Abstract:
There is provided a circuit and method for providing a supply voltage to an operational amplifier. A switch has a plurality of inputs connected to a respective plurality of supply voltages. An output of the switch is connected to a supply voltage terminal of an operational amplifier. The input of the switch is selected in dependence of the voltage levels to which a signal is to be amplified

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a technique for providing supply voltages to a supply terminal of an operational amplifier.  
         BACKGROUND TO THE INVENTION  
         [0002]    In certain applications the voltage levels to which an operational amplifier is required to drive signals is variable. Thus, for example, the operational amplifier may at times be required to drive output signals to a voltage level of 15 volts, whilst at other times it may only be necessary to drive output signals to a voltage level of 5 volts. However in order to cover the full range of possible output voltage levels, the operational amplifier is required to be provided with the voltage supply corresponding to the highest voltage level, namely in this example 15 volts.  
           [0003]    It is therefore an object of the present invention to provide an improved technique for providing a supply voltage to a supply terminal of an operational amplifier.  
         SUMMARY OF THE INVENTION  
         [0004]    In accordance with a first aspect of the present invention there is provided a circuitry for providing a supply voltage to an operational amplifier, comprising:  
           [0005]    a switch having a plurality of inputs connected to a respective plurality of supply voltages, and an output connected to a supply voltage terminal of the operational amplifier, wherein the input of the switch is selected in dependence on the voltage level to which a signal is to be amplified. Thus, advantageously the operational amplifier is only provided with a supply voltage level corresponding to the highest voltage level to which it requires to drive a signal, thereby saving power consumption when only low voltage levels are needed.  
           [0006]    A digital to analogue converter may receive digitised values, and generate a corresponding analogue signal for amplification by the operational amplifier. The input of the switch may be selected in dependence on the digitised values. The input of the switch may be selected in dependence on either the peak digitised value of the digitised values or the average digitised value of the digitised values.  
           [0007]    The input of the switch may be selected by a control signal generated in dependence on the digitised values. The digitised values may be stored in the digital to analogue converter.  
           [0008]    Preferably the switch has a first and a second input connected to a respective first and second supply voltages, a first supply voltage being lower than the second supply voltage, wherein the input of the switch is selected to be the second input if the voltage level to which the signal is to be amplified exceeds a predetermined level.  
           [0009]    The circuitry for providing the supply voltage to the operational amplifier may further comprise:  
           [0010]    a further switch having a plurality of inputs connected to a respective plurality of further supply voltages, and an output connected to a further supply voltage terminal of the operational amplifier, wherein the input of the further switch is selected in dependence on the voltage level to which the signal is to amplified.  
           [0011]    In a further aspect the present invention provides a method of providing a supply voltage to an operational amplifier, comprising the steps of:  
           [0012]    providing a plurality of supply voltages; selecting one of the supply voltages in dependence on the voltage level to which a signal is to be amplified; and connecting the selected one of the plurality of supply voltages to a supply voltage terminal of the operational amplifier.  
           [0013]    The method may further comprise the step of converting digitised values into an analogue signal for amplification by the operational amplifier. The step of selecting one of the supply voltages may be dependent upon the digitised values. The step of selecting one of the supply voltages may be dependent upon either the peak digitised value of the digitised values, or the average digitised value of the digitised values.  
           [0014]    The step of selecting one of the supply voltages may include generating a control signal in dependence of the digitised values.  
           [0015]    Preferably there is provided a first and second supply voltage, the first supply voltage being lower that the second supply voltage, wherein the second supply voltage is selected if the voltage level to which the signal is to amplified exceeds a predetermined level.  
           [0016]    The method of providing the supply voltage to the operational amplifier may further comprise the steps of:  
           [0017]    providing a plurality of further supply voltages; selecting one of the further supply voltages in dependence on the voltage level to which a signal is to be amplified; and connecting the selected one of the plurality of further supply voltages to a further supply voltage terminal of the operational amplifier.  
           [0018]    The circuitry for providing the supply voltage to the operational amplifier, or the method for providing a supply voltage to the operational amplifier, may be associated with an xDSL modem. A DSL is a digital subscriber line. The term “x” in front of DSL implies that the invention is relevant to any type of digital subscriber line technology. For example, the invention is relevant to ADSL (asymmetric digital subscriber line) technology, the VDSL (very-high-data-rate digital subscriber line) technology, HDSL (high bit rate digital subscriber line) technology etc. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    The invention will now be described with regard to a non-limiting example by way of reference to the accompanying drawings in which:  
         [0020]    [0020]FIG. 1 illustrates a first embodiment of a circuitry for providing a supply voltage to an operational amplifier in accordance with the present invention; and  
         [0021]    [0021]FIG. 2 illustrates a second embodiment of circuitry for providing a supply voltage to an operation amplifier in accordance with the present invention.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0022]    The following example of the present invention is described with specific reference to an application in an ADSL (asymmetric digital subscriber line) implementation. It will be apparent to the person skilled in the art that the present invention is generally applicable to any operational amplifier implementation.  
         [0023]    Referring to FIG. 1, the operational amplifier block according to the present invention as shown in this embodiment is generally designated by reference numeral  10 . The block comprises an operational amplifier  14  and a switch  34 . For the purposes of describing the present invention in this example there is also provided a digital to analogue converter  12  and a logic gate  16 . The digital to analogue converter  12  receives digital values on line  18 , and generates an analogue signal on lines  20  and  22  to the positive and negative inputs of the operational amplifier  14  as is well known in the art. The operational amplifier  14  has a first supply voltage terminal  36  connected to a node  38  of the switch  34 . A second supply voltage terminal  24  of the operational amplifier  14  is connected to ground. The operational amplifier  14  generates amplified analogue signals on line  26 . A switch  34  has two additional nodes  40  and  42 . Node  40  is connected to a supply voltage V CCL  on line  30 . Node  42  is connected to a supply voltage V CCH  which is connected to line  32 . An output of a digital to analogue converter  12  on line  28  is provided to logic gate  16  which in turn provides a control signal on line  30  to the switch  34 .  
         [0024]    As can be illustrated by the arrows within the switch unit  34 , the switch  34  is controllable to connect node  38  to either the node  40  or  42 . In this way the first supply terminal  36  of the operational amplifier receives either the supply voltage V CCL  on line  30  or the supply voltage of V CCH  on line  32 .  
         [0025]    The digitised values provided on line  18  to the digital to analogue converter  12  are indicative of the voltage levels to which the analogue signal on the output on line  26  of the operational amplifier are to be driven to. Thus, these values are indicative of the supply voltage level, either V CCL  or V CCH , which the first supply terminal  36  of the operational amplifier should be connected to. Thus the logic gate  16  will generate a signal on its output  30  in dependence on the value of line  18  provided to the digital to analogue converter  12  meeting certain predetermined conditions. Thus the logic gate  16  may be configured to detect a digitised value above a certain threshold value, and responsive thereto may set the signal on line  30  to switch to the higher supply voltage V CCH .  
         [0026]    Alternatively, rather than the logic gate  16  being configured to switch to a high supply voltage responsive to a single peak being detected above a threshold level, the logic gate  16  may be adapted to accumulate the digitised values and determine the average value thereof, and only response to the average value being above a peak is the control signal  30  set to switch to the higher supply voltage V CCH .  
         [0027]    Referring to FIG. 2, there is illustrated a second exemplary embodiment for controlling an operational amplifier according to the present invention. Like reference numerals are used to denote elements corresponding to elements shown in FIG. 1.  
         [0028]    Thus, as can be seen from FIG. 2, the operational amplifier block, designated by reference numeral  11 , is modified to include a further switch  50  for controlling a supply voltage provided to the second supply voltage terminal  24  of the operational amplifier  14 . The second supply voltage terminal  24  of the operational amplifier  14  is connected to a node  56  of the switch  50 . The switch  50  additionally comprises two nodes  52  and  54 . The node  52  is connected to the supply voltage level V SSL  on line  58 , and node  54  is connected to a supply voltage level V SSH  on line  60 . In addition the switch  50  receives the control signal on line  30  from the logic gate  16 .  
         [0029]    The operation of the general circuit of FIG. 2 is exactly the same as FIG. 1, with the exception that the control signal  30  now additionally controls the supply voltage applied to the second supply voltage terminal  24  of the operational amplifier. Thus, by way of example, the voltage level of V CCL  may correspond to plus five volts, and the voltage level of V SSL  correspond to minus five volts. In normal operation, the switches  34  and  50  may have a default setting in which the supply voltage terminal  36  is connected to the voltage V CCL , and the supply voltage terminal  24  is connected to the voltage V SSL.  In this example, it may be considered that the voltage level V CCH  is plus fifteen volts and the voltage level V SSH . is minus fifteen volts. Responsive to the appropriate peak or average levels being detected in the digitised signals by the logic gate  16 , the line  30  is set such that the switch  34  switches to connect the voltage V CCH  the supply voltage terminal  36 , and the switch  50  switches to connect the supply voltage terminal  24  to the supply voltage level V SSH .  
         [0030]    Thus it can be seen that with the use of the present invention power can be conserved by only applying the high voltage levels to the operational amplifier when it is required to amplify a signal to a higher voltage level.