Patent Publication Number: US-6710584-B2

Title: Series regulator

Description:
FIELD OF THE INVENTION 
     The present invention relates to a series regulator that is used for obtaining a stabilized power source in a compact device like a portable telephone. 
     BACKGROUND OF THE INVENTION 
     Series regulators are provided in the form of ICs using bipolar transistors and unipolar transistors. Series regulators using bipolar transistors will be explained below as an example. 
     FIG. 5 is a circuit diagram showing a basic structure of a conventional series regulator. As shown in FIG. 5, a power transistor  503  is connected in series between an input terminal  501  to which a non-stabilized voltage Vin output from an external starting voltage source is applied and an output terminal  502  to which a stabilized voltage Vout is output. Input ends (emitters) of transistors E 1 , E 2  and E 3  that constitute a bias current circuit are connected to a line that connects the input terminal  501  and an input end (emitter) of the power transistor  503 . 
     The transistor E 1  and the transistors E 2  and E 3  that are in diode connection have their control ends (bases) connected in common to constitute a current mirror circuit. A constant-current source  504  is provided between an output end (collector) of the transistor E 1  and the ground. An output end (collector) of the transistor E 2  is connected to a reference voltage circuit  505  and a negative-phase input end of an amplifier  506 . An output end (collector) of the transistor E 3  is connected to a bias input end of the amplifier  506 . 
     A series circuit of resistors R 1  and R 2  is provided between a line that connects an output end (collector) of the power transistor  503  and an output terminal  502  and the ground. A control end of the reistors R 1  and R 2  is connected to a positive-phase input end of the amplifier  506 . An output end of the amplifier  506  is connected to a control end (base) of the power transistor  503 . 
     In the series regulator having the above structure, when the external starting voltage source has started operation, a constant bias current is supplied to the reference voltage circuit  505  based on a current mirror operation of the transistors E 1  and E 2 , and a reference voltage is supplied to the amplifier  506  from the reference voltage circuit  505 . At the same time, a bias current is supplied to the amplifier  506  from the transistor E 3 , and the amplifier  506  starts the operation of changing the internal resistance of the power transistor  503 . The output voltage of the power transistor  503  is supplied to the amplifier  506  after being divided by the series circuit of the resistors R 1  and R 2 . 
     As a result, the amplifier  506  changes the internal resistance of the power transistor  503  based on the result of a comparison between the magnitude of the reference output voltage and the magnitude of the divided voltage, and output a stable constant output voltage Vout from the output terminal  502 . As explained above, according to the conventional series regulator, the reference voltage circuit  505  and the amplifier  506  operate based on the bias current supplied from the input side. 
     However, when the power source of the external starting voltage source is turned on, the output voltage, that is, the input voltage Vin of the series regulator, varies in many cases, as shown in FIG. 6, for example. In this case, according to the conventional series regulator, the reference voltage circuit and the amplifier operate by receiving a supply of a bias current that varies following the variation in the input voltage Vin. Therefore, there occurs a fluctuation in the reference voltage, and a ripple is generated in the output voltage Vout as shown in FIG.  6 . This becomes one of factors that aggravates a ripple removal ratio. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a series regulator capable of reducing a ripple voltage that appears in the output voltage due to a variation in the input voltage during a normal operation after a stable voltage has been obtained following the turning-on of the power source, and capable of improving a ripple removal ratio of the series regulator. 
     The series regulator according to one aspect of the present invention comprises: a power transistor connected in series between an input terminal to which a non-stabilized voltage is applied and an output terminal; an amplifier for changing an internal resistance of the power transistor based on a result of a comparison between an output voltage of the power transistor and a reference voltage, and outputting a stabilized constant voltage to the output terminal; a first bias current circuit for generating a bias current to be supplied to a reference voltage circuit that generates the reference voltage, based on a non-stabilized voltage applied to the input terminal; a resistance voltage dividing circuit for generating a divided voltage of a predetermined value from an output voltage of the power transistor; an output voltage detecting circuit including a first transistor to a control end of which there is applied a conversion voltage of a bias current that the first bias current circuit supplies to the reference voltage circuit; and a second transistor to a control end of which there is applied the divided voltage, wherein the output voltage detecting circuit having a differential structure such that the second transistor is turned on and the first transistor is turned off when the divided voltage has reached a value of the conversion voltage; a second bias current circuit for generating a bias current to be supplied to the reference voltage circuit in response to the on-operation of the second transistor, based on an output voltage of the power transistor; and a bias switching circuit for stopping a bias-current supply operation of the first bias current circuit in response to a starting of the operation of the second bias current circuit. 
     Thus, when a non-stabilized voltage has been applied to an input terminal, a bias current is supplied to a reference voltage circuit from a first bias current circuit provided at the input side. Then, an amplifier starts the control of a power transistor. In an output voltage detecting circuit, a first transistor is applied with a conversion voltage of a bias current at its control end, and is turned on. When the output voltage of the power transistor rises, and a value of a divided voltage generated by a resistance voltage dividing circuit has reached a value of a conversion voltage of the bias current, a second transistor is turned on in the output voltage detecting circuit. Therefore, a second bias current circuit starts supplying a bias current to the reference voltage circuit. At the same time, a bias switching circuit operates to stop the bias-current supply operation of the first bias current circuit. 
     The series regulator according to another aspect of the present invention comprises: a power transistor connected in series between an input terminal to which a non-stabilized voltage is applied and an output terminal; an amplifier for changing an internal resistance of the power transistor based on a result of a comparison between an output voltage of the power transistor and a reference voltage, and outputting a stabilized constant voltage to the output terminal; a resistance voltage dividing circuit for generating a divided voltage of a predetermined value from an output voltage of the power transistor; a first bias current circuit for generating a bias current to be supplied to a reference voltage circuit that generates the reference voltage, based on a non-stabilized voltage applied to the input terminal, the first bias current circuit for supplying a bias current to the reference voltage circuit during a period while a first transistor to a control end of which a conversion voltage of the bias current is applied is in on-operation; and a second bias current circuit for generating a bias current to be supplied to the reference voltage circuit, based on an output voltage of the power transistor, the second bias current circuit for supplying a bias current to the reference voltage circuit during a period while a second transistor to a control end of which the divided voltage is applied is in on-operation, wherein the first bias current circuit and the second bias current circuit are differentially structured such that the second transistor is turned on when the divided voltage has reached a value of the conversion voltage, and the first transistor is turned off following this. 
     Thus, a first bias current circuit provided at an input side and a second bias current circuit provided at an output side are differentially structured. Therefore, when a non-stabilized voltage has been applied to an input end, a first transistor is turned on, and a bias current is supplied from the first bias current circuit to a reference voltage circuit. Then, an amplifier starts controlling a power transistor. The first transistor is applied with a conversion voltage of the bias current, and continues the on-operation. A second transistor of the second bias current circuit that is differentially structured is in an off-status. When the output voltage of the power transistor rises, and a value of a divided voltage generated by a resistance voltage dividing circuit has reached a value of a conversion voltage of the bias current, the second transistor is turned on. Therefore, the second bias current circuit starts supplying a bias current to the reference voltage circuit. On the other hand, in the first bias current circuit, the first transistor is turned off. Therefore, the first bias current circuit stops supplying the bias current to the reference voltage circuit. In other words, as the first bias current circuit provided at the input side and the second bias current circuit provided at the output side are differentially structured, these bias current circuits constitute a bias switching circuit as a total system. 
     The series regulator according to another aspect of the present invention comprises: a first power transistor connected in series between an input terminal to which a non-stabilized voltage is applied and a first output terminal; a first amplifier for changing an internal resistance of the first power transistor based on a result of a comparison between an output voltage of the first power transistor and a reference voltage, and outputting a stabilized constant voltage to the first output terminal; a second power transistor connected in series between the input terminal and a second output terminal; a second amplifier for changing an internal resistance of the second power transistor based on a result of a comparison between an output voltage of the second power transistor and the reference voltage, and outputting a stabilized constant voltage to the second output terminal; a first resistance voltage dividing circuit for generating a first divided voltage of a predetermined value from an output voltage of the first power transistor, and a second resistance voltage dividing circuit for generating a second divided voltage of a predetermined value different from the first divided voltage, from an output voltage of the second power transistor; a first bias current circuit for generating a bias current to be supplied to a reference voltage circuit that generates the reference voltage, based on a non-stabilized voltage applied to the input terminal, the first bias current circuit for supplying a bias current to the reference voltage circuit during a period while a first transistor to a control end of which a conversion voltage of the bias current is applied is in on-operation; a second bias current circuit for generating a bias current to be supplied to the reference voltage circuit, based on an output voltage of the first power transistor, the second bias current circuit for supplying a bias current to the reference voltage circuit during a period while a second transistor to a control end of which the first divided voltage is applied is in on-operation; and a third bias current circuit for generating a bias current to be supplied to the reference voltage circuit, based on an output voltage of the second power transistor, the third bias current circuit for supplying a bias current to the reference voltage circuit during a period while a third transistor to a control end of which the second divided voltage is applied is in on-operation, wherein the first bias current circuit, the second bias current circuit, and the third bias current circuit are differentially structured such that only a corresponding one of the second transistor and the third transistor is turned on when either the first divided voltage or the second divided voltage having a higher value has first reached a value of the conversion voltage, and the first transistor is turned off following this. 
     Thus, a first bias current circuit provided at an input side, a second bias current circuit provided at one output side, a third bias current circuit provided at the other output side are differentially structured. Therefore, when a non-stabilized voltage has been applied to an input end, a first transistor is turned on, and a bias current is supplied from the first bias current circuit to a reference voltage circuit. Then, a first amplifier starts controlling a first power transistor, and a second amplifier starts controlling a second power transistor. The first transistor is applied with a conversion voltage of the bias current, and continues the on-operation. A second transistor of the second bias current circuit and a third transistor of the third bias current circuit that are differentially structured are in an off-status. When the output voltages of the first and second power transistors rise, and when either a first divided voltage generated by a first resistance dividing circuit or a second divided voltage generated by a second resistance dividing circuit having a higher value has first reached a value of the conversion voltage, the corresponding one of the second transistor and the third transistor is turned on. The first transistor is turned off following this. As a result, a bias current is supplied to the reference voltage circuit from the corresponding one of the second bias current circuit and the third bias current circuit. At the same time, the first bias current circuit stops supplying the bias current. Stabilized voltages are output from the two output terminals respectively. In other words, as the first bias current circuit provided at the input side, the second bias current circuit provided at one output side, and the third bias current circuit provided at the other output side are differentially structured, these bias current circuits constitute a bias switching circuit as a total system. 
     Other objects and features of this invention will become apparent from the following description with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram showing a structure of a series regulator according to a first embodiment of the present invention; 
     FIG. 2 is a circuit diagram showing a structure of a series regulator according to a second embodiment of the present invention; 
     FIG. 3 is a circuit diagram showing a structure of a series regulator according to a third embodiment of the present invention; 
     FIG. 4 is a circuit diagram showing a structure of a series regulator according to a fourth embodiment of the present invention; 
     FIG. 5 is a circuit diagram showing a basic structure of a conventional series regulator; and 
     FIG. 6 is a diagram for explaining a relationship between an input voltage and an output voltage in a process of obtaining a constant output voltage after turning on a power source in the series regulator shown in FIG.  5 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiment/s of a series regulator relating to the present invention will be explained in detail below with reference to the accompanying drawings. 
     FIG. 1 is a circuit diagram showing a structure of a series regulator according to a first embodiment of the present invention. FIG. 1 shows only the structure that is related to the first embodiment. This similarly applies to other diagrams showing the rest of embodiments. 
     As shown in FIG. 1, a power transistor  13  is connected in series between an input terminal  11  to which a non-stabilized voltage Vin output from an external starting voltage source is applied and an output terminal  12  from which a stabilized voltage Vout is output. Input ends (emitters) of transistors A 1 , A 2 , A 3 , and A 4  that constitute a bias current circuit are connected to a line that connects between the input terminal  11  and an input end (emitter) of the power transistor  13 . 
     The transistor A 1  and the transistor A 2  that are in diode connection have their control ends (bases) connected in common to constitute a current mirror circuit. A constant-current source  14  is provided between an output end (collector) of the transistor A 1  and the ground. An output end (collector) of the transistor A 2  is connected to a bias switching circuit  15 . 
     The transistor A 3  and the transistor A 4  that are in diode connection have their control ends (bases) connected in common to constitute a current mirror circuit. A bias switching circuit  15  is provided between an output end (collector) of the transistor A 3  and the ground. An output end (collector) of the transistor A 4  is connected to a reference voltage circuit  16  via a resistor R 1 . 
     An input end (collector) of a transistor A 5  is connected to a line that connects between the input terminal  11  and an input end (emitter) of the power transistor  13 . A control end (base) of the transistor A 5  is connected to an output end (collector) of the transistor A 4  via a resistor R 2 . An output end (emitter) of the transistor A 5  and an output end (emitter) of a transistor A 6  are connected to an input end (collector) of a transistor A 7 . The transistors A 5  and A 6  constitute an output voltage detecting circuit  18 . 
     The transistor A 7  has its control end (base) connected to a control end (base) of a transistor A 8 , and has its output end (emitter) connected to the ground via a resistor R 3 . The transistor A 8  has its input end (collector) connected to a line that connects between the reference voltage circuit  16  and a negative-phase input end of an amplifier  17  via a resistor R 4 . The transistor A 8  has its output end (emitter) directly connected to the ground. 
     A series circuit of resistors R 5  and R 6  and a series circuit of resistors R 7  and R 8  are provided between a line that connects between an output end (collector) of the power transistor  13  and the output terminal  12  and the ground. A connection end of the resistors R 5  and R 6  is connected to a positive-phase input end of the amplifier  17 . An output end of the amplifier  17  is connected to a control end (base) of the power transistor  13 . A connection end of the resistors R 7  and R 8  is connected to a control end (base) of the transistor A 6  via a resistor R 9 . 
     Input ends (emitters) of transistors A 9 , A 10  and A 11  that constitute a bias current circuit are connected to a line that connects between an output end (collector) of the power transistor  13  and the output terminal  12 . The transistor A 9 , the transistor A 10 , and the transistor A 11  that are in diode connection have their control ends (bases) connected in common to constitute a current mirror circuit. 
     An output end (collector) of the transistor A 11  is connected to an input end (emitter) of the transistor A 6 . An output end (collector) of the transistor A 10  is connected to a line that connects between the reference voltage circuit  16  and the negative-phase input end of the amplifier  17 . An output end (collector) of the transistor A 9  is connected to the bias switching circuit  15 . 
     The bias switching circuit  15  includes a current mirror circuit constructed of a transistor A 12  and a transistor A 13  that are in diode connection, and a current mirror circuit constructed of a transistor A 14  and a transistor A 15  that are in diode connection. The transistor A 12  and the transistor A 13  that are in diode connection have their control ends (bases) connected in common, and have their output ends (emitters) directly connected to the ground respectively. The transistor A 14  and the transistor A 15  that are in diode connection have their control ends (bases) connected in common, and have their output ends (emitters) directly connected to the ground respectively. 
     An input end (collector) of the transistor A 12  in diode connection is connected to an output end (collector) of the transistor A 9 . An input end (collector) of the transistor A 13  and an input end (collector) of the transistor A 14  are connected to the output end (collector) of the transistor A 2 . An input end (collector) of the transistor A 15  is connected to the output end (collector) of the transistor A 3 . 
     The operation of the series regulator according to the first embodiment will be explained next. When the external starting voltage source has started operation, the current mirror circuit of the transistors A 1  and A 2  and the current mirror circuit of the transistors A 12  and A 13  operate to generate a constant current. Based on this, the current mirror circuit of the transistors A 3  and A 4  operates to supply a bias current from the transistor A 4  to the reference voltage circuit  16 . 
     As the current mirror circuit of the transistors A 7  and A 8  operates based on the above operation, a conversion voltage (constant voltage) of the bias current supplied is applied to the control end (base) of the transistor A 5 , and the transistor A 5  is turned on. 
     When the constant bias current has been supplied to the reference voltage circuit  16 , the reference voltage circuit  16  supplies a reference voltage to the negative-phase input end of the amplifier  17 . Although not shown, a bias current is supplied from the input side to the amplifier  17  at the same time, and the amplifier  17  starts the operation of changing the internal resistance of the power transistor  13 . The output voltage of the power transistor  13  is divided by the series circuit of the resistors R 5  and R 6 , and this divided voltage is supplied to the positive-phase input end of the amplifier  17 . Further, the output voltage of the power transistor  13  is dived by the series circuit of the resistors R 7  and R 8 , and this divided voltage is applied to the control end (base) of the transistor A 6 . 
     When the output voltage of the power transistor  13  rises, and a value of the divided voltage applied to the control end (base) of the transistor A 6  has risen to a value of a constant voltage applied to the control end (base) of the transistor A 5 , the transistor A 6  is turned on and the transistor A 5  is turned off in the output voltage detecting circuit  18 . 
     When the transistor A 6  has been turned on, a current flows to the control ends (bases) of the transistors A 9 , A 10  and A 11  respectively, and these transistors A 9 , A 10  and A 11  are turned on. The transistor A 11  starts supplying a bias current to the reference voltage circuit  16 . At the same time, the transistor A 9  is turned on. Therefore, the current mirror circuit of the transistors A 12  and A 13  of the bias switching circuit  10  starts operating. Then, a constant current that has so far been flowing to the transistor A 14  is taken into the transistor A 13 , and the current mirror circuit of the transistors A 14  and A 15  is turned off. 
     As a result, the current mirror circuit of the transistors A 3  and A 4  is turned off, and the supply of the bias current from the transistor A 4  to the reference voltage circuit  16  is interrupted. Thereafter, the output voltage Vout is constant even when there is a variation in the input voltage Vin. Therefore, the reference voltage circuit  16  receives a supply of the bias current having no variation from the transistor A 10  at the output side. 
     As explained above, according to the first embodiment, when the output voltage Vout has reached a predetermined voltage after the power source has been turned on, the supply source of the bias current is switched immediately from the input side to the output side. Therefore, it is possible to reduce the influence on the reference voltage due to the variation in the input voltage. As a result, it is possible to reduce a ripple voltage that appears in the output voltage due to the variation in the input voltage, during a normal operation after a stable output voltage has been obtained following the turning-on of the power source. Consequently, it is possible to improve the ripple removal ratio of the series regulator. 
     FIG. 2 is a circuit diagram showing a structure of a series regulator according to a second embodiment of the present invention. As shown in FIG. 2, a power transistor  13  is connected in series between an input terminal  11  to which a non-stabilized voltage Vin output from an external starting voltage source is applied and an output terminal  12  from which a stabilized voltage Vout is output. Input ends (emitters) of transistors B 1  and B 2  that constitute a bias current circuit are connected to a line that connects between the input terminal  11  and an input end (emitter) of the power transistor  13 . 
     The transistor B 1  and the transistor B 2  that are in diode connection have their control ends (bases) connected in common to constitute a current mirror circuit. An output end (collector) of the transistor B 1  is connected to a reference voltage circuit  16  via a resistor R 1 , and is also connected to a negative-phase input end of an amplifier  17 . An output end (collector) of the transistor B 2  in diode connection is connected to an input end (collector) of a transistor B 3 . 
     The transistor B 3  has its control end (base) connected to an output end (collector) of the transistor B 1  via a resistor R 2 . An output end (emitter) of the transistor B 3  and an output end (emitter) of the transistor B 4  are connected to an input end (collector) of a transistor B 5 . The transistor B 5  has its control end (base) connected to a control end (base) of a transistor B 6  in diode connection, and has its output end (emitter) connected to the ground via a resistor R 3 . The transistor B 6  has its input end (collector) connected to a line that connects between the reference voltage circuit  16  and the negative-phase input end of the amplifier  17  via a resistor R 4 . An output end (emitter) of the transistor B 6  is directly connected to the ground. 
     A series circuit of resistors R 5  and R 6  and a series circuit of resistors R 7  and R 8  are provided between a line that connects between an output end (collector) of the power transistor  13  and the output terminal  12  and the ground. A connection end of the resistors R 5  and R 6  is connected to a positive-phase input end of the amplifier  17 . An output end of the amplifier  17  is connected to a control end (base) of the power transistor  13 . A connection end of the resistors R 7  and R 8  is connected to a control end (base) of the transistor B 4  via a resistor R 9 . 
     Input ends (emitters) of transistors B 7  and B 8  that constitute a bias current circuit are connected to a line that connects between an output end (collector) of the power transistor  13  and the output terminal  12 . The transistor B 7  and the transistor B 8  that are in diode connection have their control ends (bases) connected in common to constitute a current mirror circuit. 
     An output end (collector) of the transistor B 7  is connected to a line that connects between the reference voltage circuit  16  and the negative-phase input end of the amplifier  17 . An output end (collector) of the transistor B 8  in diode connection is connected to an input end (emitter) of the transistor B 4 . The transistors B 1  to B 4 , B 7  and B 8  constitute a bias switching circuit. 
     The operation of the series regulator according to the second embodiment will be explained next. When the external starting voltage source has started operation, the current mirror circuit of the transistors B 5  and B 6  operates to generate a constant current. Based on this, the current mirror circuit of the transistors B 1  and B 2  operates to supply a bias current from the transistor B 1  to the reference voltage circuit  16 . Based on this, a conversion voltage (constant voltage) of the bias current supplied is applied to the control end (base) of the transistor B 3 , and the transistor B 3  is turned on. 
     When the constant bias current has been supplied to the reference voltage circuit  16 , the reference voltage circuit  16  supplies a reference voltage to the negative-phase input end of the amplifier  17 . Although not shown, a bias current is supplied from the input side to the amplifier  17  at the same time, and the amplifier  17  starts the operation of changing the internal resistance of the power transistor  13 . The output voltage of the power transistor  13  is divided by the series circuit of the resistors R 5  and R 6 , and this divided voltage is supplied to the positive-phase input end of the amplifier  17 . Further, the output voltage of the power transistor  13  is dived by the series circuit of the resistors R 7  and R 8 , and this divided voltage is applied to the control end (base) of the transistor B 4 . 
     When the output voltage of the power transistor  13  rises, and a value of the divided voltage applied to the control end (base) of the transistor B 4  has risen to a value of a constant voltage applied to the control end (base) of the transistor B 3 , the transistor B 4  is turned on and the transistor B 3  is turned off. 
     When the transistor B 4  has been turned on, a current flows to the control ends (bases) of the transistors B 7  and B 8  respectively, and these transistors B 7  and B 8  are turned on. The transistor B 7  starts supplying a bias current to the reference voltage circuit  16 . At the same time, the transistor B 3  is turned off. Therefore, the current mirror circuit of the transistors B 1  and B 2  is turned off. 
     As a result, the supply of the bias current from the transistor B 1  to the reference voltage circuit  16  is interrupted. Thereafter, the output voltage Vout is constant even when there is a variation in the input voltage Vin. Therefore, the reference voltage circuit  16  receives a supply of the bias current having no variation from the transistor B 7  at the output side. 
     As explained above, according to the second embodiment, when the output voltage Vout has reached a predetermined voltage after the power source has been turned on, the supply source of the bias current is switched immediately from the input side to the output side, using a smaller number of elements than that in the first embodiment. Therefore, it is possible to reduce the influence on the reference voltage due to the variation in the input voltage, in a similar manner to that of the first embodiment. As a result, it is possible to reduce a ripple voltage that appears in the output voltage due to the variation in the input voltage, during a normal operation after a stable output voltage has been obtained following the turning-on of the power source. Consequently, it is possible to improve the ripple removal ratio of the series regulator. 
     FIG. 3 is a circuit diagram showing a structure of a series regulator according to a third embodiment of the present invention. The third embodiment shows an example of a structure of a series regulator that can obtain two outputs. 
     As shown in FIG. 3, a power transistor  13  is connected in series between an input terminal  11  to which a non-stabilized voltage Vin output from an external starting voltage source is applied and an output terminal  12  from which a stabilized voltage Vout 1  is output. A power transistor  31  is connected in series between the input terminal  11  and an output terminal  30  from which a stabilized voltage Vout 2  is output. An amplifier  32  is provided following this. One reference voltage circuit  16  can be used in common. 
     Input ends (emitters) of transistors C 1  and C 2  that constitute a bias current circuit are connected to a line that connects between the input terminal  11  and an input end (emitter) of the power transistor  13 . The transistor C 1  and the transistor C 2  that are in diode connection have their control ends (bases) connected in common to constitute a current mirror circuit. An output end (collector) of the transistor C 1  is connected to the reference voltage circuit  16  via a resistor R 1 , and is also connected to a negative-phase input end of an amplifier  17 . An output end (collector) of the transistor C 2  in diode connection is connected to an input end (collector) of a transistor C 3 . 
     The transistor C 3  has its control end (base) connected to an output end (collector) of the transistor C 2  via a resistor R 2 . An output end (emitter) of the transistor C 3  and an output end (emitter) of the transistor C 4  are connected to an input end (collector) of a transistor C 5 . The transistor C 5  has its control end (base) connected to a control end (base) of a transistor C 6  in diode connection, and has its output end (emitter) connected to the ground via a resistor R 3 . The transistor C 6  has its input end (collector) connected to a line that connects between the reference voltage circuit  16  and the negative-phase input end of the amplifier  17  via a resistor R 4 . An output end (emitter) of the transistor C 6  is directly connected to the ground. 
     A series circuit of resistors R 5  and R 6  and a series circuit of resistors R 7  and R 8  are provided between a line that connects between an output end (collector) of the power transistor  13  and the output terminal  12  and the ground. A connection end of the resistors R 5  and R 6  is connected to a positive-phase input end of the amplifier  17 . An output end of the amplifier  17  is connected to a control end (base) of the power transistor  13 . A connection end of the resistors R 7  and R 8  is connected to a control end (base) of the transistor C 4  via a resistor R 9 . 
     Input ends (emitters) of transistors C 7  and C 8  that constitute a bias current circuit are connected to a line that connects between an output end (collector) of the power transistor  13  and the output terminal  12 . The transistor C 7  and the transistor C 8  that are in diode connection have their control ends (bases) connected in common to constitute a current mirror circuit. 
     An output end (collector) of the transistor C 7  is connected to a line that connects between the reference voltage circuit  16  and the negative-phase input end of the amplifier  17 . An output end (collector) of the transistor C 8  in diode connection is connected to an input end (emitter) of the transistor C 4 . The transistors C 1  to C 4 , C 7  and C 8  constitute a bias switching circuit  33 . 
     Further, a series circuit of resistors R 10  and R 11  and a series circuit of resistors R 12  and R 13  are provided between a line that connects between an output end (collector) of the power transistor  31  and the output terminal  30  and the ground. A connection end of the resistors R 10  and R 11  is connected to a positive-phase input end of the amplifier  32 . An output end of the amplifier  32  is connected to a control end (base) of the power transistor  31 . A connection end of the resistors R 12  and R 13  is connected to a control end (base) of the transistor C 9  via a resistor R 13 . 
     Input ends (emitters) of transistors C 10  and C 11  that constitute a bias current circuit are connected to a line that connects between an output end (collector) of the power transistor  31  and the output terminal  30 . The transistor C 10  and the transistor C 11  that are in diode connection have their control ends (bases) connected in common to constitute a current mirror circuit. 
     An output end (collector) of the transistor C 10  is connected to a negative-phase input end of the amplifier  32 , and is also connected to a line that connects between the reference voltage circuit  16  and the negative-phase input end of the amplifier  17 . An output end (collector) of the transistor C 11  in diode connection is connected to an input end (emitter) of the transistor C 9 . The transistors C 9  to C 11  constitute a bias switching circuit  34 . 
     The operation of the series regulator according to the third embodiment will be explained next. When the external starting voltage source has started operation, the current mirror circuit of the transistors C 5  and C 6  operates to generate a constant current. Based on this, the current mirror circuit of the transistors C 1  and C 2  operates to supply a bias current from the transistor C 1  to the reference voltage circuit  16 . Based on this, a conversion voltage (constant voltage) of the bias current supplied is applied to the control end (base) of the transistor C 3 , and the transistor C 3  is turned on. 
     When the constant bias current has been supplied to the reference voltage circuit  16 , the reference voltage circuit  16  supplies a reference voltage to the negative-phase input ends of the amplifier  17  and the amplifier  32  respectively. Although not shown, a bias current is supplied at the same time from the input side to the amplifier  17  and the amplifier  32  respectively. Then, the amplifier  17  starts the operation of changing the internal resistance of the power transistor  13 , and the amplifier  32  starts the operation of changing the internal resistance of the power transistor  31 . 
     The output voltage of the power transistor  13  is divided by the series circuit of the resistors R 5  and R 6 , and this divided voltage is supplied to the positive-phase input end of the amplifier  17 . Further, the output voltage of the power transistor  13  is dived by the series circuit of the resistors R 7  and R 8 , and this divided voltage V 1  is applied to the control end (base) of the transistor C 4 . 
     Further, the output voltage of the power transistor  31  is divided by the series circuit of the resistors R 10  and R 11 , and this divided voltage is supplied to the positive-phase input end of the amplifier  32 . Further, the output voltage of the power transistor  31  is dived by the series circuit of the resistors R 12  and R 13 , and this divided voltage V 2  is applied to the control end (base) of the transistor C 9 . 
     In this case, resistances of the voltage-dividing circuit are set to have mutually different values for the divided voltages V 1  and V 2 . When the output voltages of the power transistors  13  and  31  rise, the divided voltages V 1  and V 2  also rise respectively. Either the divided voltage V 1  or V 2  that has a higher voltage first rises to a value of a constant voltage that is being applied to the control end (base) of the transistor C 3 . Therefore, only the transistor C 4  or C 9  that is applied with the high divided voltage (for example, the transistor C 4 ) is turned on, and the transistor C 3  is turned off following this. 
     When the transistor C 4  has been turned on, a current flows to the control ends (bases) of the transistors C 7  and C 8  respectively, and these transistors C 7  and C 8  are turned on. The transistor C 7  starts supplying a bias current to the reference voltage circuit  16 . At the same time, the transistor C 3  is turned off. Therefore, the current mirror circuit of the transistors C 1  and C 2  is turned off. 
     As a result, the supply of the bias current from the transistor C 1  to the reference voltage circuit  16  is interrupted. Thereafter, the output voltage Vout 1  is constant even when there is a variation in the input voltage Vin. Therefore, the reference voltage circuit  16  receives a supply of the bias current having no variation from the output side. A separate output voltage Vout 2  is obtained from the output terminal  30 . 
     As explained above, when the series regulator has been structured to obtain two outputs, it is also possible to switch immediately the supply source of the bias current from the input side to the output side when the output voltage Vout has reached a predetermined voltage after the power source has been turned on, like in the first and the second embodiments. Therefore, it is also possible to reduce the influence on the reference voltage due to the variation in the input voltage. 
     In the case of obtaining two outputs, when the operation of the power transistor that generates an output voltage for supplying a bias current to the reference voltage circuit  16  has been stopped by an external protection circuit, for example, the supply of the bias current is interrupted. In this case, the ripple removal ratio is aggravated. 
     In order to solve this problem, there is provided a switching circuit for switching the on/off operations between the transistors C 4  and C 9 , although not shown in the drawing. Assume, for example, the operation of the power transistor  13  has been stopped by an external protection circuit under a situation where the transistor C 4  is operating based on a size relationship of V 1 &gt;V 2  between the divided voltages V 1  and V 2 . Then, the size relationship between the divided voltages V 1  and V 2  changes to V 1 &lt;V 2 . As a result, the switching circuit detects the change in the size relationship between the divided voltages V 1  and V 2 , and immediately turns on the transistor C 9 . 
     As a bias current can be supplied immediately from the transistor C 10  to the reference voltage circuit  16 , it is possible to prevent the aggravation in the ripple removal ratio. 
     FIG. 4 is a circuit diagram showing a structure of a series regulator according to a fourth embodiment of the present invention. The fourth embodiment shows an example of a structure of a series regulator that can also switch a supply of a bias current to the amplifier. 
     As shown in FIG. 4, a power transistor  13  is connected in series between an input terminal  11  to which a non-stabilized voltage Vin output from an external starting voltage source is applied and an output terminal  12  from which a stabilized voltage Vout is output. Input ends (emitters) of transistors D 1 , D 2  and D 3  that constitute a bias current circuit are connected to a line that connects between the input terminal  11  and an input end (emitter) of the power transistor  13 . 
     The transistor D 1 , the transistor D 2  and the transistor D 3  that are in diode connection have their control ends (bases) connected in common to constitute a current mirror circuit. An output end (collector) of the transistor D 1  is connected to a bias current input end of an amplifier  17 . An output end (collector) of the transistor D 2  is connected to a reference voltage circuit  16  via a resistor R 1 , and is also connected to a negative-phase input end of the amplifier  17 . An output end (collector) of the transistor D 3  in diode connection is connected to an input end (collector) of a transistor D 4 . 
     The transistor D 4  has its control end (base) connected to an output end (collector) of the transistor D 2  via a resistor R 2 . An output end (emitter) of the transistor D 4  and an output end (emitter) of the transistor D 5  are connected to an input end (collector) of a transistor D 6 . The transistor D 6  has its control end (base) connected to a control end (base) of a transistor D 7  in diode connection, and has its output end (emitter) connected to the ground via a resistor R 3 . The transistor D 7  has its input end (collector) connected to a line that connects between the reference voltage circuit  16  and the negative-phase input end of the amplifier  17  via a resistor R 4 . An output end (emitter) of the transistor D 7  is directly connected to the ground. 
     A series circuit of resistors R 5  and R 6  and a series circuit of resistors R 7  and R 8  are provided between a line that connects between an output end (collector) of the power transistor  13  and the output terminal  12  and the ground. A connection end of the resistors R 5  and R 6  is connected to a positive-phase input end of the amplifier  17 . An output end of the amplifier  17  is connected to a control end (base) of the power transistor  13 . A connection end of the resistors R 7  and R 8  is connected to a control end (base) of the transistor D 5  via a resistor R 9 . 
     Input ends (emitters) of transistors D 8 , D 9 , and D 10  that constitute a bias current circuit are connected to a line that connects between an output end (collector) of the power transistor  13  and the output terminal  12 . The transistor D 8 , the transistor D 9 , and the transistor D 10  that are in diode connection have their control ends (bases) connected in common to constitute a current mirror circuit. 
     An output end (collector) of the transistor D 8  is connected to a line that connects between the reference voltage circuit  16  and the negative-phase input end of the amplifier  17 . An output end (collector) of the transistor D 9  is connected to a bias current input end of the amplifier  17 . An output end (collector) of the transistor D 10  in diode connection is connected to an input end (emitter) of the transistor D 5 . The transistors D 4  and D 5  constitute an output voltage detecting circuit  40 . 
     The operation of the series regulator according to the fourth embodiment will be explained next. When the external starting voltage source has started operation, the current mirror circuit of the transistors D 6  and D 7  operates to generate a constant current. Based on this, the current mirror circuit of the transistors D 1 , D 2  and D 3  operates to supply a bias current from the transistor D 1  to the amplifier  17 , and supply a bias current from the transistor D 2  to the reference voltage circuit  16 . As a result, a conversion voltage (constant voltage) of the bias current supplied is applied to the control end (base) of the transistor D 4 , and the transistor D 4  is turned on. 
     When the constant bias current has been supplied to the reference voltage circuit  16 , the reference voltage circuit  16  supplies a reference voltage to the negative-phase input end of the amplifier  17 . The amplifier  17  starts the operation of changing the internal resistance of the power transistor  13 . The output voltage of the power transistor  13  is divided by the series circuit of the resistors R 5  and R 6 , and this divided voltage is supplied to the positive-phase input end of the amplifier  17 . Further, the output voltage of the power transistor  13  is dived by the series circuit of the resistors R 7  and R 8 , and this divided voltage is applied to the control end (base) of the transistor D 5 . 
     When the output voltage of the power transistor  13  rises, and a value of the divided voltage applied to the control end (base) of the transistor D 5  has risen to a value of a constant voltage applied to the control end (base) of the transistor D 4 , the transistor D 5  is turned on and the transistor D 4  is turned off in the output voltage detecting circuit  40 . 
     When the transistor D 5  has been turned on, a current flows to the control ends (bases) of the transistors D 8 , D 9  and D 10  respectively, and these transistors D 8 , D 9  and D 10  are turned on. The transistor D 8  starts supplying a bias current to the reference voltage circuit  16 . The transistor D 9  starts supplying a bias current to the amplifier  17 . At the same time, the transistor D 4  is turned off. Therefore, the current mirror circuit of the transistors D 1 , D 2  and D 3  is turned off. 
     As a result, the supply of the bias current from the transistor D 1  to the amplifier  17  is interrupted. Further, the supply of the bias current from the transistor D 2  to the reference voltage circuit  16  is interrupted. Thereafter, the output voltage Vout is constant even when there is a variation in the input voltage Vin. Therefore, the reference voltage circuit  16  and the amplifier  17  receive a supply of the bias current having no variation from the output side respectively. 
     As explained above, according to the fourth embodiment, when the output voltage Vout has reached a predetermined voltage after the power source has been turned on, the supply source of the bias current is switched immediately from the input side to the output side. Therefore, it is possible to reduce the influence on the reference voltage due to the variation in the input voltage, more than that in the first to third embodiments. As a result, it is possible to reduce a ripple voltage that appears in the output voltage due to the variation in the input voltage, during a normal operation after a stable output voltage has been obtained following the turning-on of the power source. Consequently, it is possible to improve the ripple removal ratio of the series regulator. 
     In the fourth embodiment, FIG. 4 clearly shows a circuit that supplies a bias current from the input side to the amplifier, although this circuit is not shown in FIG. 1 to FIG. 3 that explain first to third embodiments. In the first to third embodiments, a bias current is also supplied from the input side to the amplifier in a similar circuit structure. In the fourth embodiment, there is also shown at the output side a transistor for supplying a bias current to the amplifier that is used in the second embodiment, and an example of the structure for the switching is also shown. 
     As is clear from the above explanation, in the first and third embodiments, it is also possible to provide at the output side a transistor for supplying a bias current to the amplifier, and employ a structure for switching the supply of a bias current to both the reference voltage circuit and the amplifier at the same time, in a similar method. As a result, it is possible to obtain more improved effects. 
     While the above embodiments show structures based on a bipolar transistor, the present invention is not limited to this, and it is also possible to construct a series regulator based on a unipolar transistor like FET and CMOS in a similar manner. It is needless to mention that these are also included within the scope of the present invention. 
     As explained above, according to one aspect of the present invention, when a non-stabilized voltage has been applied to an input terminal, a bias current is supplied to a reference voltage circuit from a first bias current circuit provided at the input side. Then, an amplifier starts the control of a power transistor. When the output voltage of the power transistor rises, and a value of a divided voltage generated by a resistance voltage dividing circuit has reached a value of a conversion voltage of the bias current, a second transistor is turned on in the output voltage detecting circuit. A second bias current circuit starts supplying a bias current to the reference voltage circuit. At the same time, a bias switching circuit operates to stop the bias-current supply operation of the first bias current circuit. Therefore, when the output voltage has reached a predetermined voltage after the power source has been turned on, it is possible to switch the supply source of the bias current immediately from the input side to the output side. As a result, it is possible to reduce a ripple voltage that appears in the output voltage due to the variation in the input voltage, during a normal operation after a stable output voltage has been obtained following the turning-on of the power source. Consequently, there is an effect that it is possible to improve the ripple removal ratio of the series regulator. 
     Furthermore, according to the another aspect of the present invention, a first bias current circuit provided at an input side and a second bias current circuit provided at an output side are differentially structured. Therefore, when a non-stabilized voltage has been applied to an input end, a first transistor is turned on, and a bias current is supplied from the first bias current circuit to a reference voltage circuit. Then, an amplifier starts controlling a power transistor. The first transistor is applied with a conversion voltage of the bias current, and continues the on-operation. A second transistor of the second bias current circuit that is differentially structured is in an off-status. When the output voltage of the power transistor rises, and a value of a divided voltage generated by a resistance voltage dividing circuit has reached a value of a conversion voltage of the bias current, the second transistor is turned on. Therefore, the second bias current circuit starts supplying a bias current to the reference voltage circuit. On the other hand, in the first bias current circuit, the first transistor is turned off. Therefore, the first bias current circuit stops supplying the bias current to the reference voltage circuit. It is possible to realize a bias switching circuit that has differentially structured the first bias current circuit provided at the input side and the second bias current circuit provided at the output side, by using a small number of elements. As a result, it is possible to reduce a ripple voltage that appears in the output voltage due to the variation in the input voltage, during a normal operation after a stable output voltage has been obtained following the turning-on of the power source. Consequently, there is an effect that it is possible, to improve the ripple removal ratio of the series regulator. 
     Moreover, according to still another aspect of the present invention, a first bias current circuit provided at an input side, a second bias current circuit provided at one output side, a third bias current circuit provided at the other output side are differentially structured. Therefore, when a non-stabilized voltage has been applied to an input end, a first transistor is turned on, and a bias current is supplied from the first bias current circuit to a reference voltage circuit. Then, a first amplifier starts controlling a first power transistor, and a second amplifier starts controlling a second power transistor. The first transistor is applied with a conversion voltage of the bias current, and continues the on-operation. A second transistor of the second bias current circuit and a third transistor of the third bias current circuit that are differentially structured are in an off-status. When the output voltages of the first and second power transistors rise, and when either a first divided voltage generated by a first resistance dividing circuit or a second divided voltage generated by a second resistance dividing circuit having a higher value has first reached a value of the conversion voltage, the corresponding one of the second transistor and the third transistor is turned on. The first transistor is turned off following this. A bias current is supplied to the reference voltage circuit from the corresponding one of the second bias current circuit and the third bias current circuit. At the same time, the first bias current circuit stops supplying the bias current. Stabilized voltages are output from the two output terminals respectively. Therefore, it the case of obtaining two outputs, it is also possible to switch the bias current supply source from the input side to the output side. As a result, it is possible to reduce a ripple voltage that appears in the output voltage due to the variation in the input voltage, during a normal operation after a stable output voltage has been obtained following the turning-on of the power source. Consequently, there is an effect that it is possible to improve the ripple removal ratio of the series regulator. 
     Furthermore, when a bias current is being supplied based on an output voltage of one of the first power transistor and the second power transistor, the on/off operations of the second transistor and the third transistor are switched to each other at the time of stopping the operation of the power transistor that is generating this output voltage. With this arrangement, it is possible to switch a supply source of a bias current to the other source having a different bias current. Consequently, there is an effect that it is possible to improve the ripple removal ratio of the series regulator. 
     Furthermore, the switching of a bias-current supply to the amplifier is also executed in addition to the switching of a bias-current supply to the reference voltage circuit. Consequently, there is an effect that it is possible to further improve the ripple removal ratio of the series regulator. 
     Although the invention has been described with respect to a specific embodiment for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art which fairly fall within the basic teaching herein set forth.