Abstract:
A high slew rate operational amplifier circuit of which the through current of its push-pull transistors is substantially zero is disclosed. The operational amplifier circuit preferably comprises an amplifier portion and a push-pull output amplifier including NPN and PNP output transistors. The output of the amplifier portion is transferred to the NPN output transistors base through a PNP driving transistor and to the PNP output transistors base through an NPN driving transistor. The emitters of the driving transistors are connected to respective power supply conductors through respective current sources. The through current reduction is achieved by resistors inserted between the current sources and the corresponding power supply conductors, an NPN transistor so connected with the NPN output transistor as to constitute a current mirror and a PNP transistor so connected with the PNP output transistor as to constitute another current mirror.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The invention relates to an operational amplifier with a push-pull output stage and more specifically to a high slew rate operational amplifier.  
           [0003]    2. Description of the Related Art  
           [0004]    [0004]FIG. 1 is a diagram showing a structure of a conventional operational amplifier to which the present invention relates. In FIG. 1, the operational amplifier  1  comprises a differential amplifier  10  for providing an amplified output Vio in response to the differential between input voltage V+and V−and an output push-pull amplifier  20  for providing an output Vo in response to the intermediate output Vio from the amplifier  10 . The output push-pull amplifier  20  includes a push-pull transistor stage comprising an NPN transistor T 3  having its collector connected to a positive power supply conductor Vp, a PNP transistor T 4  having its collector connected to a negative power supply conductor Vn, and serially coupled resistors R 1  and R 2  inserted between the emitters of the transistors T 3  and T 4  (hereinafter, referred to as “the push-pull transistors”). To the base of the push-pull first transistor T 3 , there are connected the cathode of a constant current source S 1  having its anode connected to the positive power supply conductor Vp and the emitter of a PNP driving transistor T 1  of which the collector is connected to the negative power supply conductor Vn. To the base of the push-pull second transistor T 4 , there are connected the anode of a constant current source S 2  having its cathode connected to the negative power supply conductor Vn and the emitter of an NPN driving transistor T 2  of which the collector is connected to the positive power supply conductor Vp. The bases of the driving transistors T 1  and T 2  are commonly connected to the output Vio of the amplifier circuit  10  and one end of a capacitor C, the other end of which is connected to the negative power supply conductor Vn.  
           [0005]    In order to raise the slew rate of the output Vo, the push-pull amplifier is so configured that the push-pull transistors T 3  and T 4  are always in ON state and accordingly always pass a certain-level current, which we call “the through current”. The present invention relates to operational amplifiers as shown in FIG. 1.  
           [0006]    Such high slew rate operational amplifiers are used, for example, in electronic control units (ECUs) of an engine and various electronic devices mounted in an automobile. Since it is necessary to make ECUs smaller and lighter, it is necessary to integrate a circuit including one or more operational amplifiers and an output circuit for driving a load into a single chip IC (integrated circuit). The calorific volume of the IC on a chip is one of the factors that limit the size of the IC. For this reason, in order to increase the integration degree of an IC, it is preferable to reduce the calorific volume of an IC. Reducing the through current which flows through the push-pull transistors T 3  and T 4  is one of effective techniques we can use to suppress the calorific volume of an operational amplifier circuit.  
           [0007]    It is an object of the invention to provide a high slew rate operational amplifier circuit that enables the through current of its push-pull transistors to be reduced substantially to zero.  
           [0008]    It is another object of the invention to provide an IC including a high slew rate operational amplifier circuit that enables the through current of its push-pull transistors to be reduced substantially to zero.  
         SUMMARY OF THE INVENTION  
         [0009]    According to the invention, a high slew rate operational amplifier circuit of which the through current of its push-pull transistors is substantially zero is provided. The invention is applicable to an operational amplifier circuit that comprises an amplifier portion for receiving a non-inverting input and an inverting input and providing an amplified output in response to the differential between the non-inverting input and the inverting input; and a push-pull amplifier for providing a final output in response to the amplified output. The push-pull amplifier preferably includes an NPN output transistor having its collector connected to a higher-potential conductor; a PNP output transistor having its collector connected to a lower-potential conductor; two serially connected resistors that are connected between emitters of the NPN and PNP output transistors, a node between the resistors serving as an output of the operational amplifier circuit, a PNP driving transistor having its base coupled with the amplified output and its emitter connected to a base of the NPN output transistor, a collector of the PNP driver transistor being connected to the lower-potential conductor; a first current source connected between the higher-potential conductor and an emitter of the PNP driving transistor; an NPN driving transistor having its base coupled with the amplified output and its emitter connected to a base of the PNP output transistor, a collector of the NPN driver transistor being connected to the higher-potential conductor; and a second current source connected between the lower-potential conductor and an-emitter of the NPN driving transistor.  
           [0010]    As circuit means for reducing a through current flowing through the NPN and PNP output transistors, the push-pull amplifier further comprises: a first resistor inserted between the higher-potential conductor and the first current source; a second resistor inserted between the second current source and the lower-potential conductor; an NPN transistor which is so connected with the NPN output transistor as to constitute a first current mirror and which has its emitter connected to a node between the second current source and the second resistor; and a PNP transistor which is so connected with the PNP output transistor as to constitute a second current mirror and which has its emitter connected to a node between the first resistor and the first current source. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0011]    Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawing, in which:  
         [0012]    [0012]FIG. 1 is a circuit diagram showing a structure of a conventional operational amplifier to which the present invention relates;  
         [0013]    [0013]FIG. 2 is a schematic circuit diagram showing an exemplary structure of an operational amplifier according to an illustrative embodiment of the invention;  
         [0014]    [0014]FIG. 3 is a diagram showing the wiring of an operational amplifier circuit that was used in simulations for comparing the through currents between the conventional operational amplifier of FIG. 1 and the inventive operational amplifier of FIG. 2;  
         [0015]    [0015]FIG. 4 is a diagram of graphs showing the result of the simulation of the inventive operational amplifier  2 ; and  
         [0016]    [0016]FIG. 5 is a diagram of graphs showing the result of the simulation of the conventional operational amplifier  1 .  
         [0017]    Throughout the drawing, the same elements when shown in more than one figure are designated by the same reference numerals. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0018]    [0018]FIG. 2 is a schematic circuit diagram showing an exemplary structure of an operational amplifier according to an illustrative embodiment of the invention. In FIG. 2, the operational amplifier  2  is identical to that of FIG. 1 except that the push-pull output amplifier  20  has been replaced with a push-pull output amplifier  20   a.  Further, the pushpull output amplifier  20   a  is identical to the amplifier  20  except that the push-pull output amplifier  20   a  has been provided with a through current reduction circuit which comprises an NPN transistor T 5 , a PNP transistor T 6  and resistors R 3  and R 4 . The resistor R 3  is inserted between the positive conductor Vp and the anode of the current source S 1 . The resistor R 4  is inserted between the cathode of the current source S 2  and the negative conductor Vn. The transistor T 5  has its collector comuected to the positive conductor Vp, its emitter connected to the node between the current source S 2  and the resistor R 4 , and its base connected to the node including the transistor T 3  base. The transistor T 6  has its collector connected to the negative conductor Vn, its emitter connected to the node between the current source S 1  and the resistor R 3 , and its base connected to the node including the transistor T 4  base.  
         [0019]    It is noted that the electric potentials Vp and Vn may be set to any suitable values as long as the positive conductor Vp is higher in potential than the negative power supply conductor Vn. Therefore, “the positive conductor” should be taken as “a higher-potential conductor” and “the negative conductor” should be taken as “a lower-potential conductor”. Either of the potentials Vp and Vn may be zero. Though the negative power supply terminal Vn is shown by using a well-known symbol indicative of the ground, the negative power supply voltage Vn has not to be 0 V.  
         [0020]    The amplifier  10  may be any suitable amplifier that includes a differential amplifier.  
         [0021]    Then, the operation of the operational amplifier  2  of FIG. 2 is described in the following. It is assumed that the operational amplifier  2  is operated, for example, in a noninverting amplification configuration in which the output Vo terminal of the operational amplifier  2  is connected to the inverting input V−terminal and an input signal V+ is applied to the non-inverting input V+ terminal as shown in FIG. 3.  
         [0022]    If the input voltage V+ goes higher from a certain voltage, then the driving transistor T 1  turns on, causing the push-pull second transistor T 4  to turn ON and keep ON till the output voltage Vo becomes equal to the input voltage V+of the non-inverting input terminal. Since the push-pull first transistor T 3  is also in the ON state in this case, the entire current flowing through the second transistor T 4  flows through the first transistor T 3 .  
         [0023]    The transistor T 5  the base of which is connected to the base of the current flowing transistor T 3  also turns ON, which cause a current to flow through the resistor R 4 . Since the transistors T 3  and T 5  have their collectors also connected together, the transistors T 3  and T 5  operate as a current mirror.  
         [0024]    Since the resistor R 4  is connected to the emitter of the transistor T 2  through the current source S 2 , this reduces the current that flows through the transistor T 2 . This causes the base-emitter voltage of the transistor T 1  to become smaller: i.e., the current of the transistor T 1  becomes smaller, which reduces the base current of the transistor T 3  and accordingly the current flowing through the transistor T 4 .  
         [0025]    If the input voltage V+goes lower from a certain voltage, then the driving transistor T 2  turns on, causing the push-pull first transistor T 3  to turn ON and keep ON till the output voltage Vo becomes equal to the input voltage V+of the non-inverting input terminal. Since the push-pull first transistor T 4  is also in the ON state in this case, the entire current flowing through the first transistor T 3  flows through the second transistor T 4 . The transistor T 6  the base of which is connected to the base of the current flowing transistor T 4  also turns ON, which cause a current to flow through the resistor R 3 . Since the transistors T 4  and T 6  have their collectors also connected together, the transistors T 4  and T 6  operate as a current mirror.  
         [0026]    Since the resistor R 3  is connected-to the emitter of the transistor T 1  through the current source S 1 , this reduces the current that flows through the transistor T 1 . This causes the base-emitter voltage of the transistor T 2  to become smaller: i.e., the current of the transistor T 2  becomes smaller, which reduces the base current of the transistor T 4  and accordingly the current flowing through the transistor T 3 .  
         [0027]    [0027]FIG. 3 is a diagram showing the wiring of an operational amplifier circuit that was used in simulations for comparing the through currents between the conventional operational amplifier of FIG. 1 and the inventive operational amplifier of FIG. 2. While the operational amplifier  2  is operated in the above-mentioned non-inverting amplification configuration for example, the through current that flows through the push-pull transistors T 3  and T 4  is monitored.  
         [0028]    [0028]FIGS. 4 and 5 shows the result of the simulations in which a rectangular pulse of 3 V in the amplitude is applied to the non-inverting input terminal. In case of the inventive operational amplifier  2 , the through current is almost always substantially 0 mA except at the leading edge and the trailing edge of the input pulse as shown in FIG. 4. On the other hand, in case of the conventional operational amplifier  1 , the through current is almost always 3.8 mA as shown in FIG. 4.  
         [0029]    Turning back to FIG. 2, in this specific example, the amplifier  10  comprises a conventional input amplifier  12  and a conventional differential amplifier  14  for example.  
         [0030]    The input amplifier  12  includes a pair of amplifiers that shares a constant current source S 10 . The inverting input V−is applied to the base of a PNP transistor T 11  through a resistor R 11 . The transistor T 11  has its collector connected to the negative conductor Vn and its emitter connected to one end of a leg of serially connected constant current source S 11  and resistor R 12  and to the gate of the following NPN transistor T 12 . The other end of the leg is connected to the positive conductor Vp. The transistor T 12  has its emitter connected to the anode of the common constant current source S 10  and its collector connected to the cathode of a constant current source S 12 , the anode of which is connected to the positive conductor Vp. The transistor-T 12  collector provides an amplified version of the inverting input V−.  
         [0031]    Similarly, the non-inverting input V+is applied to the base of a PNP transistor T 21  through a resistor R 21 . The transistor T 21  has its collector connected to the negative conductor Vn and its emitter connected to one end of a leg of serially connected constant current source S 21  and resistor R 22  and to the gate of the following NPN transistor T 22 . The other end of the leg is connected to the positive conductor Vp. The transistor T 22  has its emitter connected to the anode of the common constant current source S 10  and its collector connected to the cathode of a constant current source S 22 , the anode of which is connected to the positive conductor Vp. The transistor T 22  collector provides an amplified version of the non-inverting input V+.  
         [0032]    The differential amplifier  14  comprises, for example, two diodes D 1  and D 2 , two PNP transistors T 31  and T 32 , four NPN transistors T 33  through TT 36  and two resistors R 31  and R 32 . The emitters of the transistors T 31  and T 32  are connected to the collectors of the transistors T 22  and T 12  of the input amplifier  12 , respectively. The bases of the transistors T 31  and T 32  are connected to each other and to the cathode end of serially connected diodes D 1  and D 2  and the anode of a constant current source S 31 , the cathode of which is connected to the negative conductor Vn. The anode end of the serially connected diodes D 1  and D 2  is connected to the positive conductor Vp. The collectors of the transistors T 31  and T 32  are connected to the collectors of the transistors T 33  and T 34 , respectively. The transistors T 33  and T 34  have their bases connected to each other and to the collectors of the transistors T 31  and T 33  and their emitters connected to the collectors of the transistors T 35  and T 36 , respectively. The bases of the transistors T 35  and T 36  are connected to each other and to the collector of the transistor T 36 . The emitters of the transistors T 35  and T 36  are connected to one ends of respective resistors R 31  and R 32 , the other ends of which are connected to the negative conductor Vn. The collector node of the transistors T 32  and T 34  serves as output Vio of the differential amplifier  14 .  
         [0033]    Since the conventional input amplifier  12  and the conventional differential amplifier  14  operates in the well-known manner, the description of the operation of the amplifiers  12  and  14  is omitted.  
         [0034]    The above-described operational amplifier  2  may be implemented as a discrete circuit or as a part of an integrated circuit. For example,  
         [0035]    Many widely different embodiments of the present invention may be constructed without departing from the spirit and scope of the present invention. It should be understood that the present invention is not limited to the specific embodiments described in the specification, except as defined in the appended claims.