Abstract:
A constant voltage outputting apparatus includes a differential amplifier circuit, an amplifier circuit, a current adjustment device and a stabilization circuit. The differential amplifier circuit performs a differential amplifying operation and outputs a differential amplified voltage. The amplifier circuit amplifies the differential amplified voltage output from the differential amplifier circuit. The current adjustment device adjusts a current characteristic of the amplifier circuit. The stabilization circuit stabilizes a state of the current adjustment device. A constant voltage outputting method is also described.

Description:
BACKGROUND  
       [0001]     1. Field  
         [0002]     This patent specification relates to a method and apparatus for outputting a constant voltage to a load by using a differential amplifier circuit.  
         [0003]     2. Discussion of the Background  
         [0004]     In recent years, a lithium ion battery has been widely used as a power source for mobile devices. The operating voltage of the lithium ion battery is about 3.7 V, which is approximately three times of an operating voltage of a Ni—Cd battery or a nickel hydride battery. Therefore, the lithium ion battery can reduce the number of batteries used in a mobile device. Further, the lithium ion battery is light in weight. Accordingly, the lithium ion battery contributes to reduction in size and weight of the mobile device. When the lithium ion battery is used in the mobile device, however, an initial voltage of the lithium ion battery immediately after charging is about 4.3 V, but a final voltage of the battery after discharging is reduced to about 3.2 V. Therefore, the voltage of the lithium ion battery may need to be stabilized by a constant voltage circuit.  
         [0005]      FIG. 1  illustrates an exemplary configuration of a background constant voltage circuit. The background constant voltage circuit  11  includes a reference voltage source Rp, a bias voltage source Bp, a differential amplifier circuit Damp, an amplifier circuit Vamp, an output voltage control transistor M 8 , output voltage detection resistors R 1  and R 2 , and a current adjustment transistor M 7 . The constant voltage circuit  11  receives a voltage VBAT from a power source P and outputs an output voltage Vout to a load Lo.  
         [0006]     The differential amplifier circuit Damp performs a differential amplifying operation and outputs a voltage generated through the operation. The amplifier circuit Vamp then amplifies the voltage output from the differential amplifier circuit Damp. The output voltage control transistor M 8 , which may be a P-channel MOSFET (metal-oxide semiconductor field-effect transistor), for example, serving as an output voltage control device, receives the voltage amplified by the amplifying circuit Vamp and outputs an output voltage Vout to the load Lo. The output voltage detection resistors R 1  and R 2  detect and divide the output voltage Vout to generate a divided voltage. The divided voltage and a reference voltage Vref output from the reference voltage source Rp are input in the differential amplifying circuit Damp and used for the differential amplifying operation.  
         [0007]     The differential amplifier circuit Damp includes two differential input transistors M 1  and M 2 , a current regulation transistor M 5  and a current mirror circuit Cm 1 .  
         [0008]     The differential input transistors M 1  and M 2  may be N-channel MOSFETs, for example, and the current regulation transistor M 5  may be an N-channel MOSFET, for example, serving as a current regulation device driven by a bias voltage Vbi 1  output from the bias voltage source Bp.  
         [0009]     The current mirror circuit Cm 1  includes two transistors M 3  and M 4  connected to the power source P. The transistors M 3  and M 4  may be P-channel MOSFETs, for example. Each of the transistors M 3  and M 4  has a source connected to the power source P, and a gate connected to a drain of the transistor M 3 . Further, drains of the transistors M 3  and M 4  are connected to drains of the differential input transistors M 1  and M 2 , respectively.  
         [0010]     The differential input transistor M 1  has a gate connected to a positive terminal of the reference voltage source Rp. Meanwhile, the other differential input transistor M 2  has a gate connected to an output voltage dividing point between the output voltage detection resistors R 1  and R 2 . Sources of the differential input transistors M 1  and M 2  are connected to a drain of the current regulation transistor M 5 .  
         [0011]     The current regulation transistor M 5 , the drain of which is connected to both of the sources of the differential input transistors M 1  and M 2 , has a gate connected to the bias voltage source Bp and a source connected to a ground voltage terminal GND. The current regulation transistor M 5  regulates a drain current Id 1  of the differential input transistor M 1  and a drain current Id 2  of the differential input transistor M 2 .  
         [0012]     Further, a current adjustment transistor M 7 , which may be an N-channel MOSFET, for example, serving as a current adjustment device, forms a current mirror circuit Cm 2  together with the current regulation transistor M 5 . The current adjustment transistor M 7  is connected between the amplifier circuit Vamp described below and the ground voltage terminal GND. The current adjustment transistor M 7  has a gate connected to the bias voltage source Bp, a drain connected to a drain of an amplifier transistor M 6 , (i.e., a point Va to which an output voltage from the amplifier circuit Vamp is output) and a source connected to the ground voltage terminal GND.  
         [0013]     The amplifier transistor M 6  included in the amplifier circuit Vamp, which may be a P-channel MOSFET, for example, has a gate connected to the drain of the differential input transistor M 2 , and a source connected to the power source P.  
         [0014]     The output voltage control transistor M 8  has a gate connected to the drain of the amplifier transistor M 6 , a source connected to the power source P, and a drain connected to the predetermined load Lo via an output terminal Vr and to the output voltage detection resistors R 1  and R 2  connected in series.  
         [0015]     As described above, the output voltage detection resistors R 1  and R 2  have the output voltage dividing point connected to the gate of the differential input transistor M 2 . The output voltage detection resistor R 2  is connected to the ground voltage terminal GND.  
         [0016]     Operations of the constant voltage circuit  11  of  FIG. 1  are briefly described. When the output voltage Vout from the output terminal Vr is decreased for some reason, a gate voltage of the differential input transistor M 2  is decreased, so that the drain current Id 2  of the differential input transistor M 2  is decreased and a drain voltage Vd 2  of the differential input transistor M 2  is increased. Since the drain voltage Vd 2  of the differential input transistor M 2  is also a gate voltage of the amplifier transistor M 6 , the gate voltage of the amplifier transistor M 6  is also increased. Accordingly, a drain voltage Vd 6  of the amplifier transistor M 6  (i.e., an electric potential at the point Va to which the output voltage from the amplifier circuit Vamp is output) is decreased. Since the drain voltage Vd 6  of the amplifier transistor M 6  (i.e., the electric potential at the point Va) is output to the gate of the output voltage control transistor M 8 , a gate voltage of the output voltage control transistor M 8  is decreased, so that the output voltage Vout from the output terminal Vr is increased to a predetermined value.  
         [0017]     Conversely, when the output voltage Vout is increased for some reason, an inverse operation to the above-described operation is observed. That is, the gate voltage of the differential input transistor M 2  is increased, so that the drain current Id 2  of the differential input transistor M 2  is increased and the drain voltage Vd 2  of the differential input transistor M 2  is decreased. Since the drain voltage Vd 2  of the differential input transistor M 2  is also the gate voltage of the amplifier transistor M 6 , the gate voltage of the amplifier transistor M 6  is also decreased. Accordingly, the drain voltage Vd 6  of the amplifier transistor M 6  (i.e., the electric potential at the point Va to which the output voltage from the amplifier circuit Vamp is output) is increased. Since the drain voltage Vd 6  of the amplifier transistor M 6  (i.e., the electric potential at the point Va) is output to the gate of the output voltage control transistor M 8 , the gate voltage of the output voltage control transistor M 8  is increased, so that the output voltage Vout from the output terminal Vr is decreased to a predetermined value.  
         [0018]     In other words, in the above constant voltage circuit  11  of  FIG. 1 , even when the output voltage Vout is changed for some reason, the gate voltage of the amplifier transistor M 6  is changed in an opposite direction to a direction in which the gate voltage of the differential input transistor M 2  is changed in response to a change of the output voltage Vout. Therefore, the electric potential at the point Va is changed in an opposite direction to the direction in which the gate voltage of the amplifier transistor M 6  is changed, and the gate voltage of the output voltage control transistor M 8  is changed in the same opposite direction in which the electric potential at the point Va is changed, so that a value of the output voltage Vout from the output terminal Vr is kept constant.  
         [0019]     However, the above background constant voltage circuit  11  has a problem that, within the differential amplifier circuit Damp, a balance is lost between the drain current Id 1  of the differential input transistor M 1  and the drain current Id 2  of the differential input transistor M 2  and thus there arises an input offset voltage, which is a difference in voltage between the gate (i.e., an input terminal) of the differential input transistor M 1  and the gate (i.e., an input terminal) of the differential input transistor M 2 , causing deterioration in accuracy of the output voltage Vout. Mechanism of deterioration in accuracy of the output voltage Vout is explained below.  
         [0020]     The input offset voltage is reduced by equalizing the drain current Id 1  of the differential input transistor M 1  with the drain current Id 2  of the differential input transistor M 2 . The drain current Id 1  becomes equal to the drain current Id 2  when a drain-source voltage Vds 3  and a drain-source voltage Vds 4 , which are respectively drain-source voltages of the transistor M 3  and the transistor M 4  forming the current mirror circuit Cm 1 , are equal. The drain-source voltage Vds 3  of the transistor M 3  is equal to a gate-source voltage Vgs 3  of the transistor M 3 , and the drain-source voltage Vds 4  of the transistor M 4  is equal to a gate-source voltage Vgs 6  of the amplifier transistor M 6 . Therefore, the gate-source voltage Vgs 3  of the transistor M 3  should be equalized with the gate-source voltage Vgs 6  of the amplifier transistor M 6 .  
         [0021]     The drain-source voltage Vds 4  of the transistor M 4 , which is also the gate-source voltage Vgs 6  of the amplifier transistor M 6 , can be expressed as in the first formula Vds 4 =Vgs 6 =−{square root}(2×Id 6 /β6)+Vth 6 , wherein β(beta)6 is a transconductance coefficient of the amplifier transistor M 6 , and Vth 6  is a threshold voltage of the amplifier transistor M 6 .  
         [0022]     The gate-source voltage Vgs 3  of the transistor M 3  can be expressed as in the second formula Vds 3 =Vgs 3 =−{square root}(2×Id 3 /β3)+Vth 3 , wherein β3 is a transconductance coefficient of the transistor M 3 , and Vth 3  is a threshold voltage of the transistor M 3 .  
         [0023]     A condition under which a value of the first formula becomes equal to a value of the second formula can be expressed as in the third formula β6/β3=Id 6 /Id 3 .  
         [0024]     Normally, a device size of each of the differential input transistors M 1  and M 2 , the transistors M 3  and M 4 , the current regulation transistor M 5 , and the amplifier transistor M 6  is determined so as to satisfy the third formula.  
         [0025]     For example, when a lithium ion battery is used as the power source P, a voltage VBAT of the lithium ion battery starts gradually decreasing from the initial voltage of about 4.3 V down to the final voltage of about 3.2 V. When the lithium ion battery is thus discharged, the output voltage from the amplifier circuit Vamp (i.e., the voltage at the point Va) also gradually decreases. This is because a value of a gate-source voltage Vgs 8  of the output voltage control transistor M 8  is kept constant when a value of a current IL flowing through the load Lo is constant, as observed from the fourth formula Vgs 8 =−{square root}(2×Id 8 /β8)+Vth 8 , wherein β8 is a transconductance coefficient of the output voltage control transistor M 8 , and Vth 8  is a threshold voltage of the output voltage control transistor M 8 .  
         [0026]     That is, the output voltage from the amplifier circuit Vamp (i.e., the electric potential at the point Va), which is equal to Vgs 8 , changes by approximately a voltage of 1.1 V from the voltage of about 4.3 V to the voltage of about 3.2 V. Further, even when the voltage VBAT of the power source P is constant, if the current IL flowing through the load Lo changes, the gate-source voltage Vgs 8  of the output voltage control transistor M 8  changes. As a result, the output voltage from the amplifier circuit Vamp (i.e., the voltage at the point Va) changes. The output voltage from the amplifier circuit Vamp or the voltage at the point Va is also a drain-source voltage Vds 7  of the current adjustment transistor M 7 . Even when a gate-source voltage Vgs 7  of the current adjustment transistor M 7  is constant, if the drain-source voltage Vds 7  of the current adjustment transistor M 7  changes, a drain current Id 7  of the current adjustment transistor M 7  changes due to a channel length modulation effect. The change of the drain current Id 7  results in a change of a drain current Id 6  of the amplifier transistor M 6 , since the drain current Id 7  of the current adjustment transistor M 7  is equal to the drain current Id 6  of the amplifier transistor M 6 .  
         [0027]     On the other hand, a drain-source voltage Vds 5  of the current regulation transistor M 5  can be expressed as in the fifth formula Vds 5 =Vref−Vgs 1 =Vref−({square root}(2×Id 1 /β1)+Vth 1 ) indicating a relationship between the reference voltage Vref and the gate-source voltage Vgs 1  of the differential input transistor M 1 , wherein β1 is a transconductance coefficient of the differential input transistor M 1 , and Vth 1  is a threshold voltage of the differential input transistor M 1 .  
         [0028]     The gate-source voltage Vgs 1  of the differential input transistor M 1  takes an almost constant value. It is therefore determined from the fifth formula that the value of the drain-source voltage Vds 5  of the current regulation transistor M 5  is almost constant regardless of variation in the voltage VBAT of the power source P or variation in the current IL flowing through the load Lo. Accordingly, a drain current Id 5  of the current regulation transistor M 5  also takes an almost constant value.  
         [0029]     As described above, the gate-source voltage Vgs 6  of the amplifier transistor M 6  is also the drain-source voltage Vds 4  of the transistor M 4 . Therefore, when the gate-source voltage Vgs 6  of the amplifier transistor M 6  is changed, the drain-source voltage Vds 4  of the transistor M 4  is also changed. As a result, a drain current Id 4  of the transistor M 4  is changed due to the channel length modulation effect.  
         [0030]     The drain current Id 4  of the transistor M 4  is equal to the drain current Id 2  of the differential input transistor M 2 , and a sum of the drain current Id 1  of the differential input transistor M 1  and the drain current Id 2  of the differential input transistor M 2  is equal to the drain current Id 5  of the current regulation transistor M 5 . Further, the value of the drain current Id 5  of the current regulation transistor M 5  is constant, as described above. Therefore, when the drain current Id 2  of the differential input transistor M 2  is changed, the drain current Id 1  of the differential input transistor M 1  is changed in an inverse direction to a direction in which the drain current Id 2  is changed. As a result, a difference in voltage arises between the gate-source voltage Vgs 1  of the differential input transistor M 1  and the gate-source voltage Vgs 2  of the differential input transistor M 2 . This difference in voltage results in the input offset voltage and causes a change in the output voltage Vout.  
         [0031]     Usually, the output voltage Vout is added with a voltage value obtained by multiplying the value of the input offset voltage by (R 1 +R 2 )/R 2 , as an error margin.  
       SUMMARY  
       [0032]     This patent specification describes a novel constant voltage outputting apparatus. In one example, a novel constant voltage outputting apparatus includes a differential amplifier circuit, an amplifier circuit, a current adjustment device and a stabilization circuit. The differential amplifier circuit is configured to perform a differential amplifying operation and output a differential amplified voltage. The amplifier circuit is configured to amplify the differential amplified voltage output from the differential amplifier circuit. The current adjustment device is configured to adjust a current characteristic of the amplifier circuit. The stabilization circuit is configured to stabilize a state of the current adjustment device.  
         [0033]     This patent specification further describes another constant voltage outputting apparatus. In one example, this constant voltage outputting apparatus includes a reference voltage source, two output voltage detection resistors, a differential amplifier circuit, an amplifier circuit, a current adjustment device, a stabilization circuit and an output voltage control device. The reference voltage source is configured to output a reference voltage. The two output voltage detection resistors are configured to detect and divide an output voltage to generate a feedback voltage. The differential amplifier circuit is configured to receive an input voltage, the reference voltage and the feedback voltage, perform a differential amplifying operation, and output a differential amplified voltage. The amplifier circuit is configured to amplify the differential amplified voltage output from the differential amplifier circuit. The current adjustment device is configured to adjust a current characteristic of the amplifier circuit. The stabilization circuit is configured to stabilize a state of the current adjustment device. The output voltage control device is configured to receive the differential amplified voltage amplified by the amplifier circuit and control output of the output voltage to an external load based on the input voltage in accordance with the differential amplified voltage.  
         [0034]     The differential amplifier circuit may include a current mirror circuit, two differential input transistors and a current regulation device. The current mirror circuit may be configured to generate mirror currents based on the input voltage. The two differential input transistors may be configured to be connected to the current mirror circuit and perform the differential amplifying operation based on the mirror currents, the reference voltage and the feedback voltage. The current regulation device may be configured to regulate a current characteristic of each of the two differential input transistors.  
         [0035]     The stabilization circuit may include a stabilization transistor having a constant gate electric potential and being connected in series with the current adjustment device.  
         [0036]     The stabilization circuit may include a bias voltage source configured to output a bias voltage, and a stabilization transistor configured to be placed between the amplifier circuit and the current adjustment device, and configured to have a gate connected to the bias voltage source and a source connected to a drain of the current adjustment device.  
         [0037]     The stabilization circuit may include a depression-type stabilization transistor configured to be placed between the amplifier circuit and the current adjustment device, and configured to have a gate connected to a source of the current adjustment device and a source connected to a drain of the current adjustment device.  
         [0038]     The stabilization circuit may include a constant current source, a first bias voltage generation device, a stabilization transistor and a second bias voltage generation device. The first bias voltage generation device may be configured to output, based on a current output from the constant current source, a first bias voltage to a gate of the current adjustment device and a gate of the current regulation device. The stabilization transistor may be configured to be placed between the amplifier circuit and the current adjustment device, and configured to have a source connected to a drain of the current adjustment device. The second bias voltage generation device may be configured to output, based on a current output from the constant current source, a second bias voltage to a gate of the stabilization transistor. A gate and a drain of the second bias voltage generation device may be connected to the constant current source, and a gate and a drain of the first bias voltage generation device may be connected to a source of the second bias voltage generation device.  
         [0039]     The stabilization circuit may include a stabilization transistor configured to be placed between the amplifier circuit and the current adjustment device, and configured to have a gate connected to the reference voltage source and a source connected to a drain of the current adjustment device.  
         [0040]     This patent specification further describes a novel constant voltage outputting method. In one example, a novel constant voltage outputting method includes providing a differential amplifier circuit configured to receive an input voltage a reference voltage, and a feedback voltage generated by dividing an output voltage, providing an amplifier circuit and a current adjustment device, inserting a stabilization circuit between the amplifier circuit and the current adjustment device, performing a differential amplifying operation with the differential amplifier circuit to output a differential amplified voltage, amplifying the differential amplified voltage with the amplifier circuit, adjusting a current characteristic of the amplifier circuit, stabilizing a state of the current adjustment device, and controlling output of the output voltage to an external load based on the input voltage in accordance with the differential amplified voltage amplified by the amplifier circuit.  
         [0041]     The differential amplifier circuit may include a current mirror circuit, two differential input transistors and a current regulation device. The current mirror circuit may be configured to generate mirror currents based on the input voltage. The two differential input transistors may be configured to be connected to the current mirror circuit and perform the differential amplifying operation based on the mirror currents, the reference voltage and the feedback voltage. The current regulation device may be configured to regulate a current characteristic of each of the two differential input transistors.  
         [0042]     The stabilization circuit may include a stabilization transistor having a constant gate electric potential and being connected in series with the current adjustment device.  
         [0043]     The stabilization circuit may include a bias voltage source configured to output a bias voltage, and a stabilization transistor configured to have a gate connected to the bias voltage source and a source connected to a drain of the current adjustment device.  
         [0044]     The stabilization circuit may include a depression-type stabilization transistor configured to have a gate connected to a source of the current adjustment device and a source connected to a drain of the current adjustment device.  
         [0045]     The stabilization circuit may include a constant current source, a first bias voltage generation device, a stabilization transistor and a second bias voltage generation device. The first bias voltage generation device may be configured to output, based on a current output from the constant current source, a first bias voltage to a gate of the current adjustment device and a gate of the current regulation device. The stabilization transistor may be configured to have a source connected to a drain of the current adjustment device. The second bias voltage generation device may be configured to output, based on a current output from the constant current source, a second bias voltage to a gate of the stabilization transistor. A gate and a drain of the second bias voltage generation device may be connected to the constant current source, and a gate and a drain of the first bias voltage generation device may be connected to a source of the second bias voltage generation device.  
         [0046]     The stabilization circuit may include a stabilization transistor configured to have a gate connected to a reference voltage source and a source connected to a drain of the current adjustment device. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0047]     A more complete appreciation of the disclosure and many of the advantages thereof are readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:  
         [0048]      FIG. 1  is a circuit diagram illustrating an exemplary configuration of a background constant voltage circuit;  
         [0049]      FIG. 2  is a circuit diagram illustrating an exemplary configuration of a constant voltage circuit according to an embodiment of this disclosure;  
         [0050]      FIG. 3  is a circuit diagram illustrating an exemplary configuration of a constant voltage circuit according to another embodiment;  
         [0051]      FIG. 4  is a circuit diagram illustrating an exemplary configuration of a constant voltage circuit according to still another embodiment; and  
         [0052]      FIG. 5  is a circuit diagram illustrating an exemplary configuration of a constant voltage circuit according to still yet another embodiment. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0053]     In describing preferred embodiments illustrated in the drawings, specific terminology is employed for the purpose of clarity. However, the disclosure of this patent specification is not intended to be limited to the specific terminology so used and it is to be understood that substitutions for each specific element can include any technical equivalents that operate in a similar manner.  
         [0054]     Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views,  FIG. 2  illustrates a configuration of a constant voltage circuit  21  according to an exemplary embodiment. Description is omitted for components of the constant voltage circuit  21  which are also components of the background constant voltage circuit shown in  FIG. 1 .  
         [0055]     As illustrated in  FIG. 2 , the present constant voltage circuit  21  includes, as a stabilization circuit, a bias voltage source Bp 2  and a stabilization transistor M 9 . The stabilization transistor M 9 , which may be a P-channel MOSFET, for example, stabilizes a state of the current adjustment transistor M 7  such as the value of the drain current Id 7 .  
         [0056]     The bias voltage source Bp 2  has a negative voltage terminal connected to the ground voltage terminal GND, and a positive voltage terminal for outputting a bias voltage Vbi 2 .  
         [0057]     The stabilization transistor M 9  has a gate connected to the positive voltage terminal of the bias voltage source Bp 2 , a drain connected to the drain of the amplifier transistor M 6  (i.e., the point Va), and a source connected to the drain of the current adjustment transistor M 7 .  
         [0058]     In the constant voltage circuit  21  of  FIG. 2 , the drain-source voltage Vds 7  of the current adjustment transistor M 7  (i.e., a voltage at a point Vb) is stabilized as explained below. A value of a drain-source voltage Vds 9  of the stabilization transistor M 9  is obtained by subtracting a value of a gate-source voltage Vgs 9  of the stabilization transistor M 9  from a value of the bias voltage Vbi 2 . Namely, Vds 9  can be expressed as Vds 9 =Vbi 2 −Vgs 9 . A value of the drain current Id 9  of the stabilization transistor M 9  is constant and equal to a value of the drain current Id 7  of the current adjustment transistor M 7 . Since the value of the bias voltage Vbi 2  applied to the gate of the stabilization transistor M 9  is also kept constant, the gate-source voltage Vgs 9  of the stabilization transistor M 9  takes a constant value. This constant value of the gate-source voltage Vgs 9  of the stabilization transistor M 9  allows the drain-source voltage Vds 7  of the current adjustment transistor M 7  to take a constant value.  
         [0059]     Accordingly, even when the voltage VBAT of the power source P or the current IL of the load Lo is changed and thus the output voltage from the amplifier transistor M 6  (i.e., the voltage at the point Va) is changed, the drain-source voltage Vds 7  of the current adjustment transistor M 7  (i.e., the voltage at the point Vb) is stabilized. Therefore, the drain current Id 7  of the current adjustment transistor M 7  is unchanged and stabilized. As a result, the drain current Id 6  of the amplifier transistor M 6  is not changed, so that the value of the gate-source voltage Vgs 6  of the amplifier transistor M 6  is kept constant. Accordingly, the channel length modulation effect is reduced, and the drain current Id 4  of the transistor M 4  is stabilized. Further, the difference in voltage does not arise between the gate-source voltage Vgs 1  of the differential input transistor M 1  and the gate-source voltage Vgs 2  of the differential input transistor M 2 , so that the input offset voltage is reduced without altering the balance between the current flowing through the differential input transistor M 1  and the current flowing through the differential input transistor M 2 .  
         [0060]     In the constant voltage circuit  21  of  FIG. 2 , the stabilization transistor M 9  having a constant gate voltage stabilizes the drain current Id 7  of the current adjustment transistor M 7 . As a result, the drain current Id 6  of the amplifier transistor M 6  is stabilized, so that the value of each of the drain voltage Vd 4  and the drain current Id 4  of the transistor M 4  becomes constant and the input offset voltage is reduced. Accordingly, even if the voltage VBAT of the power source P or the current IL flowing through the load Lo is changed, accuracy in regulating the output voltage Vout is improved.  
         [0061]     Referring to  FIG. 3 , a constant voltage circuit  31  according to another embodiment is described. Description is omitted for components of the constant voltage circuit  31  which are also components of the background constant voltage circuit  11  shown in  FIG. 1 .  
         [0062]     As illustrated in  FIG. 3 , the constant voltage circuit  31  includes, as a stabilization circuit, a depression-type stabilization transistor DM 9 , which may be a D-N-channel MOSFET, for example.  
         [0063]     The stabilization transistor DM 9  has a gate connected to the source of the current adjustment transistor M 7 , which is at a side of the ground voltage terminal GND, a drain connected to the drain of the amplifier transistor M 6 , which is the point Va, and a source connected to the drain of the current adjustment transistor M 7 .  
         [0064]     The value of the drain-source voltage Vds 7  of the current adjustment transistor M 7  is obtained by subtracting a value of a gate-source voltage Vgs 9  of the stabilization transistor DM 9  from a value of a gate voltage Vg 9  of the stabilization transistor DM 9 . Namely, Vds 7  can be expressed as Vds 7 =Vg 9 −Vgs 9 . The current adjustment transistor M 7  operates in a saturation region, keeping the value of the drain-source voltage Vds 7  constant. In other words, in accordance with the operation of the stabilization transistor DM 9 , the current adjustment transistor M 7  operates in the saturation region to obtain a necessary drain-source voltage Vds 7 . As a result, the drain current Id 7  of the current adjustment transistor M 7  is unchanged and stabilized, so that the drain current Id 6  of the amplifier transistor M 6  is not changed, keeping the value of the gate-source voltage Vgs 6  of the amplifier transistor M 6  constant. Accordingly, the drain current Id 4  of the transistor M 4  is stabilized, and the difference in voltage does not arise between the gate-source voltage Vgs 1  of the differential input transistor M 1  and the gate-source voltage Vgs 2  of the differential input transistor M 2 . As a result, the input offset voltage is reduced, without altering the balance between the current flowing through the differential input transistor M 1  and the current flowing through the differential input transistor M 2 .  
         [0065]     Similar to the case of the constant voltage circuit  21  of  FIG. 2 , in the constant voltage circuit  31  of FIG.  3 , the state of the current adjustment transistor M 7  is stabilized in the saturation region, and the drain current Id 7  of the current adjustment transistor M 7  is stabilized. As a result, the drain current Id 6  of the amplifier transistor M 6  is stabilized, so that the input offset voltage is reduced. Therefore, even if the voltage VBAT of the power source P or the current IL flowing through the load Lo is changed, the accuracy in regulating the output voltage Vout is improved. Further, since the constant voltage circuit  31  of  FIG. 3  does not require a circuit element for generating the bias voltage Vbi 2 , the constant voltage circuit  31  consumes a smaller amount of current than the constant voltage circuit  21  of  FIG. 2  does.  
         [0066]     Referring to  FIG. 4 , a constant voltage circuit  41  according to still another embodiment is described. Description is omitted for components of the constant voltage circuit  41  which are also components of the background constant voltage circuit  11  shown in  FIG. 1 .  
         [0067]     As illustrated in  FIG. 4 , the constant voltage circuit  41  includes, as a stabilization circuit, a constant current source I 1 , a bias voltage generation transistor M 10 , a stabilization transistor M 9  and a bias voltage generation transistor M 11 . Each of the bias voltage generation transistor M 10  and the stabilization transistor M 9  may be an N-channel MOSFET, for example, while the bias voltage generation transistor M 11  may be a P-channel MOSFET, for example.  
         [0068]     The constant current source I 1  is connected to the power source P. The bias voltage generation transistor M 10  has a gate connected to the gate of the current regulation transistor M 5 , a drain connected via the bias voltage generation transistor M 11  to the constant current source I 1 , and a source connected to the ground voltage terminal GND. Further, a bias circuit Bs 1  is provided to connect the drain of the bias voltage generation transistor M 10  to the gate of the bias voltage generation transistor M 10 , and to connect the drain of the bias voltage generation transistor M 10  to the gate of the current regulation transistor M 5 . The bias circuit Bs 1  is further connected to the gate of the current adjustment transistor M 7 . The bias voltage generation transistor M 10  outputs the bias voltage Vbi 1  to the gate of the current regulation transistor M 5  and to the gate of the current adjustment transistor M 7 .  
         [0069]     The stabilization transistor M 9  has a drain connected to the drain of the amplifier transistor M 6  (i.e., the point Va), a source connected to the drain of the current adjustment transistor M 7 , and a gate connected to a gate of the bias voltage generation transistor M 11 .  
         [0070]     The bias voltage generation transistor M 11 , the gate of which is connected to the gate of the stabilization transistor M 9 , has a drain connected to the constant current source I 1  and a source connected to the drain of the bias voltage generation transistor M 10 . Further, a bias circuit Bs 2  is provided to connect the drain of the bias voltage generation transistor M 11  to the gate of the bias voltage generation transistor M 11 , and to connect the drain of the bias voltage generation transistor M 11  to the gate of the stabilization transistor M 9 . The bias voltage generation transistor M 11  outputs the bias voltage Vbi 2  to the gate of the stabilization transistor M 9 .  
         [0071]     The current regulation transistor M 5  operates based on the bias voltage Vbi 1  to keep a constant value of each of the drain current Id 1  of the differential input transistor M 1  and the drain current Id 2  of the differential input transistor M 2 . On the other hand, the value of the drain-source voltage Vds 7  of the current adjustment transistor M 7  is obtained by subtracting the value of the gate-source voltage Vgs 9  of the stabilization transistor M 9  from a sum of a value of a gate-source voltage Vgs 10  of the bias voltage generation transistor M 10  and a value of a gate-source voltage Vgs 11  of the bias voltage generation transistor M 11 . Namely, Vds 7  can be expressed as Vds 7 =Vgs 10 +Vgs 11 −Vgs 9 . If the area size of each of the current adjustment transistor M 7 , the stabilization transistor M 9 , the bias voltage generation transistor M 10  and the bias voltage generation transistor M 11  is appropriately set, the drain-source voltage Vds 9  of the stabilization transistor M 9  is stabilized in accordance with the bias voltage Vbi 2 , and the current adjustment transistor M 7  operates in the saturation region, so that the value of the drain-source voltage Vds 7  of the current adjustment transistor M 7  is kept constant. Accordingly, the drain current Id 7  of the current adjustment transistor M 7  is not changed, and thus the drain current Id 6  of the amplifier transistor M 6  is stabilized. As a result, the drain current Id 4  of the transistor M 4  is stabilized, so that the input offset voltage is reduced.  
         [0072]     In the constant voltage circuit  41  of  FIG. 4 , in accordance with the bias voltage Vbi 2 , the stabilization transistor M 9  causes the current adjustment transistor M 7  to operate in the saturation region such that the value of the drain-source voltage Vds 7  of the current adjustment transistor M 7  is kept constant. Accordingly, the drain current Id 6  flowing through the amplifier transistor M 6  is stabilized, and the input offset voltage is reduced. As a result, even if the voltage VBAT of the power source P or the current IL flowing through the load Lo is changed, the accuracy in regulating the output voltage Vout can be improved.  
         [0073]     Referring to  FIG. 5 , a constant voltage circuit  51  according to still yet another embodiment is described. Description is omitted for components of the constant voltage circuit  51  which are also components of the background constant voltage circuit  11  shown in  FIG. 1 .  
         [0074]     The constant voltage circuit  51  of  FIG. 5  is similar to the constant voltage circuit  21  of  FIG. 2  in that the stabilization transistor M 9  is provided as a stabilization circuit, but the constant voltage circuit  51  of  FIG. 5  is different from the constant voltage circuit  21  of  FIG. 2  in that the gate of the stabilization transistor M 9  is connected to the reference voltage source Rp.  
         [0075]     When the constant voltage circuit  51  of  FIG. 5  is in a stable state, the drain current Id 5  flowing through the current regulation transistor M 5  that outputs currents to be supplied to the transistors M 1  to M 5 , which serve as error amplifiers, is determined largely by the drain-source current Ids 1  of the differential input transistor M 1 , the reference voltage Vref biased to the gate of the differential input transistor M 1 , and the threshold voltage and the transconductance coefficient of the differential input transistor M 1 . Therefore, if a ratio between the drain-source current Ids 9  flowing through the stabilization transistor M 9  and the drain-source current Ids 1  flowing through the differential input transistor M 1  is determined, it is possible to equalize an electric potential of the drain voltage Vd 5  and an electric potential of the drain voltage Vd 7 , which are respective electric potentials of the current regulation transistor M 5  and the current adjustment transistor M 7  forming the current mirror circuit Cm 2 , by using the reference voltage Vref as a voltage to be biased to the gate of the stabilization transistor M 9  and adjusting the type and area size of the stabilization transistor M 9 .  
         [0076]     The source of the current regulation transistor M 5  and the source of the current adjustment transistor M 7  are connected to the ground voltage terminal GND. If the electric potential of the drain voltage Vd 5  is equal to the electric potential of the drain voltage Vd 7 , the drain-source current Ids 7  having a current value in proportion to an area size ratio between the current regulation transistor M 5  and the current adjustment transistor M 7  flows. Further, if the differential input transistor M 1  and the stabilization transistor M 9  are formed to have a similar area size and similar characteristics (e.g., both of the transistors M 1  and M 9  are N-channel MOSFETs), a change in the electric potential of the source caused by a change in a temperature characteristic, the reference voltage Vref, or the like, also becomes similar between the differential input transistor M 1  and the stabilization transistor M 9 . As a result, consistency against an environmental variation between a constant current flowing through the current regulation transistor M 5  and a constant current flowing through the current adjustment transistor M 7  is improved. As a result, stability of the output voltage Vout output from the constant voltage circuit  51  is improved.  
         [0077]     The constant voltage circuit  51  of  FIG. 5  has an advantage of stabilizing the drain current Id 6  of the amplifier transistor M 6  and reducing the input offset voltage so that the accuracy in regulating the output voltage Vout is improved. In addition, since the constant voltage circuit  51  does not require the bias voltage source Bp 2 , the constant voltage circuit  51  has another advantage of reducing the number of circuit elements and the amount of current consumption so as to reduce man-hours and production costs required for producing the constant voltage circuit  51  and a running cost required for operating the constant voltage circuit  51 , as in the case of the constant voltage circuit  31  of  FIG. 3 .  
         [0078]     In each of the above embodiments, a transistor formed by an N-channel MOSFET may also be formed by a P-channel MOSFET, and a transistor formed by a P-channel MOSFET may also be formed by an N-channel MOSFET.  
         [0079]     Furthermore, the use of the transistors M 1  to M 7  and M 9 , which are used for error amplification, is not limited within the constant voltage circuits  21 ,  31 ,  41  and  51 , but the transistors are also applicable to a general operational amplifier circuit. If the transistors M 1  to M 7  and M 9  are used in such a general operational amplifier circuit, occurrence of the offset voltage in input terminals can be suppressed, and gains of the operational amplifier circuit can be substantially improved. As a result, performance of the operational amplifier circuit can be substantially improved.  
         [0080]     The above-described embodiments are illustrative, and numerous additional modifications and variations are possible in light of the above teachings. For example, elements and/or features of different illustrative and exemplary embodiments herein may be combined with each other and/or substituted for each other within the scope of this disclosure and appended claims. It is therefore to be understood that within the scope of the appended claims, the disclosure of this patent specification may be practiced otherwise than as specifically described herein.  
         [0081]     This patent specification is based on Japanese patent application No. 2004-015724 filed on Jan. 23, 2004 in the Japan Patent Office, the entire contents of which are incorporated by reference herein.