Abstract:
In a band gap reference voltage circuit, a band gap cell circuit composed of two transistors is driven with different current densities under a bias condition in which first and second reference voltages output in accordance with the operating states of the two transistors are equal to each other, thereby outputting a band gap reference voltage from a reference voltage output line. A differential amplifying circuit that is supplied with the first and second reference voltages as differential input signals subjects the differential input signals thus supplied to differential amplification. A level shift circuit is connected between a power supply line and the reference voltage output line and supplied with an output voltage of the differential amplifying circuit to carry out a level shift operation on the output voltage concerned.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application is based upon, claims the benefit of priority of, and incorporates by reference the contents of Japanese Patent Application No. 2004-292476 filed on Oct. 5, 2004. 
   TECHNICAL FIELD 
   The technical field relates to a band gap reference voltage circuit having a band gap cell circuit for outputting a reference voltage by driving two transistors with different current densities. 
   BACKGROUND 
     FIG. 6  shows a specific circuit construction of a band gap reference voltage circuit disclosed in JP-A-2003-157119. A band gap reference voltage circuit  1  comprises a band gap cell circuit  2 , a differential pair  3 , a current mirror circuit portion  4 , a gain forming portion  5  and an emitter follower circuit portion  6 . 
   In the band gap cell circuit  2 , a series circuit comprising a resistor R 11  and an NPN transistor T 11 , and a series circuit comprising a resistor R 12 , an NPN transistor T 12  and a resistor R 13  are connected to each other in parallel between a reference voltage output line VBG and the ground. The bases of transistors T 11  and T 12  are commonly connected to the collector of the transistor T 11 . The resistance values of the resistors R 11 , R 12  and R 13  are adjusted so that the transistors T 11  and T 12  are driven with different current densities (that is, asymmetrical current is supplied to the transistors T 11  and T 12 ), whereby the band gap cell circuit  2  acts to compensate for the characteristic variation with respect to the temperature. 
   The differential pair  3  comprises an NPN transistor T 13  having the base to which the collector (connection point A) of the transistor T 11  is connected, an NPN transistor T 14  having the base to which the collector (connection point B) of the transistor T 12  is connected, and a resistor R 14  connected between the emitter of each of the transistors T 13  and T 14  and the ground. 
   The current mirror circuit portion  4  comprises PNP transistors T 15  and T 16  whose bases are connected to each other. The emitters of the transistors T 15  and T 16  are connected to the reference voltage output line VBG through resistors R 15  and R 16 , and the collectors of the transistors T 15  and T 16  are connected to the collectors of the transistors T 13  and T 14 , respectively. The same level current is supplied to the transistors T 15  and T 16 . 
   The gain forming portion  5  has a PNP transistor T 17  and an NPN transistor T 18 . The emitter of the transistor T 17  is connected to the reference voltage output line VBG through an resistor R 17 , the collector thereof is connected to the ground through a resistor R 18  and the base thereof is connected to the collector of the transistor T 14 . The transistor T 18  is disposed to apply a gain to amplify variation of current supplied to the transistor T 14  through the transistor T 17 . The collector of the transistor T 18  is connected to the power source VCC through a resistor R 19 , the base thereof is connected to the collector of the transistor T 17 , and the emitter thereof is connected to the ground. 
   The emitter follower circuit portion  6  comprises the resistor R 19  and the NPN transistor T 19 . the collector of the transistor T 19  is connected to the power source VCC, the base thereof is connected to the collector of the transistor T 18 , and the emitter thereof is connected to the reference voltage output line VBG. The differential pair  3 , the current mirror circuit portion  4 , the gain forming portion  5  and the emitter follower circuit portion  6  constitute an operational amplifier  7 . 
   Capacitors C 1  to C 3  are provided for phase compensation to prevent oscillation of the operational amplifier  7 . The capacitor C 1  is connected between the collector and base of the transistor T 14 , the capacitor C 2  is connected between the collectors of the transistor T 14  and T 17 , and the capacitor C 3  is connected between the collectors of the transistors T 17  and T 18 . 
   Next, the operation of the band gap reference voltage circuit  1 . When the collector currents of the transistors T 11  and T 12  are represented by Ic 1  and Ic 2 , and the base-emitter voltages (junction voltages) of the transistors T 11  and T 12  are represented by VBE 11  and VBE 12 , the current Ic 2  flowing in the resistor R 13  is equal to the current value corresponding to the differential voltage of the respective base-emitter voltages VBE 11  and VBE 12 , and represented by the following equation.
 
 Ic 2=( VBE 11 −VBE 12)/ R 13
 
   Furthermore, when the base currents of the transistors T 11  and T 12  are represented by Ib 1  and Ib 2  respectively and the emitter currents of the transistors T 11  and T 12  are represented by Ie 1  Ie 2  respectively, the respective base currents Ib 1  and Ib 2  are sufficiently small and thus can be neglected as compared with the respective collector currents Ic 1  and Ic 2 , and thus the respective emitter currents Ie 1 , Ie 2  can be regarded as being equal to the collector currents Ic 1  and Ic 2 , respectively. Accordingly, when the base-emitter voltages VBE 11  and VBE 12  are varied due to characteristic variation of the transistors T 11  and T 12 , the collector current Ic 2  flowing in the resistor R 13  varies in connection with the variation of the base-emitter voltage VBE 11 , VBE 12 , and thus the relationship between the potentials (reference voltage) of the connection points A and B is varied. The potentials of the connection points A and B are applied as the base voltages of the two transistors T 13  and T 14  constituting the differential pair  3 . 
   Here, when the collector currents of the transistors T 13  and T 14  are represented by I 1  and I 2  respectively and the current flowing in the resistor R 14  connected to the collectors of the transistors T 13  and T 14  is represented by I, the currents I 1  and I 2  are basically equal to I/2 because the collector currents I 3  and I 4  of the transistors T 15  and T 16  are equal to each other. For example, when the current I 2  flowing in the transistor T 14  is about to increase to be larger than I/2, the collector currents I 3  and  14  of the transistors T 15  and T 16  must keep the same value, and thus an insufficient current component is compensated by the base current of the transistor T 17 . Accordingly, the collector current I 5  of the transistor T 17 , that is, the current flowing in the resistor R 18  is increased, and in connection with this current increase, the collector current I 6  of the transistor T 18  is also increased. 
   The collector current I 6  corresponds to the current I 7  flowing in the resistor R 19 , and thus the base potential and the emitter potential of the transistor T 19  is reduced by the increase of the collector currents I 6  and I 7 . By the above action, the potentials at the connection points A and B are adjusted, and the output voltage VBG is fed back so that the potentials are controlled to be fixed. The emitter follower circuit portion  6  is subjected to level shift by only the amount corresponding to the base-emitter voltage to set the output voltage VBG. That is, in the band gap reference voltage circuit  1 , the collector potentials of the transistors T 11  and T 12  in the band gap cell circuit  2  are amplified by the differential pair  3  and the current mirror circuit portion  4 , and further amplified by the transistors T 17  and T 18  in the gain forming portion  5 . 
   The band gap reference voltage circuit  1  thus constructed is designed so that amplification is carried out at plural stages in the operational amplifier  7 . Therefore, the total gain of the circuit is increased, and also phase-delay is more liable to occur because the operation of each circuit portion is delayed, so that the circuit may fall into an oscillation operation with an extremely high probability. Therefore, the capacitors C 1  to C 3  are needed for phase compensation to prevent oscillation. When a semiconductor integrated circuit is constructed, capacitors occupy a very large area, and thus the circuit scale is increased. In addition, the start-up of the circuit operation when power is turned on is further delayed. 
   In JP-A-2003-157119, it is illustrated that only one capacitor for phase compensation is disposed. However, it is experimentally obvious that if three capacitors C 1  to C 3  are disposed as shown in  FIG. 6 , it would be actually difficult to surely suppress the oscillation operation. 
   SUMMARY 
   In view of the foregoing, it is an object to provide a band gap reference voltage circuit that can further reduce the number of connections of capacitors for phase compensation or further reduce the capacitance needed to suppress oscillation. 
   According to a band gap reference voltage circuit of a first aspect, first and second reference voltages in a band gap cell circuit are applied as differential input signals, and an output voltage of a differential amplifying circuit for carrying out differential amplification on these input signals is directly input to a level shift circuit without being passed through a gain forming portion unlike the conventional construction, thereby generating and outputting a band gap reference voltage. 
   That is, in the conventional band gap reference voltage circuit, the reason why a gain forming portion is needed resides in that there is achieved such an advantage that the offset voltage of the operational amplifying portion can be reduced to a lesser level by increasing the gain and also the operation voltage range can be set to a broader range. In an actual application of the reference voltage circuit, attention is not paid to these characteristics at all times. 
   Accordingly, if the circuit is designed so that the output voltage of the differential amplifying circuit is directly subjected to level shift, the gain is reduced and a phase difference allowance degree is further increased, so that the number of connections of capacitors for phase compensation can be reduced or the capacitance needed to prevent oscillation can be reduced. Accordingly, the circuit scale can be reduced, and the response speed of the circuit operation can be further increased. 
   According to a band gap reference voltage circuit of a second aspect, in the level shift circuit, an element to which the output voltage of the differential amplifying circuit is applied is constructed by a MOSFET. That is, when the output voltage is directly subjected to level shift without being amplified, the offset voltage is apt to increase. Therefore, with the above construction, current is hardly supplied to the gate of MOSFET serving as a voltage driving type element, and thus unbalance of current in the differential pair of the differential amplifying circuit hardly occurs. Accordingly, the offset voltage can be reduced to a less level, and the output precision of the reference voltage can be enhanced. 
   According to a band gap reference voltage circuit of a third aspect, a phase compensating capacitor is connected between a ground-side terminal and a signal input terminal of a transistor disposed at the amplification output side out of the transistors constituting the differential pair. With this construction, by connecting only one capacitor having relatively low capacitance, the phase difference allowance degree can be more sufficiently secured while suppressing the increase of the circuit scale as much as possible. 
   According to a band gap reference voltage circuit of a fourth aspect, the transistor constituting the differential amplifying circuit is constructed by adding an SOI (Silicon On Insulator) structure with a trench insulating separation structure. That is, the differential amplifying circuit portion has a risk that unbalance of current occurs due to occurrence of unconsidered current leak or formation of a parasite transistor, so that the offset voltage is increased. Therefore, with respect to at least the transistor constituting the differential amplifying circuit, occurrence of the current leak is suppressed at maximum by adopting the device structure as described above, and the operation characteristic can be stabilized with keeping the offset balance optimally. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages will become more apparent from the following detailed description made with reference to the accompanying drawings. In the drawings: 
       FIG. 1  is a diagram showing the construction of a band gap reference voltage circuit according to a first embodiment; 
       FIG. 2  is a diagram showing a second embodiment, which corresponds to  FIG. 1 ; 
       FIG. 3  is a diagram showing a third embodiment, which corresponds to  FIG. 1 ; 
       FIG. 4A  is a cross-sectional view showing a semiconductor structure of an PNP transistor achieved by adding an SOI structure with a trench insulating separation structure, and  FIG. 4B  is a cross-sectional view showing a junction separation structure; 
       FIG. 5  is a diagram showing a fourth embodiment, which corresponds to  FIG. 1 ; and 
       FIG. 6  is a diagram showing a prior art, which corresponds to  FIG. 1 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Preferred embodiments will be described hereunder with reference to the accompanying drawings. 
   First Embodiment 
   A first embodiment will be described with reference to  FIG. 1 . The same parts as  FIG. 6  are represented by the same reference numerals, and the description thereof is omitted, and only the different portions will be described. The construction of a band gap reference voltage circuit  11  shown in  FIG. 1  is achieved by replacing the portions corresponding to the differential pair  3  and the current mirror circuit portion  4  of the band gap reference voltage circuit  1  shown in  FIG. 6  by a differential amplifying circuit  12 , the portions corresponding to the gain forming portion  5  and the emitter follower circuit portion  6  by a level shift circuit  13  and deleting the phase compensating capacitors C 1  to C 3 . 
   The differential amplifying circuit  12  is constructed by a differential pair  14  and a current mirror circuit portion  15 . The differential pair  14  is constructed by two PNP transistors T 21  and T 22  whose emitters are commonly connected to the power source VCC through a resistor R 21 . The current mirror circuit portion  15  is constructed by two NPN transistors T 23  and T 24  which are connected to each other in a current mirror style, and the collectors of the transistors T 23  and T 24  are connected to the collectors of the transistors T 21  and T 22  respectively while the emitters thereof are connected to the ground. 
   The level shift circuit  13  is constructed by not only the transistor T 19  and the resistor R 19  constituting the emitter follower circuit portion  6 , but also a PNP transistor T 25  having a collector connected to the base of the transistor T 19  through a resistor R 22  and an emitter connected to the ground. The base of the transistor T 25  is connected to the collector of the transistor T 24 . In the differential amplifying circuit  12 , the connection relationship of the constituent parts corresponding to the resistor R 21 , the differential pair  14  and the current mirror circuit portion  15  is inverted to that of the construction of  FIG. 6 . However, this portion has no feature as a circuit, and may be replaced by the same differential amplifying circuit  3  as shown in  FIG. 6 . 
   According to the embodiment thus constructed, the output voltage of the differential amplifying circuit  12  to which the potentials (reference voltage) at the connection points A and B of the band gap cell circuit  2  are applied as a differential input signal is directly input to the level shift circuit  13  without being passed through the gain forming portion  5  unlike the conventional construction, whereby the gain of the whole circuit is reduced and the phase different allowance degree is more increased. 
   As a result, the oscillation operation can be suppressed even when the phase-compensating capacitors C 1  to C 3  needed in the prior art are deleted, and the circuit scale of the band gap reference voltage circuit  11  can be reduced. In addition to the deletion of the capacitors C 1  to C 3 , the response speed of the circuit operation can be increased by deleting the gain forming portion  5  and reducing the number of the circuit elements. 
   Second Embodiment 
     FIG. 2  shows a second embodiment of the present invention. The same parts as the first embodiment are represented by the same reference numerals, and the description thereof is omitted. Only the different portions will be described. A band gap reference circuit according to a second embodiment is achieved by replacing the level shift circuit  13  of the band gap reference voltage circuit  11  of the first embodiment by a level shift circuit  17 . In the level shift circuit  17 , the transistor T 25  is replaced by a transistor T 26  comprising MOSFET. 
   That is, in the level shift circuit  13  of the band gap reference voltage circuit  11  of the first embodiment, the output voltage of the differential amplifying circuit  12  is received by the transistor T 25 , and unbalance of current occurs in the differential amplifying circuit by the degree corresponding to the current flowing into the base of the transistor T 25 , so that the offset voltage is apt to increase. Therefore, in the second embodiment, the output voltage of the differential amplifying circuit  12  is received by the transistor T 26  which is constructed by a MOSFET and serves as a voltage driving type element. That is, current hardly flows into the gate of the transistor T 26 , and thus occurrence of an offset voltage in the differential amplifying circuit  12  can be suppressed, and the output precision of the reference voltage VBG can be enhanced. 
   Third Embodiment 
     FIGS. 3 and 4  show a third embodiment of the present invention, and only the different portion from the second embodiment will be described. A band gap reference voltage circuit  18  of the third embodiment is achieved by inserting a phase compensating capacitor C 11  between the collector and base of the transistor T 22  disposed at the amplification output side in the differential amplifying circuit  12  constituting the band gap reference voltage circuit  16  of the second embodiment. That is, by adding the capacitor C 11 , the band gap reference voltage circuit  18  can be provided with a larger phase difference allowance degree. 
   The addition of the capacitor C 11  of the third embodiment is determined on the basis of a simulation result achieved by carrying out simulations as to where a capacitor should be connected in order to achieve the highest effect when it is permitted to provide only one capacitor having small capacitance. 
   In the third embodiment, each of the transistor elements constituting the band gap reference voltage circuit  18  is constructed by adding the SOI (Silicon On Insulator) structure with the trench insulating separation structure. Here,  FIG. 4B  is a cross-sectional view showing a case where the PNP transistor is constructed by the junction separation structure. That is, when isolation is carried out by a P-type area  21 , the substrate  22  (P−) of wafer is connected to the ground which is kept to the lowest potential of the circuit, whereby the P-type area  21  disposed so as to surround the device and the N− area  23  in the device are set to be inversely biased. 
   On design, the operation expected to the PNP transistor controls the current between the emitter and the collector in accordance with the base current. However, in the structure shown in  FIG. 4B , under a high temperature atmosphere, current leak occurs at the substrate  22  side from the N− area  23  serving as the base, and the emitter and the collector may be conducted to each other irrespective of the base current which is actually made to flow. Furthermore, a parasite transistor is formed so that P− of the substrate  22  serves as the collector of the PNP transistor, and the current of the circuit may pulled out by the parasite transistor. That is, in the band gap reference voltage circuit  18 , when the transistors T 21  and T 22  constituting the differential pair  14  suffer such an effect as described above, the reference voltage VBG is destabilized. 
   Therefore, in the third embodiment, the PNP transistor is constructed by adding the SOI structure with the trench insulating separation structure. That is, SiO 2  oxide film  25  is formed on the substrate  24 , an N+ layer  26  on the SiO 2  oxide film  25 , and trenches are formed so as to surround the device forming area and extend to the oxide film  25  as shown in  FIG. 4A . SiO 2  oxide film  28  is filled in the trenches  27 . In this construction, no current leak occurs and no parasite transistor unlike the junction separation structure shown in  FIG. 4B , and thus the operation characteristics of the band gap reference voltage circuit  18  can be stabilized under a high temperature atmosphere. 
   Fourth Embodiment 
     FIG. 5  shows a fourth embodiment of the present invention, and only different portion from the second embodiment will be described. A band gap reference voltage circuit  31  of the fourth embodiment is designed so that the transistors T 21  and T 22  are replaced by transistors T 31  and T 32  comprising P-channel MOSFETs, the transistors T 23  and T 24  are replaced by transistors T 33  and T 34  comprising N-channel MOSFETs and the transistor T 19  is replaced by a transistor T 35  comprising an N-channel MOSFET in the band gap reference voltage circuit  16  of the second embodiment. The respective circuit portions at which the elements are replaced constitute a differential pair  32 , a current mirror circuit portion  33  and a level shift circuit  34 . A differential amplifying circuit  35  is constructed by the differential pair  32  and the current mirror circuit portion  33 . 
   According to the fourth embodiment thus constructed, the dispersion of the offset voltage is apt to slightly increase as compared with the second embodiment, however, substantially the same action and effect can be achieved. 
   The present invention is not limited to the embodiments which are described above or illustrated in the drawings, and the following modifications may be made. 
   In the constructions of the first, second and fourth embodiments, the transistor achieved by adding the SOI structure with the trench insulating separation structure as in the case of the third embodiment may be used. Furthermore, the phase compensating capacitor C 11  may be added like the third embodiment. 
   In the third embodiment, it is not necessarily applied to all the elements that the trench insulating separation structure is added to the SOI structure to form a transistor, and it may be applied at least elements constituting the differential amplifying circuit  12 . This is because prevention of current leak for the differential amplifying circuit  12  is effective to suppress the offset voltage. 
   Alternatively, the construction of the third embodiment may be designed with a transistor formed by the junction separation structure. 
   The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.