Abstract:
A power control apparatus for minimizing energy losses in a radio transmitter uses a variable-output switched-mode power supply to control a voltage applied to an output power amplifier stage of the radio transmitter. By monitoring a current drawn by the output power amplifier operating in a Class D mode, an error signal is generated that regulates the applied voltage, such that only a minimally-sufficient voltage is supplied to the output amplifier for a given transmitted output power.

Description:
DESCRIPTION 
     The present invention relates to an electronic apparatus comprising: 
     a power amplifier for producing a transmit power, formed by: 
     a final stage having a supply input for receiving a supply power, 
     a measuring circuit for measuring an electrical magnitude applied to said supply input, 
     a variable power supply circuit controlled by said measuring circuit for controlling the said transmit power. 
     The invention also relates to an amplifier suitable for such an apparatus. 
     The invention finds applications notably in mobile radiotelephony devices for which reduced power consumptions are required to lengthen the discharge period of the accumulator that feeds these devices. The high-frequency transmission amplifier consumes much energy. It is thus interesting to use high-efficiency power amplifier classes. This is the case with class D and others. 
     U.S. Pat. No. 5,497,125 describes a power amplifier of this type whose output power is controlled on the basis of a measurement of the power consumption. However, in this known amplifier there is a risk that there is non-negligible power dissipation in the current measuring circuit. Furthermore, the change of output power is effected via a variable gain preamplifier that also consumes a little energy. 
     The present invention proposes an apparatus of the type defined in the opening paragraph which offers reduced power consumption of the control circuit and is largely immune to noise. 
     Therefore, such an apparatus is characterized in that the said variable power supply circuit comprises: 
     a switched-mode power supply circuit having a switched-mode control input for defining its output voltage. 
     The idea of the invention is to utilize a switched-mode power supply that has the best efficiency. 
     An important characteristic feature of the invention comprises that said variable power supply circuit further includes a series power supply circuit that has a series control input for defining its output voltage and to which is connected a slaved circuit, so that the voltage difference between the input and the output of the series power supply circuit is minimized. This characteristic feature makes that the series pass transistor which is included in series power supply circuits works to saturation and thus has little energy consumption. Furthermore, the advantage is obtained that the switching noises of the switched-mode power supply are filtered without too much energy consumption. 
     These and other aspects of the invention are apparent from and will be elucidated, by way of non-limitative example, with reference to the embodiment(s) described hereinafter. 
    
    
     In the drawings: 
     FIG. 1 shows an apparatus in accordance with the invention, 
     FIG. 2 shows the manufacturing diagram of the amplifier of the apparatus shown in FIG. 1, 
     FIG. 3 shows a more detailed diagram of the amplifier according to the invention, 
     FIG. 4 explains the current tapping circuit, and 
     FIG. 5 explains the structure of an amplifier working in class D. 
    
    
     FIG. 1 shows an electronic apparatus in accordance with the invention. This apparatus is a transceiver such as a mobile radio in a cellular network. It comprises a transceiver assembly  5  which has a transmission part  7  and a receiving part  9  for transmitting signals and for receiving them by means of an antenna  10 . The signals to be transmitted notably come from a microphone  12  and the signals that are received are notably intended for a loudspeaker  14 . A processing element  17  ensures the interface between the low-frequency signals relating to the microphone  12  and the loudspeaker  14  and the assembly  5 . The whole is supplied with power via an accumulator  25 . 
     The transceiver assembly comprises a high-frequency amplifier  20  which produces relatively powerful signals. 
     This amplifier  20  is shown in detail in FIG.  2 . It comprises a final stage  30  intended to amplify the signals applied to its input terminal  31  so as to supply them to the antenna  10 . This amplifier is fed through its power supply input  32  by the accumulator  25  followed by a variable power supply regulation circuit  35 . This power supply is controlled by a comparator circuit  37  which measures the current supplied by the final stage  30  so as to compare it with a reference value “ref1” produced by a first reference voltage generator  38 . A current-tapping circuit  40  taps a well-defined fraction of the supply current from this final stage. This current defines the power produced by this final stage. 
     In accordance with the invention, the variable power supply circuit  35  comprises a switched-mode power supply circuit  50 . 
     FIG. 3 shows in more detail the embodiment of the invention. To the switched-mode power supply circuit  50 , whose input is directly connected to a terminal of the accumulator  25 , is associated a series power supply circuit  52 . The structure of this circuit  52  is profited from for realizing the current-tapping circuit in a manner that will be explained with the aid of FIG.  4 . 
     According to the embodiment represented in FIG. 3, the switched-mode power supply circuit  50  receives a switch control voltage that depends on the difference between the voltages &lt;&lt;Ve&gt;&gt; and &lt;&lt;Vs&gt;&gt; which exists between the input and output of the series power supply circuit  52 . This difference is formed by a subtracter circuit  55 . The output voltage of this circuit  55  is compared with a reference voltage “ref2” produced by a second reference voltage generator  57  by means of a subtracter circuit  59  which produces voltage at the serial command of the power supply circuit  52 . The generator  57  produces a sufficiently low voltage to avoid energy dissipation via the series power supply circuit  52  since it sets the difference &lt;&lt;Vs−Ve&gt;&gt; to a low value. The output voltage &lt;&lt;Vs&gt;&gt; of the series power supply circuit is set by the comparator circuit  37 , as has already been observed. 
     Thus, as a result of this double slave control, the first one involving the switched-mode power supply circuit  50  providing a good energy efficiency, and the second one involving the series power supply circuit  52  eliminating the switching disturbances of the switched-mode power supply circuit  50 , not much energy is spent, because the voltage between its input and its output is kept low. 
     FIG. 4 shows how the tapping circuit  40  is built up. Reference  60  indicates the pass transistor of the series circuit. This transistor has a plurality of emitter zones: the largest part Z 1  is used for feeding the amplifier  30  and a single zone Z 2  is used for being applied to the comparator  37 . 
     FIG. 5 diagrammatically shows the operation of an amplifier  30  operating in class D. Such an amplifier is formed from a switching circuit  70  which is opened and closed by means of a control circuit  71 . Thus, the switch is opened and closed as a function of the signals applied to the input terminal  31 . Thus, via the power supply circuits  50  and  52  the supply voltage is applied directly to the load which is formed by the antenna  10 . 
     Thanks to the invention, one has gained control of the power produced by the amplifier  30 . This power is defined by the generator  38  which may produce variable references according to the needs.