realizing high power added efficiency in each of these two modulation modes. BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description which is to be read in conjunction with the accompanying drawings, in which: Fig. 1 is a block diagram showing an arrangement of a power amplifier in accordance with a first embodiment of the present invention; Fig. 2 is a circuit diagram showing detailed arrangement of a power amplification circuit shown in Fig. 1; Fig. 3 is a graph showing changes of input/output characteristics and power added efficiency of the power amplifier shown in Fig. 1 obtained by switching a DC bias voltage Vdd in the power amplification circuit; Fig. 4 is a block diagram showing an arrangement of a power amplifier in accordance with a second embodiment of the present invention; Fig. 5 is a graph showing changes of input/output characteristics and power added efficiency of the power amplifier shown in Fig. 4 obtained by switching a DC bias voltage Vdd in the power amplification circuit; Fig. 6 is a graph showing an example of input/output characteristics and power added efficiency in accordance with a conventional power amplifier; and Fig. 7 is a graph showing an example of input/output characteristics and power added efficiency in accordance with another conventional power amplifier. DETAILED DESCRIPTION OF THE INVENTION Hereinafter, preferred embodiments of the present invention will be explained in greater detail with reference to the accompanying drawings. (First Embodiment) Fig. 1 is a block diagram showing an arrangement of a power amplifier in accordance with the first embodiment of the present invention, which is preferably used for an FDMA/TDMA dual-mode radio (wireless) telephone transmitter. In Fig. 1, a reference numeral 1 represents a positive DC power unit; a reference numeral 2 represents a switch; a reference numeral 3 represents a micro processor; a reference numeral 4 represents a negative DC power unit; and a reference numeral 5 represents a power amplification circuit. The positive DC power unit 1 supplies two kinds of DC voltages, 4.8 V and 6.0 V. More specifically, 4.8 V is produced by serial four cells of nickel-cadmium battery, while 6.0 V is produced by serial five cells of the nickel-cadmium battery. The positive DC power unit 1, generating such two kinds of DC voltages, can be replaced by a voltage regulator circuit having the same function. The switch 2 acts as a selector for selecting either of these two kinds of DC voltages generated from the positive DC power unit 1. The micro processor 3 controls the switch 2 in compliance with the program and determines a preferable DC voltage to be selected by the switch 2 in accordance with the type of modulation mode. More specifically, the micro processor 3 controls the switch 2 to select 4.8 V when the modulation mode is an analog type (i.e. FDMA) and to select 6.0 V when the modulation mode is a digital type (i.e. TDMA). The negative DC power unit 4 generates a DC bias voltage Vgg of -3.6 V. The power amplification circuit 5 amplifies radio frequency signals. RF IN represents an RF signal input terminal of the power amplification circuit 5, while RF OUT represents an RF signal output terminal of the power amplification circuit 5. Furthermore, the power amplification circuit 5 has a Vdd terminal supplying the DC power voltage (4.8 V or 6.0 V) selected by the switch 2, and a Vgg terminal supplying the DC bias voltage Vgg (-3.6 V) from the negative DC power unit 4. Fig. 2 is a circuit diagram showing detailed arrangement of the power amplification circuit 5 shown in Fig. 1. In Fig. 2, reference numerals 11 and 12 represent field effect transistors (FET); reference numerals 13-18 represent micro strip lines (i.e. inductors); C1-C5 are capacitors; and R1-R4 are resistances. The DC power voltage, entered from the Vdd terminal, is supplied to drain electrodes of the field effect transistors 11 and 12 via the micro strip lines 14 and 17, respectively. The DC bias voltage, entered from the Vgg terminal, determines the bias of gate electrodes of the field effect transistors 11 and 12. A preferable transistor type adoptable as such field effect transistors 11 and 12 is, for example, GaAs-MESFET since it allows us to handle RF signals of approximately 1 GHz. Instead, it is also possible to use bipolar transistors, MOSFETs, or hetero-bipolar transistors (HBTs) which utilize the junction between heterogeneous materials, such as GaAs and AlGaAs. Fig. 3 is a graph showing changes of input/output characteristics and power added efficiency of the power amplifier shown in Fig. 1 obtained by switching the DC power voltage Vdd in the power amplification circuit 5. According to the input/output characteristics (dotted line in Fig. 3) in the analog modulation (i.e. FDMA) mode, its saturation output power is 1.5 W. According to the input/output characteristics (solid line in Fig. 3) of the digital modulation (i.e. TDMA) mode, its saturation output power is 2.8 W. By switching the power voltage to be supplied to the power amplification circuit 5, the power added efficiency becomes approximately 60% for the FDMA mode and approximately 40% for the TDMA mode when the output power is 1.5 W. As explained above, according to this embodiment, the power added efficiency in the FDMA mode can be improved by a degree of 20% compared with the conventional example of Fig. 6. Thus, the effect is so tremendous. (Second Embodiment) Fig. 4 is a block diagram showing an arrangement of a power amplifier in accordance with the second embodiment of the present invention, which is preferably used for an FDMA/CDMA dual-mode radio (wireless) telephone transmitter. This embodiment is different from the Fig. 1 embodiment in that the DC power voltages produced from the positive DC power unit 1 are 3.6 V and 4.8 V. The micro processor 3 controls the switch 2 to select 4.8 V when the modulation type is an analog type (i.e. FDMA) and to select 3.6 V when the modulation type is a digital type (i.e. CDMA). The negative DC power unit 4 and the power amplification circuit 5 have the same construction as those of the above-described first embodiment. Fig. 5 is a graph showing changes of input/output characteristics and power added efficiency of the power amplifier shown in Fig. 4 obtained by switching the DC power voltage Vdd in the power amplification circuit 5. According to the input/output characteristics (dotted line in Fig. 5) of the analog modulation (i.e. FDMA) mode, its saturation output power is higher than that of the input/output characteristics (solid line in Fig. 5) in the digital modulation (i.e. CDMA) mode. By switching the power voltage to be supplied to the power amplification circuit 5, the power added efficiency becomes approximately 60% for the FDMA mode when the output power is 1.5 W and approximately 40% for the CDMA mode when the output power is 0.5 W. As explained above, according to this embodiment, the power added efficiency in the CDMA mode can be improved by a degree of 10% compared with the conventional example of Fig. 7. Thus, the effect is so tremendous. The values Vdd and Vgg adopted in the above-described embodiments can be varied in accordance with the individual internal arrangement of the power amplification circuit 5. Furthermore, the present invention can be applied to other devices, such as power amplifiers of the radio transmitters dedicated to data transmission, other than the wireless telephone transmitters dedicated to voice transmission. The present embodiments as described are therefore intended to be only illustrative and not restrictive, since the scope of the invention is defined by the appended claims rather than by the description preceding them, and all changes that fall within the metes and bounds of the claims, or equivalents of such metes and bounds, are therefore intended to be embraced by the claims.

Claims: A power amplifier used for a radio transmitter operative in both an analog modulation mode and a digital modulation mode, comprising: a power unit circuit (1) for producing two kinds of DC voltages; a switch circuit (2) for selecting either of said two kinds of DC voltages produced from said power unit circuit (1); a control circuit (3) for controlling said switch circuit (2) in accordance with a type of modulation mode; and a single power amplification circuit (5) having a power amplifying element (12) for amplifying radio frequency signals by applying as a power-source voltage, the DC voltage selected by said switch circuit (3) to an output terminal of said power amplifying element (12). The power amplifier in accordance with claim 1, wherein said power amplification circuit (5) is used for a wireless telephone transmitter. The power amplifier in accordance with claim 1, wherein said switch circuit (2) selects a higher DC voltage to be applied as a power-source voltage to the output terminal of said power amplifying element (12) of the power amplification circuit (5) in the digital modulation mode, while selects a lower DC voltage in the analog modulation mode. The power amplifier in accordance with claim 3, wherein Frequency Division Multiple Access (FDMA) system is used as an access system for said analog modulation mode, while Time Division Multiple Access (TDMA) system is used as an access system for said digital modulation mode. The power amplifier in accordance with claim 1, wherein said switch circuit (2) selects a lower DC voltage to be applied as a power-source voltage to the output terminal of said power amplifying element (12) of the power amplification circuit (5) in the digital modulation mode, while selects a higher DC voltage in the analog modulation mode. The power amplifier in accordance with claim 5, wherein Frequency Division Multiple Access (FDMA) system is used as an access system for said analog modulation mode, while Code Division Multiple Access (CDMA) system is used as an access system for said digital modulation mode. An amplifying method for amplifying radio frequency signals in a power amplification circuit used for a radio transmitter operative in both an analog modulation mode and a digital modulation mode, comprising the steps of producing two kinds of DC voltages to be applied as a power-source voltage to an output terminal of a power amplifying element of the power amplification circuit; and selecting either of said two kinds of DC voltages in accordance with a type of mode, thereby controlling input/output characteristics of said power amplification circuit (5). The amplifying method in accordance with claim 7, wherein said power amplification circuit is used for a wireless telephone transmitter.

Inventors: KANAZAWA KUNIHIKO, KOIZUMI HARUHIKO, SUGIMURA AKIHISA, TATEOKA KAZUKI, KANAZAWA, KUNIHIKO, KOIZUMI, HARUHIKO, SUGIMURA, AKIHISA, TATEOKA, KAZUKI
Applicants: MATSUSHITA ELECTRONICS CORP, MATSUSHITA ELECTRONICS CORPORATION
Application year: 1995
Main classification: H03G3
IPC codes: H04B1, H03F1, H04Q7, H03G3, H04B7, H04J3