Abstract:
A transmitting apparatus, in particular for a mobile radio base station, is provided. The apparatus includes at least one power amplifier which has at least one amplifier element and one device for forcing a quiescent current to flow into the amplifier element. The level of the quiescent current can be varied in dependence of the operating state of the transmitting apparatus.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]    This application claims priority of European Application No. 02251368.3 filed on Feb. 27, 2002. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The invention relates to a transmitting apparatus, for example for a mobile radio base station, and to a power amplifier in particular for use in such a transmitting apparatus.  
           [0004]    2. Description of the Related Art  
           [0005]    Radio frequency power amplifiers which are required, for example, in mobile radio stations for the GSM mobile radio system are distinguished by a constant quiescent current, which ensures a mean power loss in the mobile radio station.  
         SUMMARY OF THE INVENTION  
         [0006]    The invention provides a transmitting apparatus and a power amplifier, by means of which the mean power loss of a power amplifier can be reduced.  
           [0007]    In one embodiment of the invention, the quiescent current of the power amplifier is not kept constant during operation of a mobile radio base station, but to optimally match the level of the quiescent current to different operating states of the transmitting device.  
           [0008]    In another embodiment, a transmitting apparatus is provided which has at least one power amplifier. The power amplifier contains at least one amplifier element and means for forcing a quiescent current to flow into the amplifier element. The transmitting apparatus varies the level of the quiescent current as a function of the operating state of the transmitting device. Phase modulators, in particular GMSK (Gaussian Minimum Shift Keying) modulators are used for modulation of transmission signals in a mobile radio base station. These modulators ensure pure phase modulation of the transmission signal, that is to say the amplitude of the modulated signals is constant within a TDMA time slot. In other words, the modulated signal does not contain any amplitude modulation components. This situation can be used in an advantageous manner to build radio frequency power amplifiers with a very high efficiency, for example of about 50%. The radio frequency power transistors used in radio frequency power amplifiers generally have a linearity response which is dependent on the quiescent current. Thus a low quiescent current results in poor linearity with high efficiency. On the other hand, a high quiescent current ensures that the radio frequency power transistor is highly linear, although its efficiency is poor. Power amplifiers with poor linearity, which is achieved with a low quiescent current of approximately 10% of the maximum current of the power amplifier, with the amplifier being operated in the so-called C-mode, can therefore be used for amplification of purely phase-modulated signals, which have a constant envelope. One advantageous feature of the C-mode, as already mentioned, is the high efficiency of the power amplifier. New modulation methods will now have to be introduced to satisfy future requirements for high data rates in mobile radio. One such modulation method is, for example, the 8-PSK modulation used in the EDGE Standard. Signals which have been modulated, for example, by means of 8-PSK modulation now also have amplitude-modulated components in addition to phase-modulated components. The amplitude-modulated components are subject to signal distortion if they are amplified using a non-linear power amplifier. Power amplifiers for amplifying signals which have both amplitude-modulated components and phase-modulated components must therefore be operated with a high quiescent current, for example of approximately 50% of the maximum current of the power amplifier, to ensure sufficient linearity of the power amplifier. Transmitting apparatuses with conventional power amplifiers which are operated with a constant quiescent current lead to an increased power loss when using different modulation methods, for example GMSK and 8-PSK modulation.  
           [0009]    The power loss in the transmitting apparatus when using different modulation methods can now be reduced by providing controlling means which cause the quiescent current level to be set as a function of the modulation method which is applied to the signal to be amplified.  
           [0010]    By way of example, as mentioned, the transmitting apparatus contains a pure phase modulator, such as a GMSK modulator and a phase and amplitude modulator, such as an 8-PSK modulator. Since, as explained, 8-PSK modulation demands a power amplifier with high linearity, a high quiescent current must be forced to flow into the amplifier element. However, a high quiescent current leads to poorer power-amplifier efficiency. For the situation where a GMSK modulated signal is produced by the transmitting apparatus, a low quiescent current is sufficient, as already mentioned, since such modulation methods operate reliably even with the poor power amplifier linearity.  
           [0011]    The power loss in the transmitting apparatus or the power amplifier can be reduced further by using the controlling means to set the quiescent current level to zero during a time in which there is no transmission, that is to say when no signal whatsoever is being transmitted. This avoids heat losses being produced unnecessarily.  
           [0012]    The use of means for setting the quiescent current level of a power amplifier as a function of the operating state of the transmitting apparatus allows the power loss, and hence the complexity for its cooling, to be reduced considerably. In consequence, a number of transmitting and receiving modules can also be implemented in one base station.  
           [0013]    For the situation of the transmitting apparatus operating using the time-division multiplex mode (TDMA mode), the controlling means can be implemented such that it cause the quiescent current level to be set on a time slot basis. If the quiescent current level is set or switched on a time slot basis, then it is expedient to reset the quiescent current between successive time slots, in particular in the so-called guard periods.  
           [0014]    The quiescent current can be set or switched discontinuously by using hard switching, for example between three states, or else continuously by slowly increasing or reducing a control voltage by means of a digital/analogue converter.  
           [0015]    For example, a power amplifier has at least one amplifier element and means for forcing a quiescent current to flow into the amplifier element, in which case the level of the quiescent current can be varied as a function of the state of the signal to be amplified.  
           [0016]    In this context, the expression the state of the signal to be amplified means not only the modulation method which is applied to the signal to be amplified, but also a state in which there is no signal.  
           [0017]    Accordingly, one advantageous development provides controlling means which causes the quiescent current level to be set to zero during a time in which there is no transmission.  
           [0018]    The controller can also set the quiescent current level as a function of the modulation method which is applied to the signal to be amplified.  
           [0019]    The invention allows the power loss in the amplifier element, for example in an output stage power transistor, to be reduced, since an optimum quiescent current is always set for the type of modulation used in each particular case. In particular, higher output power levels can be produced during EDGE operation than for a GMS system. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    The invention will be explained in more detail in the following text with reference to an exemplary embodiment and in conjunction with the drawings, in which:  
         [0021]    [0021]FIG. 1 shows a schematic block diagram of a radio frequency power amplifier according to the invention, for use in a mobile radio base station,  
         [0022]    [0022]FIG. 2 shows the waveform of the output power of a signal which is modulated by means of a GMSK and 8-PSK modulator, and  
         [0023]    [0023]FIG. 3 shows the waveform of the quiescent current for the power waveform shown in FIG. 2. 
     
    
     DETAILED DESCRIPTION  
       [0024]    [0024]FIG. 1 shows a radio frequency power amplifier  10 , which can be used, for example, in a mobile radio base station in a GSM mobile radio system. On the input side, the radio frequency power amplifier  10  has a controller  20 , for example in the form of a digital signal processor DSP. A modulated, digital, complex signal, for example, is applied to the input of the controller  20  and an example of its power waveform is shown in FIG. 2. The I-component of this complex signal that appears at one output of the controller  20  is supplied via a D/A converter  30  to the first input of an IQ mixer  40 , which is known per se and operates in analogue form, while, in contrast, the Q-component of the complex signal, which is produced at another output of the controller  20 , is supplied via a D/A converter  35  to the second input of the IQ mixer  40 . The IQ mixer  40  has two multiplexers  42  and  44 , which are connected to an oscillator  60 . In order to introduce a 90° phase shift between the I-component and the Q-component of the signal to be transmitted, the output signal from the oscillator  60  is passed via a 90° phase shifter  45  to one input of the multiplier  44 . The output signal from the multipliers  42  and  44  is supplied to an adder  46 . A coupling capacitor  70  provides DC isolation between the IQ mixer  40  and the radio frequency power transistor  80 . The output signal from the radio frequency power transistor  80  is supplied via a coupling capacitor  75  to a transmitting antenna  50  of a mobile radio station. The radio frequency power transistor  80  may be, for example, an FET transistor.  
         [0025]    The quiescent current is controlled by a control circuit. Via a further output of the controller  20 , a digital voltage value is converted via a D/A converter  90  to an analogue control voltage which, in conjunction with a resistor  100 , results in a current flow, which flows as the quiescent current to the gate connection of the radio frequency power transistor  80 .  
         [0026]    The method of operation of the radio frequency power transistor  10  shown in FIG. 1 will be explained in more detail in the following text.  
         [0027]    In order to make it possible to comply with the future requirement for higher data rates in mobile radio, it is necessary to introduce new modulation methods. With regard to the GSM mobile radio system, a decision has in this case been made in favor of the EDGE Standard, which is based on 8-PSK modulation. EDGE systems are based on a time-division multiplex mode (TDMA mode), in which the data to be transmitted is transmitted in time slots. In order to make it possible to continue to operate, for example, mobile radio stations which are not EDGE compatible, it is provided that time slots which, for example, contain both GMSK modulated signals and 8-PSK modulated signals, can be located in mixed form on a carrier frequency. FIG. 2 shows an example of the power waveform of one such signal. In detail, the first and third time slots show the envelope of a GMSK modulated signal while, in contrast, the second and fifth time slots show an 8-PSK-modulated signal. The fourth time slot indicates that no signal is transmitted at all.  
         [0028]    In order to make it possible to reduce the power loss in a mobile radio base station when using, for example, two different modulation methods, as mentioned above, the radio frequency power amplifier  10  illustrated in FIG. 1 is implemented such that the level of the quiescent current which is supplied via the resistor  100  to the power transistor  80  is set as a function of the modulation method being used at that time. Furthermore, the controller  20  is implemented such that no quiescent current is forced to flow into the gate connection of the radio frequency power transistor  80  when a time slot is found in which no signals are transmitted. The controller or the digital signal processor  20  is also able to identify pauses, or so-called guard periods, between successive time slots. Since the output power of the transmission signal is reduced during this time, it is possible to switch the quiescent current level in this time, since switching processes do not produce any interference spectral components in the output signal.  
         [0029]    At the time t 1 , which is shown in FIG. 2 and is located within a guard period, the controller  20  identifies the fact that the following time slot contains a GMSK-modulated signal. A voltage value is then transmitted from the controller  20  to the D/A converter  90 , which converts this voltage value to an analogue control voltage, such that a small quiescent current is forced to flow via the resistor  100  into the radio frequency power amplifier  80 . This is because, as already mentioned, GMSK signals do not contain any amplitude modulation components, so that it is permissible for the radio frequency power amplifier  80  to have poorer linearity without this producing unacceptable signal distortions. At the time t 2 , which is likewise shown in FIG. 2 and once again occurs in a guard period, the controller  20  learns that an 8-PSK modulated signal will be transmitted in the subsequent second time slot. Since 8-PSK-modulated signals contain both phase-modulated components and amplitude modulation components, the radio frequency power amplifier  80  must have better linearity than a GMSK modulated signal. In consequence, the controller applies a higher voltage value to the D/A converter  90 , which converts this to a corresponding analogue control voltage, such that a higher quiescent current is forced to flow via the resistor  100  into the radio frequency power transistor  80 . At the time t 3 , the controller  20  learns that a GMSK-modulated signal will be transmitted once again in the next time slot. In consequence, in the manner already described, it causes a lower quiescent current to be supplied to the radio frequency power transistor  80 .  
         [0030]    At the time t 4 , which is illustrated in FIG. 2, the controller  20  knows that no signal will be transmitted in the next time slot. In consequence, the controller  20  applies a digital voltage value of zero to the D/A converter  90 , which ensures that no quiescent current flows into the radio frequency power amplifier  80 . During this time period, no heat losses are produced in the radio frequency power amplifier  10  either.  
         [0031]    [0031]FIG. 3 shows the quiescent current flowing into the power transistor  80  whose level has been set as a function of the state of the transmission signal to be amplified, as shown in FIG. 2, or as a function of the operating state of the transmitting apparatus in the mobile radio station.