Abstract:
A buffer amplifier having a wide output voltage range includes a first source follower circuit having a first current source and a first transistor, and a second source follower circuit having a second current source and a second transistor. The first source follower circuit has an output terminal connected to a gate of a third transistor and a source of a fourth transistor. The second source follower circuit has an output terminal connected to a gate of a fifth transistor and a source of a sixth transistor. First and second voltages are respectively supplied to the gates of the fourth and sixth transistors. The sixth transistor is operated in place of the fifth transistor in a low voltage range, and the fourth transistor is operated in place of the third transistor in a high voltage range.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to a buffer amplifier that generates an output voltage in accordance with an input voltage, and more particularly, to an amplifier circuit having a wide output voltage range. 
         [0002]    In the prior art, a buffer amplifier that outputs an output signal amplified at a gain of one time is used in accordance with the provided input signal. When an operational amplifier is used as the buffer amplifier, feedback is necessary. Thus, the circuit becomes more complicated. To solve this problem, the use of a driver circuit having low power consumption and high voltage accuracy has been discussed (refer, for example, to Japanese Laid-Open Patent Publication No. 2005-217949,  FIG. 3 ). Such a driver circuit includes a voltage generation unit for generating a basic output voltage that is in accordance with an input voltage, a first buffer circuit for outputting an output voltage in accordance with the basic output voltage output from the voltage generation unit, and a second buffer circuit for generating a voltage in accordance with the output voltage. The second buffer circuit consumes more power than the first buffer circuit. The driver circuit further includes a quasi-buffer circuit having substantially the same properties as the first buffer circuit. The quasi-buffer circuit generates a quasi-voltage in accordance with the basic output voltage provided by the voltage generation unit. The basic output voltage is controlled based on the simulation voltage. 
         [0003]    There has also been discussion of a buffer amplifier having an output voltage range that can be widened (refer, for example, to Japanese Laid-Open Patent Publication No. 2002-185269,  FIG. 1 ). Such a buffer amplifier includes an n-channel source follower circuit and a p-channel source follower circuit, each of which receives an input voltage. The buffer amplifier further includes an output stage circuit connected to the outputs of the two source follower circuits to generate an output voltage. A first current mirror circuit supplies an output current to the output of the output stage circuit using the output current of the n-channel source follower circuit as an input current. A second current mirror circuit supplies an output current to the output of the output stage circuit using the output current of the p-channel source follower circuit as an input current. 
         [0004]    However, the output voltage range (swing) is not sufficient for the following reasons. Referring to  FIG. 14 , a schematic diagram of the driver circuit disclosed in Japanese Laid-Open Patent Publication No. 2005-217949 is shown. In this driver circuit, an output of a pulse generator  500  is amplified by a first buffer circuit  510  and a second buffer circuit  520 . In this circuit, however, the output voltage of the second buffer circuit  520  is restricted by the base-emitter voltage of a transistor  531  when the input voltage is in a high range and restricted by a transistor  532  when the input voltage is low. 
         [0005]      FIG. 15  is a schematic circuit diagram of a buffer amplifier of Japanese Laid-Open Patent Publication No. 2002-185269. The buffer amplifier includes current mirror circuits  640  and  650  to widen or increase the swing. The current mirror circuits  640  and  650 , respectively, include transistors  641  and  642  and transistors  651  and  652  to generate output voltages in a range in which source follower circuits  610  and  620  do not function. 
         [0006]    In order to operate the transistor  641 , however, the drain voltage of the transistor  611  must be decreased as much as possible. When the input voltage becomes high and approaches the power supply voltage, the drain voltage of the transistor  611  also becomes high. Thus, the transistor  641  cannot be sufficiently driven. 
         [0007]    In this case, the drive capacity of the current mirror circuit  640  can be ensured by increasing the threshold voltage of the transistor  611 . However, this would lower the operational speed since the source-drain voltage of the transistor  611  is small. Such a problem also arises with the source follower circuit  620  and the current mirror circuit  650  when the input voltage is low. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a circuit diagram of a series regulator according to a preferred embodiment of the present invention; 
           [0009]      FIG. 2  is a circuit diagram of a buffer amplifier shown in  FIG. 1 ; 
           [0010]      FIG. 3  is a circuit diagram illustrating the operation of the buffer amplifier in an intermediate voltage range; 
           [0011]      FIGS. 4A to 4C  are charts showing voltage characteristics of the buffer amplifier in the intermediate voltage range, in which  FIG. 4A  shows the output voltage,  FIG. 4B  shows the current flowing through transistors P 1 , N 1 , P 4 , and N 4 , and  FIG. 4C  shows the current flowing through transistors P 2  and N 2 ; 
           [0012]      FIG. 5  is a circuit diagram showing the operation of the buffer amplifier in a low voltage range; 
           [0013]      FIGS. 6A to 6C  are charts showing voltage characteristics of the buffer amplifier in the low voltage range, in which  FIG. 6A  shows the output voltage,  FIG. 6B  shows the current flowing through the transistors P 1 , N 1 , P 4 , and N 4 , and  FIG. 6C  shows the current flowing through the transistors P 2  and N 2 ; 
           [0014]      FIG. 7  is a circuit diagram showing the operation of the buffer amplifier in a high voltage range; 
           [0015]      FIGS. 8A to 8C  are charts showing voltage characteristics of the buffer amplifier in the high voltage range, in which  FIG. 8A  shows the output voltage,  FIG. 8B  shows the current flowing through the transistors P 1 , N 1 , P 4 , and N 4 , and  FIG. 8C  shows the current flowing through the transistors P 2  and N 2 ; 
           [0016]      FIG. 9  is a circuit diagram of a prior art series regulator; 
           [0017]      FIGS. 10A to 10D  are charts showing frequency characteristics of a series regulator, in which  FIG. 10A  shows the gain of a circuit in the prior art,  FIG. 10B  shows the phase of the circuit in the prior art,  FIG. 10C  shows the gain of a circuit in the preferred embodiment, and  FIG. 10D  shows the phase of the circuit in the preferred embodiment; 
           [0018]      FIGS. 11A to 11D  are charts showing input voltage characteristics of a series regulator, in which  FIG. 11A  shows the output voltage of a circuit in the prior art,  FIG. 11B  shows the output current of the circuit in the prior art,  FIG. 11C  shows the output voltage of a circuit in the preferred embodiment, and  FIG. 11D  shows the output current of the circuit in the preferred embodiment; 
           [0019]      FIG. 12  is a circuit diagram of a buffer amplifier according to another embodiment of the present invention; 
           [0020]      FIG. 13  is a circuit diagram of a buffer amplifier according to a further embodiment of the present invention; 
           [0021]      FIG. 14  is a circuit diagram of a prior art buffer amplifier; and 
           [0022]      FIG. 15  is a circuit diagram of another prior art buffer amplifier. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0023]    The present invention provides a buffer amplifier having a wide output voltage range. 
         [0024]    In one embodiment, the present invention provides an amplifier circuit including first and second source follower circuits supplied with an input voltage. A first output transistor includes a control terminal, which is provided with an output of the first source follower circuit, and a first terminal, which is supplied with a power supply voltage. A second output transistor includes a control terminal, which is provided with an output of the second source follower circuit, and a first terminal, which is supplied with a base voltage. A first auxiliary transistor includes a second terminal, which is connected to an output terminal of the first source follower circuit, and a control terminal, which receives a first bias voltage. A second auxiliary transistor includes a second terminal, which is connected to an output terminal of the second source follower circuit, and a control terminal, which receives a second bias voltage. An output means outputs a voltage at a connection node of second terminals of the first and second output transistors and first terminals of the first and second auxiliary transistors. 
         [0025]    A buffer amplifier according to the present invention will be discussed with reference to  FIGS. 1 to 11 . In one embodiment of the present invention, the buffer amplifier includes first and second auxiliary transistors. The first auxiliary transistor operates when the input voltage is close to the power supply voltage, and the second auxiliary transistor operates when the input voltage is close to ground potential. In one embodiment, the buffer amplifier is formed by a field-effect transistor element, the gate terminal, drain terminal, and source terminal of which respectively function as a control terminal, a first terminal, and a second terminal. 
         [0026]    Referring now to  FIG. 1 , the buffer amplifier of one embodiment of the present invention is applied to a series regulator SR 1 . The series regulator SR 1  includes an amplifier  10 , a resistor R 1 , a capacitor C 1 , a buffer amplifier  20 , a resistor R 2 , and a resistor R 3 . 
         [0027]    The amplifier  10  receives an input signal of the series regulator SR 1 . The output of the amplifier  10  is provided to the buffer amplifier  20 . The resistor R 1  and the capacitor C 1  are arranged between the output terminal of the amplifier  10  and a ground line L 1 . 
         [0028]    The output of the buffer amplifier  20  is provided to the gate terminal of an output transistor  30 . The output transistor  30  is formed by a field-effect transistor element having a p-channel type (first conductivity type) MOS structure. The source terminal of the output transistor  30  is connected to a power supply line L 2 . The drain terminal of the output transistor  30  is connected to the ground line L 1  via the resistors R 2  and R 3 . The voltage at the drain terminal of the output transistor  30  is the output voltage. This output voltage is divided by the resistors R 2  and R 3  and fed back to the amplifier  10 . 
         [0029]    The configuration of the buffer amplifier  20  will now be described with reference to  FIG. 2 . The buffer amplifier  20  is connected to the power supply line L 2 , which is supplied with power supply voltage, and a ground line L 1 , which is supplied with a base voltage. The buffer amplifier  20  includes an input terminal supplied with a voltage V 1  and an output terminal for outputting a voltage V 2 . 
         [0030]    The input terminal of the buffer amplifier  20  is connected to the gate terminals of transistors P 1  and N 1 , which are thus supplied with the voltage V 1 . The transistor P 1  is formed by a transistor element (first transistor), which has a p-channel type (first conductivity type) MOS structure, and the transistor N 1  is formed by a transistor element (second transistor), which has an n-channel type (second conductivity type) MOS structure. 
         [0031]    The source terminal of the transistor P 1  is supplied with current from a current source CS 1  (first current source), which is connected to the power supply line L 2  for receiving the power supply voltage VCC. The drain terminal of the transistor P 1  is connected to the ground line L 1 . The current source CS 1  and the transistor P 1  form a first source follower circuit. 
         [0032]    The drain terminal of the transistor N 1  is connected to the power supply line L 2 . The source terminal of the transistor N 1  is connected to the ground line L 1  via a second current source CS 2 . The second current source CS 2  and the transistor N 1  form a second source follower circuit. 
         [0033]    A connection node of the current source CS 1  and the transistor P 1  (i.e., the output terminal of the first source follower circuit) is connected to the gate terminal of a transistor N 2  and the source terminal of a transistor P 4 . A connection node of the current source CS 2  and the transistor N 1  (i.e., the output terminal of the second source follower circuit) is connected to a gate terminal of a transistor P 2  and a source terminal of a transistor N 4 . The transistors N 2  and N 4  are transistor elements each having an n-channel type MOS structure, and the transistors P 2  and P 4  are transistor elements having a p-channel type MOS structure. 
         [0034]    The transistor N 2  functions as a first output transistor, the drain terminal of which is connected to the power supply line L 2 . The source terminal of the transistor N 2  is connected to the source terminal of the transistor P 2 . The transistor P 2  functions as a second output transistor, the drain terminal of which is connected to the ground line L 1 . 
         [0035]    The transistor P 4  functions as the first auxiliary transistor, the gate terminal of which is supplied with voltage V 11  serving as a first bias voltage. The transistor N 4  functions as the second auxiliary transistor, the gate terminal of which is supplied with voltage V 12  serving as a second bias voltage. The voltage V 11  is set based on a value obtained by subtracting the threshold voltage of the transistor P 1  from the power supply voltage. More specifically, a threshold voltage Vth is subtracted from the power supply voltage VCC, and a value that is less than the obtained difference by a predetermined value is set as the voltage V 11 . Further, the voltage V 12  is set based on a value obtained by adding the threshold voltage of the transistor N 1  to the base voltage. More specifically, a value that is greater by a predetermined value than the threshold voltage Vth is set as the voltage V 12 . 
         [0036]    The drain terminal of the transistor P 4  and the drain terminal of the transistor N 4  are connected to each other. The output terminal of the buffer amplifier  20  (output means) is connected to a connection node of the transistors N 4  and P 4  and to a connection node of the transistors P 2  and N 2 . 
         [0037]    The operation of the buffer amplifier  20  for an intermediate voltage range, low voltage range, and high voltage range will now be discussed with reference to  FIGS. 3 to 8 . 
       [Intermediate Voltage Range] 
       [0038]    First, with reference to  FIGS. 3 and 4 , the operation of the buffer amplifier  20  when the voltage V 1  is in the intermediate voltage range (range of threshold voltage Vth to voltage [VCC-Vth]) will be discussed. 
         [0039]    In this case, the transistors P 1  and N 1  are activated so that currents IP 1  and IN 1  flow, as shown in  FIG. 4B . Referring to  FIG. 3 , the gate-source voltage of the transistors P 4  and N 4  is small. Thus, the transistors P 4  and N 4  are deactivated and currents IP 4  and IN 4  become “0”. Currents IP 2  and IN 2 , which are shown in  FIG. 4C , flow to the transistors P 2  and N 2 . 
         [0040]    Such a circuit is referred to as a diamond buffer circuit. This circuit is formed by the first source follower circuit, which includes the transistor P 1  and the current source CS 1 , and the second source follower circuit, which includes the transistor N 1  and the current source CS 2 . The transistors N 2  and P 2  are driven by the output of each source follower circuit, and the voltage V 2  (voltage V 1 ) is output, as shown in  FIG. 4A . 
       [Low Voltage Range] 
       [0041]    With reference to  FIGS. 5 and 6 , the operation of the buffer amplifier  20  when the input voltage is in the low voltage range (range of ground voltage to threshold voltage Vth) will be discussed. 
         [0042]    When the voltage V 1  input to the buffer amplifier  20  becomes low, the gate terminal voltage of the transistor N 1  approaches the ground voltage GND. When the difference between the voltage V 1  and the ground voltage GND becomes less than or equal to the threshold voltage Vth of the transistor N 1 , the transistor P 1  remains activated but the transistor N 1  is deactivated, as shown in  FIG. 5 . In this case, the transistor P 4  remains deactivated. However, the current I 2  is supplied from the source terminal of the transistor N 4  to the current source CS 2 . This lowers the voltage at the source terminal of the transistor N 4 . The transistor N 4  is activated when the gate-source voltage of the transistor N 4  becomes greater than the threshold voltage Vth. 
         [0043]    As a result, the transistor N 4  operates in place of the transistor P 2 , and the current IN 4  flows as shown in  FIG. 6B . The amount of current IN 4  is the same as the current I 2  of the current source CS 2 . The switching to the transistor N 4  is quickly performed by setting the voltage V 12  supplied to the transistor N 4  at a high value. 
         [0044]    When there is no input or output of current from the output terminal, the transistor N 2 , the transistor N 4 , and the current source CS 2  function as a source follower circuit, and the current IN 2  flows to the transistor N 2  as shown in  FIG. 6C . The source follower circuit is driven by the outputs of the transistor P 1  and the current source CS 1 , and the voltage V 2 , which is in accordance with the voltage V 1 , is output as shown in  FIG. 6A . 
       [High Voltage Range] 
       [0045]    With reference to  FIGS. 7 and 8 , the operation of the buffer amplifier  20  when the input voltage is in the high voltage range (range of voltage [VCC-Vth] to power supply voltage VCC) will be discussed. 
         [0046]    When the voltage V 1  input to the buffer amplifier  20  becomes high, the gate terminal voltage of the transistor P 1  approaches the power supply voltage VCC. When the difference between the voltage V 1  and the power supply voltage VCC becomes less than or equal to the threshold voltage Vth of the transistor P 1 , the transistor N 1  remains activated. However, the transistor P 1  is deactivated, as shown in  FIG. 7 . In this case, the transistor N 4  remains deactivated. However, the current I 1  of the current source CS 1  is supplied to the source terminal of the transistor P 4 . This increases the voltage at the source terminal of the transistor P 4 . The transistor P 4  is activated when the gate-source voltage of the transistor P 4  becomes greater than the threshold voltage Vth. 
         [0047]    As a result, the transistor P 4  then operates in place of the transistor N 2 , and the current IP 4  flows as shown in  FIG. 8B . The current IP 4  is the same as the current I 1  of the current source CS 1 . The switching of the transistor P 4  is quickly performed by setting the voltage V 11  supplied to the transistor P 4  at a low value. 
         [0048]    When there is no input or output of current from the output terminal, the transistor P 2 , the transistor P 4 , and the current source CS 1  function as a source follower circuit, and the current IP 2  flows to the transistor P 2  as shown in  FIG. 8C . The source follower circuit is driven by the outputs of the transistor N 1  and the current source CS 2 , and the voltage V 2  corresponding to the voltage V 1  is output, as shown in  FIG. 8A . 
       [Operation of Series Regulator] 
       [0049]    The buffer amplifier  20  is used in the series regulator SR 1  as shown in  FIG. 1 . In this case, the output transistor  30  occupies a large area in the series regulator SR 1 . Thus, even if the output current of the series regulator SR 1  is “0”, the output voltage of the buffer amplifier  20  must rise to the vicinity of the power supply voltage. Otherwise, the output transistor  30  will not be sufficiently deactivated and thereby produce leakage current. Such leakage current may increase the output voltage. In order to reduce the influence of such leakage current, the resistance values of the resistors R 2  and R 3  must be decreased or a bleeder resistor must be added to the output. This would, however, increase current consumption. 
         [0050]    If the output voltage of the buffer amplifier  20  is brought close to the ground voltage GND, current would easily flow to the output transistor  30 . Therefore, a smaller output transistor  30  can be used to obtain the same output current. When the same output transistor  30  is used, a larger output current can be obtained. 
         [0051]    To this end, it is preferable that the swing of the output of the buffer amplifier  20  be as large as possible between the ground voltage GND and the power supply voltage VCC. 
         [0052]    With reference to  FIG. 9 , a series regulator SR 0  that does not include the buffer amplifier  20  will be described to demonstrate the effect of the buffer amplifier  20 . If the output transistor  30  was large, a large capacitor C 2  would exist between the source and gate terminals of the output transistor  30 . When operating such a series regulator SR 0 , as shown in  FIG. 10A , in the gain-frequency characteristics, a pole PL 1  is generated by the output capacitor C 3  and a pole PL 2  is generated by the capacitor C 2 . The capacitor C 1  is much smaller than the capacitor C 2 . Thus, the capacitor C 1  does not influence the pole PL 2 . 
         [0053]    If the two poles PL 1  and PL 2  are close, a phase margin (phase when the gain is 0 dB) cannot be sufficiently ensured in the phase frequency characteristics shown in  FIG. 10B . Therefore, it is preferable that the two poles PL 1  and PL 2  be sufficiently spaced apart to ensure the phase margin and stably operate the series regulator. 
         [0054]    Insertion of the buffer amplifier  20  disconnects the two capacitors C 1  and C 2 . If the buffer amplifier  20  can sufficiently drive the capacitor C 2 , the pole generated by the capacitor C 2  moves to a higher frequency range. The pole generated by the resistor R 1  and the capacitor C 1  at the input of the buffer amplifier  20  also exists in a higher frequency range than the pole PL 1 . Thus, the pole PL 2  generated at the input or the output of the buffer amplifier  20  moves to a higher frequency than the pole PL 1 . As a result, the two poles PL 1  and PL 2  can be separated and the gain can be maintained at a high frequency, as shown in  FIG. 10C . The phase margin can also be ensured, as shown in  FIG. 10D . 
         [0055]    The amplifier circuit of the above-described embodiment has the advantages described below. 
         [0056]    The transistor N 4  operates in place of the transistor P 2  in the low voltage range, and the transistor P 4  operates in place of the transistor N 2  in the high voltage range. In the buffer amplifier of the prior art, the output voltage is restricted by the input voltage due to the influence of the threshold voltage of the output transistor in the series regulator for the low voltage range and the high voltage range, as shown in  FIG. 11A . As a result, the output current is restricted even if the input voltage is low, and leakage current is produced when the input voltage becomes high, as shown in  FIG. 11B . In comparison, by using the transistors P 4  and N 4 , the output voltage is fully swung with respect to the input voltage without being affected by the threshold voltage of the transistors P 2  and N 2 , as shown in  FIG. 11C . As a result, the maximum output current is increased, and leakage current is suppressed, as shown in  FIG. 11D . 
         [0057]    It should be apparent to those skilled in the art that the present invention may be embodied in many other specific forms without departing from the spirit or scope of the invention. Particularly, it should be understood that the present invention may be embodied in the following forms. 
         [0058]    In the above-described embodiment, the buffer amplifier  20  is used in the series regulator. However, the application of the buffer amplifier  20  is not limited in any manner. 
         [0059]    In the above-described embodiment, the current source CS 1  is directly connected to the transistor P 1 , and the current source CS 2  is directly connected to the transistor N 1 . Instead, as shown in  FIG. 12 , a buffer amplifier  21  may include a transistor P 3 , which is arranged between the current source CS 1  and the transistor P 1 , and a transistor N 3 , which is arranged between the current source CS 2  and the transistor N 1 . In this case, the transistor P 1  is formed by a transistor element having a p-channel type MOS structure, and the transistor N 1  is formed by a transistor element having an n-channel type MOS structure. The voltage V 11  is input to the gate terminal of the transistor P 3 , and the voltage V 12  is input to the gate terminal of the transistor N 3 . In this case, the first and the second source follower circuits include the transistors P 3  and N 3 . 
         [0060]    In the above-described embodiment, the transistors P 1  and N 1  are formed by transistor element having a MOS structure. Instead, as shown in  FIG. 13 , a buffer amplifier  22  may include transistors BP 1  and BN 1 , which are bipolar transistor elements. In this case, a pnp type transistor element is used as the transistor BP 1 , which serves as the first transistor, and an npn type transistor element is used as the transistor BN 1 , which serves as the second transistor. 
         [0061]    The present examples and embodiments are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalence of the appended claims.