Abstract:
A power supply device that reduces the loss of a load device, such as an amplifier, which is a supply destination of a power supply voltage. The power supply device includes a controller  14  for controlling a power supply voltage supplied to a load device  12  variably in accordance with an input voltage of the load device  12  (output of a signal source  13 ).

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a power supply device supplying voltage to a load device and to a method of supplying power supply voltage to the load device. 
         [0003]    2. Description of the Related Art 
         [0004]    Power supply devices include a device outputting a constant voltage and a device outputting voltage that changes over time. The present invention is of the latter type. In this field, a technology to control the loss in a load device (such as an amplifier) to which the voltage is supplied is known. 
         [0005]    For example, Japanese Patent Application Publication No. 2001-69241 “Circuit For Transmitting Ringer Signal” discloses a ringer signal transmission apparatus for transmitting a ringer signal. The ringer signal transmission apparatus reduces the loss of an amplifier circuit by synchronizing a power supply voltage supplied to the amplifier circuit amplifying a ringer signal with the amplitude of the ringer signal and changing the power supply voltage continuously. 
       SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
       [0006]    It is an object of the invention to provide a power supply device that can reduce the loss of a load device such as an amplifier that is a supply destination of the power supply voltage. 
       Means to Solve the Problems 
       [0007]    The power supply apparatus of the present invention is a power supply device supplying a power supply voltage to a load device such as an amplifier and comprises control means for controlling the power supply voltage supplied to the load device variably in a stepwise manner according to an input voltage of the load device. 
         [0008]    In a load device such as an amplifier with an output voltage being proportional to an input voltage, the output voltage can be predicted by referencing to the input voltage. Therefore, by controlling the power supply voltage variably so as to reduce the difference from the input voltage (output voltage), it is possible to reduce the difference between the power supply voltage supplied to the load device and the output voltage of the load device, and the loss of the load device can be reduced. 
       EFFECT OF THE INVENTION 
       [0009]    According to the present invention, it is possible to reduce the loss of load devices that are a supply destination of the power supply voltage. Consequently, it is possible to extend the device lifetime as well as to reduce power consumption of the load device, and reduction in size, weight and cost of the load device can be realized. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  illustrates an overview configuration of a power supply circuit relating to the first embodiment of the present invention. 
           [0011]      FIG. 2  illustrates a more detailed configuration of the power supply circuit of  FIG. 1 . 
           [0012]      FIG. 3  is a diagram showing the results of a simulation of power supplied to the amplifier under the condition that the power supply voltage supplied to the amplifier changes in a stepwise manner and under the condition that the power supply voltage supplied to the amplifier is constant (25V). 
           [0013]      FIG. 4  is a diagram showing the results of a simulation of power supplied to the amplifier under the condition that the power supply voltage supplied to the amplifier changes in a stepwise manner and under the condition that the power supply voltage supplied to the amplifier is constant (25V). 
           [0014]      FIG. 5  shows a first modification of the power supply circuit of the first embodiment according to the present invention. 
           [0015]      FIG. 6  shows a second modification of the power supply circuit of the first embodiment according to the present invention. 
           [0016]      FIG. 7  illustrates a more detailed configuration of the power supply circuit of  FIG. 6 . 
           [0017]      FIG. 8  is a flowchart illustrating a processing carried out by the controller in the second modification of the first embodiment according to the present invention. 
           [0018]      FIG. 9  illustrates an example of a timing diagram of the operation of switches in the power supply circuit which control the level on the power supply voltage based on the processing carried out by the controller in the flowchart in  FIG. 8 . 
           [0019]      FIG. 10  is a diagram of an overview configuration of the power supply circuit relating to the second embodiment of the present invention. 
           [0020]      FIG. 11  shows a first modification of the power supply circuit of the second embodiment according to the present invention. 
           [0021]      FIG. 12  shows a second modification of the power supply circuit of the second embodiment according to the present invention. 
           [0022]      FIG. 13  shows a third modification of the power supply circuit of the second embodiment according to the present invention. 
           [0023]      FIG. 14  is a diagram showing another method for determining the decrease of the power supply voltage supplied to the amplifier in the configuration where a load resistor and a switch are added. 
           [0024]      FIG. 15  shows a fourth modification of the power supply circuit of the second embodiment according to the present invention. 
           [0025]      FIG. 16  is a diagram showing further details of the main part of  FIG. 15 . 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0026]    In the following description, details of the preferred embodiments of the present invention will be described with reference to the accompanying drawings. 
         [0027]      FIG. 1  is a diagram of an overview configuration of a power supply circuit relating to the first embodiment of the present invention. The power supply circuit supplies a power supply voltage to an amplifier  12  that serves as a load. 
         [0028]    The power supply circuit comprises a signal source  13  for outputting a signal that changes periodically or aperiodically, an amplifier  12  and a controller  14  that input the output of the signal source  13 , DC voltage supplies  15 - 1 ,  15 - 2 ,  15 - 3  and  15 - 4  arranged in parallel, switches (transistors)  16 - 1 ,  16 - 2 ,  16 - 3  and  16 - 4  that correspond to the respective DC voltage supplies, and diodes  17 - 1 ,  17 - 2 ,  17 - 3  and  17 - 4  provided in the subsequent stage of each switch for rectification. 
         [0029]      FIG. 2  is a diagram showing a further detailed configuration of the power supply circuit of  FIG. 1 . 
         [0030]      FIG. 2  shows a configuration of the controller  14  of  FIG. 1 . In  FIG. 2 , compared with  FIG. 1 , a DC power supply  21  generating a voltage supplied as a default value to the amplifier  12  that serves as a load and a diode  22  provided to the subsequent stage of the DC power supply  21  for rectification are additionally provided. 
         [0031]    In  FIG. 2 , the DC voltage supplies  15 - 1 ,  15 - 2 ,  15 - 3  and  15 - 4  are arranged in parallel and generate voltages of 10V, 15V, 20V, and 25V, respectively, for example. The default value of the voltage that the DC power supply  21  generates is set to be smaller than the minimum voltage value of the voltage values generated by the DC voltage supplies  15 - 1 ,  15 - 2 ,  15 - 3  and  15 - 4  arranged in parallel. In this example, the voltage value generated by the DC voltage supply  21  is set to 5V. As a result, any of the five level voltages of 5V, 10V, 15V, 20V and 25V are supplied as a power supply voltage to the amplifier  12  that serves as a load. 
         [0032]    It should be noted that in the present embodiment, the output voltage V out  of the load circuit (amplifier  12 ) is proportional to the input voltage in the load circuit (amplifier  12 ), and it is possible to predict the output voltage on the basis of the value of the input voltage. 
         [0033]    As shown in  FIG. 2 , the controller  14  comprises comparators  24 - 1 ,  24 - 2 ,  24 - 3  and  24 - 4 . The comparator  24 - 1  compares a voltage that is a threshold of the output of a DC voltage supply  23 - 1  with a voltage output from the signal source  13 , and outputs a signal to the switch  16 - 1  to allow the switch (transistor)  16 - 1  to conduct, if the output voltage of the signal source  13  is equal to or higher than the output threshold voltage of the DC voltage supply  23 - 1 . 
         [0034]    The comparator  24 - 2  compares a voltage that is a threshold of the output of a DC voltage supply  23 - 2  and a voltage output from the signal source  13 , and outputs a signal to the switch  16 - 2  to allow the switch (transistor)  16 - 2  to conduct, if the output voltage of the signal source  13  is equal to or higher than the output threshold voltage of the DC voltage supply  23 - 2 . 
         [0035]    The comparator  24 - 3  compares a voltage that is a threshold of the output of a DC voltage supply  23 - 3  and a voltage output from the signal source  13 , and outputs a signal to the switch  16 - 3  to allow the switch (transistor)  16 - 3  to conduct, if the output voltage of the signal source  13  is equal to or higher than the output threshold voltage of the DC voltage supply  23 - 3 . 
         [0036]    The comparator  24 - 4  compares a voltage that is a threshold of the output of a DC voltage supply  23 - 4  and a voltage output from the signal source  13 , and outputs a signal to the switch  16 - 4  to allow the switch (transistor)  16 - 4  to conduct, if the output voltage of the signal source  13  is equal to or higher than the output threshold voltage of the DC voltage supply  23 - 4 . 
         [0037]    It should also be noted that in the present embodiment, as shown in  FIG. 2 , the output voltage V out  of the load circuit (amplifier  12 ) can be feedback (a′-a′) such that it is compared to the output threshold voltages of the DC voltage supplies  23 - 1 - 23 - 4  in the comparators  24 - 1 - 24 - 4 , respectively, in order determine the power supply voltage that is supplied to the load circuit (amplifier  12 ). 
         [0038]    The voltage (threshold) that the DC voltage supply  23 - 1  outputs is set to be a slightly smaller value (e.g. 4.5V) than the default voltage (5V) output by the DC voltage supply  21  that outputs the default value voltage. 
         [0039]    The voltage (threshold) that the DC voltage supply  23 - 2  outputs is set to be a slightly smaller value (e.g. 9.5V) than the voltage output by the DC voltage supply  15 - 1  (10V). 
         [0040]    The voltage (threshold) that the DC voltage supply  23 - 3  outputs is set to be a slightly smaller value (e.g. 14.5V) than the voltage output by the DC voltage supply  15 - 2  (15V). 
         [0041]    The voltage (threshold) that the DC voltage supply  23 - 4  outputs is set to be a slightly smaller value (e.g. 19.5V) than the voltage output by the DC voltage supply  15 - 3  (20V). 
         [0042]    It should be noted that when the voltage value output by the DC voltage supply  23 - 4  is set in the manner described above, the effective value of the voltage that the signal source  13  outputs has to be limited to a value smaller than 25V/√2=approximately 17.677 in order to operate the power supply circuit effectively. For example, when a voltage with its effective value being 17V is output from the signal source  13 , the maximum amplitude is 17×√2=approximately 24V. 
         [0043]    When the voltage that the signal source  13  outputs gradually increases, the power supply circuit performs the following operations. That is, when the voltage output by the signal source  13  exceeds the default value voltage (5V) output by the DC voltage supply  21 , the voltage output by the DC voltage supply  15 - 1  (10V) can be supplied to the amplifier  12  as a power supply voltage. When the voltage output by the signal source  13  exceeds the voltage output by the DC voltage supply  15 - 1  (10V), the voltage output by the DC voltage supply  15 - 2  (15V) can be supplied to the amplifier  12  as a power supply voltage. 
         [0044]    When the voltage output by the signal source  13  exceeds the voltage output by the DC voltage supply  15 - 2  (15V), the voltage output by the DC voltage supply  15 - 3  (20V) can be supplied to the amplifier  12  as a power supply voltage. When the voltage output by the signal source  13  exceeds the voltage output by the DC voltage supply  15 - 3  (20V), the voltage output by the DC voltage supply  15 - 4  (25V) can be supplied to the amplifier  12  as a power supply voltage. 
         [0045]    Additionally, when the voltage output by the signal source  13  gradually decreases, the power supply circuit performs the following operations. That is, when the voltage output by the signal source  13  falls below the output threshold voltage of the DC voltage supply  23 - 4 , which is slightly lower than the voltage output by the DC voltage supply  15 - 3  (20V), the voltage output by the DC voltage supply  15 - 3  (20V) can be supplied to the amplifier  12  as a power supply voltage. When the voltage output by the signal source  13  falls below the output threshold voltage of the DC voltage supply  23 - 3 , which is slightly lower than the voltage output by the DC voltage supply  15 - 2  (15V), the voltage output by the DC voltage supply  15 - 2  (15V) can be supplied to the amplifier  12  as a power supply voltage. 
         [0046]    When the voltage output by the signal source  13  falls below the output threshold voltage of the DC voltage supply  23 - 2 , which is slightly lower than the voltage output by the DC voltage supply  15 - 1  (10V), the voltage output by the DC voltage supply  15 - 1  (10V) can be supplied to the amplifier  12  as a power supply voltage. When the voltage output by the signal source  13  falls below the output threshold voltage of the DC voltage supply  23 - 1 , which is slightly lower than the voltage output by the DC voltage supply  21  (5V), the voltage output by the DC voltage supply  21  (5V) can be supplied to the amplifier  12  as a power supply voltage. 
         [0047]    In the following description, a case that the power supply voltage supplied to the amplifier changes in a stepwise manner is compared with a case that the power supply voltage supplied to the amplifier is constant (25V) in terms of the amount of voltage that the power supply circuit expends in the load circuit (amplifier  12 ). 
         [0048]      FIG. 3  is a diagram ( 1 ) showing the results of a simulation of power supplied to the amplifier under the condition that the power supply voltage supplied to the amplifier changes in a stepwise manner and under the condition that the power supply voltage supplied to the amplifier is constant (25V). 
         [0049]      FIG. 3  assumes that the maximum amplitude of the input signal of the amplifier (output of the signal source) is approximately the same as the maximum value of the voltage that can be output from the power supply circuit. 
         [0050]    Given that the effective value V o  of the input signal of the amplifier is 17Vrms and the load resistance of the amplifier is 7Ω, the current I o , flowing into the amplifier is 17Vrms/70. When the power supply voltage supplied to the amplifier is V in  the power P supplied to the amplifier can be represented in the following equation. 
         [0000]        P=V   in   ×I   o   (1) 
         [0051]    According to the above equation (1), each of the supplied power P(DC) in a case of the power supply voltage supplied to the amplifier being a constant value (25V) and the supplied power P(stepwise) in a case of the power supply voltage supplied to the amplifier changing in a stepwise manner is calculated as follows. 
         [0000]        P ( DC )=60.7 W 
         [0000]        P (stepwise)=55.6 W 
         [0052]    The output power P o  of the amplifier can be represented by the following equation. 
         [0000]        P   o   =V   o   ×I   o   (2) 
         [0053]    According to the above equation (2), the output power P o  can be calculated as follows. 
         [0000]        P   o =17Vrms×17Vrms/7Ω=41.3 W 
         [0054]    Therefore, each of a loss Pr (DC) when the power supply voltage supplied to the amplifier is a constant value (25V) and a loss Pr (stepwise) when the power supply voltage supplied to the amplifier changes in a stepwise manner is calculated as follows. 
         [0000]        Pr ( DC )=60.7 W−41.3 W=19.4 W 
         [0000]        Pr (stepwise)=55.6 W−41.3 W=14.3 W 
         [0055]    In addition, each of an efficiency η(DC) when the power supply voltage supplied to the amplifier is a constant value (25V) and an efficiency η(stepwise) when the power supply voltage supplied to the amplifier changes in a stepwise manner is calculated as follows. 
         [0000]      η( DC )=41.3 W/60.7 W=68% 
         [0000]      η(stepwise)=41.3 W/55.6 W=74% 
         [0056]      FIG. 4  is a diagram ( 2 ) showing the results of a simulation of power supplied to the amplifier under the condition that the power supply voltage supplied to the amplifier changes in a stepwise manner and under the condition that the power supply voltage supplied to the amplifier is constant (25V). 
         [0057]      FIG. 4  assumes that the maximum amplitude of the input signal of the amplifier (output of the signal source) is approximately half of the maximum value of the voltage that can be output from the power supply circuit. 
         [0058]    Given that the effective value V o  of the input signal of the amplifier is 8Vrms and the load resistance of the amplifier is 7Ω, the current I o  flowing into the amplifier is 8Vrms/7Ω. 
         [0059]    According to the above equation (1), each of the supplied power P (DC) in a case of the power supply voltage supplied to the amplifier being a constant value (25V) and the supplied power P(stepwise) in a case of the power supply voltage supplied to the amplifier changing in a stepwise manner is calculated as follows. 
         [0000]        P ( DC )=28.6 W 
         [0000]        P (stepwise)=14.8 W 
         [0060]    According to the above equation (2), the output power P o  can be calculated as follows. 
         [0000]        P   o =8Vrms×8Vrms/7Ω2=9.1 W 
         [0061]    Therefore, each of a loss Pr (DC) when the power supply voltage supplied to the amplifier is a constant value (25V) and a loss Pr (stepwise) when the power supply voltage supplied to the amplifier changes in a stepwise manner is calculated as follows. 
         [0000]        Pr ( DC )=28.6 W−9.1 W=19.5 W 
         [0000]        Pr (stepwise)=14.8 W−9.1 W=5.7 W 
         [0062]    In addition, each of an efficiency η(DC) when the power supply voltage supplied to the amplifier is a constant value (25V) and an efficiency η(stepwise) when the power supply voltage supplied to the amplifier changes in a stepwise manner is calculated as follows. 
         [0000]      η( DC )=9.1 W/28.6 W=32% 
         [0000]      η(stepwise)=9.1 W/14.8 W=61% 
         [0063]    As seen in the above, under the assumption that the maximum amplitude of the input signal of the amplifier (output of the signal source) is approximately the same as the maximum value of the voltage that can be output from the power supply circuit, if the power supply voltage supplied to the amplifier is changed in a stepwise manner, the efficiency is improved for 6% (74%-68%) compared with the case that the power supply voltage supplied to the amplifier is constant. 
         [0064]    Furthermore, under the assumption that the maximum amplitude of the input signal of the amplifier (output of the signal source) is approximately half of the maximum value of the voltage that can be output from the power supply circuit, if the power supply voltage supplied to the amplifier is changed in a stepwise manner, the efficiency is improved for 29% (61%-32%) compared with the case that the power supply voltage supplied to the amplifier is constant. 
         [0065]    In other words, by supplying the power supply voltage to an amplifier that rarely outputs a large value using the power supply circuit of the present embodiment, a great improvement of the efficiency of the amplifier can be achieved. 
         [0066]    As shown in  FIG. 2 , each of the diodes  17 - 1 ,  17 - 2 ,  17 - 3 ,  17 - 4  protects the respective DC voltage supplies  15 - 1 - 15 - 4  by preventing the current from flowing back to the respective switches  16 - 1 - 16 - 4  and flowing ultimately to the corresponding DC voltage supplies  15 - 1 - 15 - 4  if the respective switches  16 - 1  and  16 - 4  are allowed to conduct. In addition, diode  22  prevents current from flowing back to the DC power supply  21 . 
         [0067]    As previously discussed, if the output voltage of the signal source  13  is equal to or higher than the output threshold voltage of the DC voltage supply  23 - 3  but lower than the output threshold voltage of the DC voltage supply  23 - 4 , then the voltage output by the DC voltage supply  15 - 3  (20V) can be supplied to the amplifier  12  as a power supply voltage. In this case, the output voltage of the signal source  13  would be greater than the output threshold voltages of each of the DC voltage supplies  23 - 1 ,  23 - 2  and  23 - 3 . Therefore, the output voltage of the signal source  13  would be greater than the thresholds of each of the comparators  24 - 1 ,  24 - 2  and  24 - 3  and each of the switches (transistors)  16 - 1 ,  16 - 2  and  16 - 3  would be set to conduct. However, diodes  17 - 1  and  17 - 2  would prevent the current flowing from the DC voltage supply  15 - 3  to the load device (amplifier  12 ) from flowing back through the switches (transistors)  16 - 1  and  16 - 2  and to the DC voltage supplies  15 - 1  and  15 - 2 , respectively. In addition, diode  17 - 4  would prevent current from flowing back to switch (transistor)  16 - 4 . However, since switch (transistor)  16 - 4  is not set to conduct, diode  17 - 4  may be omitted. 
         [0068]      FIG. 5  shows a first modification of the configuration of the controller  14  of  FIGS. 1 and 2 . In  FIG. 5 , compared with  FIG. 2 , the diode  17 - 4  has been omitted, since the power supply circuit according to the first embodiment is able to operate without the diode  17 - 4 . That is, the switch (transistor)  16 - 4  is switched to conduct only when the output voltage of the signal source  13  is greater than the threshold of the DC voltage supply  23 - 4 . In this case, the current flows from the DC voltage supply  15 - 4  to the load  12  via switch (transistor)  16 - 4 . In the other cases, when the output voltage of the signal source  13  is lower than the threshold of the DC voltage supply  23 - 4 , the switch (transistor)  16 - 4  is not switched to conduct (open) and therefore the current is precluded from following back to the DC voltage supply  15 - 4  by the switch (transistor)  16 - 4 . 
         [0069]      FIGS. 6 and 7  shows a second modification of the configuration of the controller  14  of  FIGS. 1 and 2 , respectively. In  FIGS. 6 and 7 , compared with  FIGS. 1 and 2 , the controller  14  includes a central processing unit (CPU)  100 , but diodes  17 - 1 - 17 - 4  have been omitted. The CPU  100  monitors the output of each of the comparators  24 - 1 - 24 - 4  and based on such outputs selects a single switch (transistor) to conduct and prohibits the remaining switches from conducting, or prohibits all the switches from conducting. Therefore, the diodes  17 - 1 - 17 - 4  are not necessary, since while current flows through the selected switch, the remaining non-selected switches block the current from flowing back to and damaging the respective DC voltage supplies. 
         [0070]    When the voltage that the signal source  13  outputs gradually increases, the power supply circuit performs the following operations. For example, as illustrated in  FIG. 8 , in step S 1 , the CPU  100  determines whether the comparator  24 - 1  outputs a signal (whether the output voltage of the signal source  13  is equal to or higher than the output threshold voltage of the DC voltage supply  23 - 1 ). If the comparator  24 - 1  does not output a signal, the CPU  100  in step S 2  prohibits all of the switches  16 - 1 - 16 - 4  from conducting and the default voltage (5V) output by the DC voltage supply  21  is supplied to the load device (amplifier  12 ) as a power supply voltage. For example, as shown in the timing diagram of  FIG. 9  before time t 1 , the default voltage (5V) is supplied as the power supply voltage to the load device (amplifier  12 ). 
         [0071]    If the comparator  24 - 1  does output a signal, then the CPU  100  determines in step S 3  whether the comparator  24 - 2  outputs a signal (whether the output voltage of the signal source  13  is equal to or higher than the output threshold voltage of the DC voltage supply  23 - 2 ). If comparator  24 - 2  does not output a signal, then in step S 4  the CPU  100  sends a signal to switch  16 - 1  to allow it to conduct and sends a signal to each of the remaining switches  16 - 2 ,  16 - 3  and  16 - 4  to prohibit them from conducting. For example, as shown in the timing diagram of  FIG. 9 , at time t 1 , when the switch  16 - 1  is set to conduct, the 10V output voltage of the DC voltage supply  15 - 1  is supplied as the power supply voltage to the load device (amplifier  12 ). 
         [0072]    If the CPU  100  determines in step S 3  that the comparator  24 - 2  does output a signal, then the CPU  100  determines in step S 5  whether comparator  24 - 3  outputs a signal (whether the output voltage of the signal source  13  is equal to or higher than the output threshold voltage of the DC voltage supply  23 - 3 ). If the comparator  24 - 3  does not output a signal, then in step S 6  the CPU  100  sends a signal to switch  16 - 2  to allow it to conduct and sends a signal to each of the remaining switches  16 - 1 ,  16 - 3  and  16 - 4  to prohibit them from conducting. For example, as shown in the timing diagram of  FIG. 9 , at time t 2  when the switch  16 - 2  is set to conduct, the 15V output voltage of the DC voltage supply  15 - 2  is supplied as the power supply voltage to the load device (amplifier  12 ). 
         [0073]    If the CPU  100  determines in step S 5  that the comparator  24 - 3  does output a signal, then the CPU determines in step S 7  whether comparator  24 - 4  outputs a signal (whether the output voltage of the signal source  13  is equal to or higher than the output threshold voltage of the DC voltage supply  23 - 4 ). If comparator  24 - 4  does not output a signal, then in step S 8  the CPU  100  sends a signal to switch  16 - 3  to allow it to conduct and sends a signal to each of the remaining switches  16 - 1 ,  16 - 2  and  16 - 4  to prohibit them from conducting. For example, as shown in the timing diagram of  FIG. 9 , at time t 3  when the switch  16 - 3  is set to conduct, the 20V output voltage of the DC voltage supply  15 - 3  is supplied as the power supply voltage to the load device (amplifier  12 ). 
         [0074]    However, unlike the power supply circuit of  FIG. 2 , when the 20V output voltage of the DC voltage supply  15 - 3  is supplied as the power supply voltage to the load device (amplifier  12 ) only switch  16 - 3  is set to conduct. Therefore, diodes  17 - 1  and  17 - 2  are not needed in  FIG. 7  to prevent the current flowing from the DC voltage supply  15 - 3  to the load device (amplifier  12 ) from flowing back through to the respective DC voltage supplies  15 - 1  and  15 - 2 , since the switches (transistors)  16 - 1  and  16 - 2  do not conduct. 
         [0075]    If the CPU  100  determines in step S 7  that the comparator  24 - 4  does output a signal, then in step S 9  the CPU  100  sends a signal to switch  16 - 4  to allow it to conduct and sends a signal to each of the remaining switches  16 - 1 ,  16 - 2  and  16 - 3  to prohibit them from conducting. For example, as shown in the timing diagram of  FIG. 9 , at time t 4  when the switch  16 - 4  is set to conduct, the 25V output voltage of the DC voltage supply  15 - 4  is supplied as the power supply voltage to the load device (amplifier  12 ). 
         [0076]    Additionally, when the voltage output by the signal source  13  gradually decreases, the power supply circuit performs the following operations. For example, when the CPU  100  determines that the comparator  24 - 4  no longer outputs a signal in step S 7 , the CPU  100  in step S 8  sends a signal to switch  16 - 3  to allow it to conduct and sends a signal to each of the remaining switches  16 - 1 ,  16 - 2  and  16 - 4  to prohibit them from conducting. For example, as shown in the timing diagram of  FIG. 9 , at time t 5  when the switch  16 - 3  is set to conduct, the 20V output voltage of the DC voltage supply  15 - 3  is supplied as the power supply voltage to the load device (amplifier  12 ). 
         [0077]    Next, when the CPU  100  determines that the comparator  24 - 3  no longer outputs a signal in step S 5 , the CPU  100  in step S 6  sends a signal to switch  16 - 2  to allow it to conduct and sends a signal to each of the remaining switches  16 - 1 ,  16 - 3  and  16 - 4  to prohibit them from conducting. For example, as shown in the timing diagram of  FIG. 9 , at time t 6  when the switch  16 - 2  is set to conduct, the 15V output voltage of the DC voltage supply  15 - 2  is supplied as the power supply voltage to the load device (amplifier  12 ). 
         [0078]    Next, when the CPU  100  determines that the comparator  24 - 2  no longer outputs a signal in step S 3 , the CPU  100  in step S 4  sends a signal to switch  16 - 1  to allow it to conduct and sends a signal to each of the remaining switches  16 - 2 ,  16 - 3  and  16 - 4  to prohibit them from conducting. For example, as shown in the timing diagram of  FIG. 9 , at time t 7  when the switch  16 - 1  is set to conduct, the 10V output voltage of the DC voltage supply  15 - 1  is supplied as the power supply voltage to the load device (amplifier  12 ). 
         [0079]    Finally, when the CPU  100  determines that the comparator  24 - 1  no longer outputs a signal in step S 1 , the CPU  100  in step S 2  sends a signal to each of the switches  16 - 1 ,  16 - 2 ,  16 - 3  and  16 - 4  to prohibit them from conducting. For example, as shown in the timing diagram of  FIG. 9 , at time t 8  when none of the switches conduct, the 5V output default voltage of the DC voltage supply  21  is supplied as the power supply voltage to the load device (amplifier  12 ). 
         [0080]      FIG. 10  is a diagram of an overview configuration of the power supply circuit relating to the second embodiment of the present invention. 
         [0081]    In the first embodiment, the DC voltage supplies  15 - 1 ,  15 - 2 ,  15 - 3  and  15 - 4  are arranged in parallel. However, in the second embodiment, the DC voltage supplies  31 - 1 ,  31 - 2 ,  31 - 3  and  31 - 4  are loaded in series. As a result, it is possible to improve the use efficiency of the DC voltage supply. 
         [0082]    Because operations of the switches  16 - 1 ,  16 - 2 ,  16 - 3  and  16 - 4  and the controller  14  are the same as those in the first embodiment, the explanations are omitted. 
         [0083]    Further, as shown in  FIG. 10 , the second embodiment, like the first embodiment, also includes diodes  17 - 1 ,  17 - 2 ,  17 - 3 ,  17 - 4  each of which protects the respective DC voltage supplies  31 - 1 - 31 - 4  by preventing the current from flowing back to the respective switches  16 - 1 - 16 - 4  and flowing ultimately to the corresponding DC voltage supplies  15 - 1 - 15 - 4  if the respective switches  16 - 1  and  16 - 4  are allowed to conduct. 
         [0084]    However, in the second embodiment, the diodes  17 - 1 ,  17 - 2  and  17 - 3  provide the additional function of preventing a short circuit if more than one of the switches  16 - 1 - 16 - 4  are set to conduct. For example, if the output voltage of the signal source  13  is equal to or higher than the output threshold voltage of the DC voltage supply  23 - 3 , then the voltage output by the DC voltage supply  31 - 3  (20V) can be supplied to the amplifier  12  as a power supply voltage. In this case, the output voltage of the signal source  13  would be greater than the output threshold voltages of each of the DC voltage supplies  23 - 1 ,  23 - 2  and  23 - 3 . Therefore, the output voltage of the signal source  13  would be greater than the thresholds of each of the comparators  24 - 1 ,  24 - 2  and  24 - 3  and each of the switches (transistors)  16 - 1 ,  16 - 2  and  16 - 3  would be set to conduct. However, if diodes  17 - 1  and  17 - 2  were omitted from the power supply circuit, then the 10V from the power supply  31 - 1  would be supplied as the power supply voltage to the load (amplifier  12 ) instead of the 20V from the DC voltage supply  31 - 3 , since switch  16 - 1  would be closed, which would cause a short circuit from the DC power supply voltage  31 - 1  to the load (amplifier  12 ). In other words, in the example shown in  FIG. 10 , without diodes  17 - 1 - 17 - 3 , the power supply voltage would always be 10V from the DC voltage supply  31 - 1 . 
         [0085]      FIG. 11  shows a first modification (1) of the second embodiment of the present invention. In  FIG. 11  compared with  FIG. 10 , the diode  17 - 4  has been omitted, since the power supply circuit according to the second embodiment is able to operate without the diode  17 - 4 . That is, the switch (transistor)  16 - 4  is switched to conduct only when the output voltage of the signal source  13  is greater than the threshold of the DC voltage supply  23 - 4 . In this case, the current flows from the DC voltage supply  31 - 4  to the load  12 . In the other cases, when the output voltage of the signal source  13  is lower than the threshold of the DC voltage supply  23 - 4 , the switch (transistor)  16 - 4  is not switched to conduct (open) and therefore the current is precluded from following back to and damaging the DC voltage supply  31 - 4  by the switch (transistor)  16 - 4 . 
         [0086]      FIG. 12  shows a second modification (2) of the second embodiment of the present invention. In  FIG. 12 , compared with  FIGS. 10 and 11 , the controller  14  includes a central processing unit (CPU)  100 , but the diodes  17 - 1 - 17 - 4  have been omitted. Because operations of the CPU  100  and the switches  16 - 1 - 16 - 4  are the same as those in the first embodiment, the explanations are omitted. 
         [0087]    Note that in the power supply circuit of the present embodiment, there is a problem that even when the power supply voltage supplied to the amplifier is switched to decrease in a stepwise manner as the voltage input to the amplifier decrease, the power supply voltage supplied to the amplifier is maintained due to the capacity of the amplifier, and the amplifier is burdened with the decreased amount of the extra voltage. 
         [0088]    In the following description, the solution of the problem is explained with reference to  FIG. 13-FIG .  16 . 
         [0089]      FIG. 13  shows a modification example (3) of the power supply circuit of the second embodiment according to the present invention. 
         [0090]    In the power supply circuit shown in  FIG. 13 , a load resistor  33  and a switch (transistor)  32  are added in order to adjust the power supply voltage supplied to the amplifier  12 . The controller  14  additionally comprises a determination circuit (not shown in the drawing) for determining whether the power supply voltage supplied to the amplifier  12  is decreased or not by referencing to outputs of the comparator  24 - 1 ,  24 - 2 ,  24 - 3  and  24 - 4 , for example. When the determination circuit determines that the power supply voltage supplied to the amplifier  12  is decreased, by allowing the switch  32  to conduct, the power supply voltage supplied to the amplifier  12  is consumed by the load resistor  33 . 
         [0091]    As described above, when the determination circuit determines that the power supply voltage supplied to the amplifier  12  is decreased, by removing the power supply voltage supplied to the amplifier  12 , it is possible to prevent the power supply voltage from being maintained due to the capacity C (shown by the dashed lines in  FIG. 13 ) of the amplifier  12 . 
         [0092]    Alternatively, diodes  41 - 1 ,  41 - 2 ,  41 - 3  and  41 - 4  provided respectively in parallel to the diodes  17 - 1 ,  17 - 2 ,  17 - 3  and  17 - 4  and a comparator  42  can be added instead of adding the determination circuit in the controller  14  as shown in  FIG. 14 . 
         [0093]    In such a case, the comparator  42  compares a power supply voltage that went through the diodes  41 - 1   41 - 2 ,  41 - 3  and  41 - 4  (a first power supply voltage) with a power supply voltage in the input side of the amplifier  12  (a second power supply voltage), determines that the power supply voltage supplied to the amplifier  12  is decreased if the first power supply voltage closer to the DC voltage supplies  31 - 1 ,  31 - 2 ,  31 - 3  and  31 - 4  is smaller than the second power supply voltage, and outputs a signal to operate the switch  32 . As a result, the power supply voltage supplied to the amplifier  12  can be consumed by the load resistor  33 . 
         [0094]      FIG. 15  shows a modification example (4) of the power supply circuit of the second embodiment according to the present invention. 
         [0095]    In the power supply circuit shown in  FIG. 15 , a DC/DC converter  34  is added in order to increase the power supply voltage supplied to the amplifier  12 . The DC/DC converter  34  sends back the increased power supply voltage of the amplifier  12  to the controller  14  to be used as one of the DC voltage supplies. 
         [0096]    According to the above example, when it is determined that the power supply voltage supplied to the amplifier  12  is decreased, it is possible to remove the power supply voltage supplied to the amplifier  12 , and therefore, it is possible to prevent the power supply voltage from being maintained due to the capacity of the amplifier  12 . 
         [0097]    Note that in  FIG. 15 , the DC/DC converter  34  increases the power supply voltage supplied to the amplifier  12  to the value sliqhtly larger than the voltage value that the DC voltage supply  31 - 1  generates (10V) and outputs the voltage. Consequently, when the switch (transistor)  16 - 1  is operated, the output of the DC/DC converter  34  is used as the power supply voltage. 
         [0098]      FIG. 16  is a diagram showing further details of the main part of  FIG. 15 . 
         [0099]    The operations of the diodes  41 - 1 ,  41 - 2 ,  41 - 3  and  41 - 4  and the comparator  42  in  FIG. 16  are the same as those in  FIG. 14 , and therefore the explanations are omitted. 
         [0100]    In  FIG. 16 , the comparator  42  compares a power supply voltage that went through the diodes  41 - 1   41 - 2 ,  41 - 3  and  41 - 4  (a first power supply voltage) with a power supply voltage in the input side of the amplifier  12  (a second power supply voltage), determines that the power supply voltage supplied to the amplifier  12  is decreased if the first power supply voltage closer to the DC voltage supplies  31 - 1 ,  31 - 2 ,  31 - 3  and  31 - 4  is smaller than the second power supply voltage, and outputs a signal to start the DC/DC converter  34 . As a result, it is possible to further remove the power supply voltage supplied to the amplifier  12  via the DC/DC converter  34 . 
         [0101]    It should be noted that in the explanation above, the number of the DC voltage supplies is four; however, the number of the DC voltage supplies can be any number that is two and above. 
         [0102]    The configurations additionally explained in  FIG. 13-FIG .  16  can be implemented in the power supply circuit of the first embodiment as well as the power supply circuit of the second embodiment.