Abstract:
An operational amplification circuit includes a differential amplification circuit portion that amplifies a differential input, and an output circuit portion that outputs the amplified output using a signal amplified in the differential amplification circuit portion. The differential amplification circuit portion is provided with a pair of first transistors to which signals are differentially input, and second and third transistors which are connected to current paths of the pair of first transistors and which constitute current mirror circuits with respect to each other. The output circuit portion is provided with a fourth transistor, a gate of which is connected to a drain of the second transistor, and an amplified output is output from a drain of the fourth transistor.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present invention contains subject matter related to and claims priority to Japanese Patent Application JP 2009-128848 filed in the Japanese Patent Office on May 28, 2009, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to an operational amplification circuit having a differential amplification circuit portion which uses a current mirror circuit. 
     2. Related Art 
       FIG. 4  is a diagram illustrating an example of an operational amplification circuit having a current mirror circuit in the related art. The operational amplification circuit is provided with a differential amplification circuit portion  1  in which a pair of MOS transistors T 20  and T 21  are differentially connected, a current mirror circuit portion  2 , a constant current circuit portion  3 , and an output circuit portion  4 . The output circuit portion  4  is formed of a MOS transistor T 22  and a load resistor  5  connected to a drain output terminal of the MOS transistor T 22 . A gate of the MOS transistor T 22  is connected to a drain output terminal of the MOS transistor T 20 . 
     In the operational amplification circuit configured as described above, a differential amplification output of the drain of the MOS transistor T 20  of the differential amplification circuit portion  1  is applied to the gate of the MOS transistor T 22  of the output circuit portion  4 , and an electric current corresponding to a gate voltage of the MOS transistor T 22  flows in between the source and the drain of the MOS transistor T 22 , thereby outputting an output voltage V OUT . 
     Examples of the related art are disclosed, for example, in Japanese Unexamined Patent Application Publication No. 5-63455 and Japanese Unexamined Patent Application Publication No. 11-127037. 
     SUMMARY 
     According to an aspect of the disclosure, there is provided an operational amplification circuit including a differential amplification circuit portion that amplifies a differential input and an output circuit portion that outputs the amplified output using a signal amplified in the differential amplification circuit portion. The differential amplification circuit portion is provided with a pair of first transistors to which signals are differentially input, and second and third transistors which are connected to current paths of the pair of first transistors and which constitute current mirror circuits with respect to each other. The output circuit portion is provided with a fourth transistor, a gate of which is connected to a drain of the second transistor, and an amplified output is output from a drain of the fourth transistor. A fifth transistor, a gate of which is connected to the drain of the second transistor, is provided between the second transistor and the ground, and a sixth transistor, a gate of which is connected to the drain of the third transistor, is provided between the third transistor and the ground. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram illustrating an operational amplification circuit according to an embodiment of the disclosure. 
         FIG. 2  is a graph illustrating a test result of current variation between lots of operational amplification circuits of the invention and operational amplification circuits in the related art. 
         FIG. 3  is a graph illustrating a test result of current variation between wafers of the operation amplification circuits of the invention and the operational amplification circuits in the related art. 
         FIG. 4  is a circuit diagram illustrating the operational amplification in the related art. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Hereinafter, an embodiment of the invention will be described in detail with reference to the accompanying drawings. 
       FIG. 1  is a diagram illustrating a configuration of an operational amplification circuit according to an embodiment of the invention. The operational amplification circuit  1  is provided with first and second differential amplification circuit portions  10 - 1  and  10 - 2 , and an output circuit portion  20  formed of a current mirror type push-pull circuit. The first and second differential amplification circuits  10 - 1  and  10 - 2  have the same configuration, and thus the same reference numerals and signs are given to the corresponding elements. 
     The first differential amplification circuit portion  10 - 1  is provided with a pair of PMOS transistors T 1   a  and T 1   b  to which signals (Vinn and Vinp) are differentially input. Sources of the PMOS transistors T 1   a  and T 1   b  are connected to a constant current source  12 , a drain of the PMOS transistor T 1   a  is connected to a drain of a second NMOS transistor T 2  constituting a current mirror circuit, and a drain of the PMOS transistor T 1   b  is connected to a drain of a third NMOS transistor T 3  constituting the same mirror circuit. A positive input signal Vinp is input to a gate of the PMOS transistor T 1   a , and a negative input signal Vinn is input to a gate of the PMOS transistor T 1   b.    
     Sources of the second and third NMOS transistors T 2  and T 3  constituting the mirror circuits are connected to the ground through drains and sources of fifth and sixth NMOS transistors T 5  and T 6 . A gate of the fifth NMOS transistor T 5  is connected to the drain of the second NMOS transistor T 2 , and a gate of the sixth NMOS transistor T 6  is connected to the drain of the third NMOS transistor T 3 . 
     The output circuit portion  20  is formed of a current mirror type push-pull circuit. Sources of eighth and ninth PMOS transistors T 8  and T 9  constituting current mirror circuits are connected to a power supply terminal Vdd, a drain of the eighth PMOS transistor T 8  is connected to the ground through the drain and source of the fourth NMOS transistor T 4 , and a drain of the ninth PMOS transistor T 9  is connected to the ground through the drain and source of the seventh NMOS transistor T 7 . The gate of the fourth NMOS transistor T 4  is connected to the drain of the PMOS transistor T 1   a  in the first differential amplification circuit portion  10 - 1 , and the differential output voltage of the first differential amplification circuit portion  10 - 1  is applied to the gate of the fourth NMOS transistor T 4 . The gate of the seventh NMOS transistor T 7  is connected to the drain of the PMOS transistor T 1   b  in the second differential amplification circuit portion  10 - 2 , and the differential output voltage of the second differential amplification circuit  10 - 2  is applied to the gate of the seventh NMOS transistor T 7 . 
     Next, the operation of the embodiment configured as described above will be described. 
     The differential input signals Vinp and Vinn are input to the first and second differential amplification circuit portions  10 - 1  and  10 - 2 . In the first and second differential amplification circuit portions  10 - 1  and  10 - 2 , a bias current flows which corresponds to the gate voltage of the differential input signal Vinp applied to the gate of the PMOS transistor T 1   a , and a bias current flows which corresponds to the gate voltage of the differential input signal Vinn applied to the gate of the PMOS transistor T 1   b . At this time, the same electric current flows in the second and third NMOS transistor T 2  and T 3  constituting the current mirror circuit, and the gate voltage corresponding to the differential current between the bias current flowing in the PMOS transistor T 1   a  and the bias current flowing in the PMOS transistor T 1   b  is applied to the fourth and seventh NMOS transistors T 4  and T 7  of the output circuit portion  20 . The electric current corresponding to the gate voltage flows between the source and the drain of the fourth NMOS transistor T 4 , and the electric current corresponding to the gate voltage flows between the source and the drain of the seventh NMOS transistor T 7 . The same electric current flows in the eighth and ninth NMOS transistors T 8  and T 9  constituting the current mirror circuit, and the output voltage Vout corresponding to the differential current between the electric current flowing in the fourth NMOS transistor T 4  and the electric current flowing in the seventh NMOS transistor T 7  is taken out of the drain of the fourth NMOS transistor T 4 . 
     In the embodiment, as for the second NMOS transistor T 2 , the fifth NMOS transistor T 5  is connected in series, and the gate of the fifth NMOS transistor T 5  is connected to the drain (the gate of the fourth NMOS transistor T 4 ) of the second NMOS transistor T 2 . For this reason, when the gate voltage with respect to the fourth NMOS transistor T 4  tends to vary in a direction to increase, the gate voltage of the fifth NMOS transistor T 5  also varies in the same direction. When the gate voltage of the fifth NMOS transistor T 5  increases, the electric current flowing in the fifth NMOS transistor T 5  increases, and the gate potential of the fourth and fifth NMOS transistors T 4  and T 5  decrease. Accordingly, the gate voltage of the fourth NMOS transistor T 4  converges to a predetermined voltage. When the gate voltage of the fourth NMOS transistor T 4  tends to vary in the direction to decrease, the gate voltage of the fifth NMOS transistor T 5  varies to the same voltage in the same direction. Accordingly, the gate voltage of the fourth NMOS transistor T 4  converges to a predetermined voltage. 
     In addition, as for the third NMOS transistor T 3  constituting the current mirror circuit with the second NMOS transistor T 2 , the sixth NMOS transistor T 6  is connected in series, and the gate of the sixth NMOS transistor T 6  is connected to the drain of the third NMOS transistor T 3 . For this reason, the gate voltage of the fourth NMOS transistor T 4  converges to the gate voltages of the second and third NMOS transistors T 2  and T 3 . 
     As described above, even when the gate voltage of the fourth NMOS transistor T 4  varies due to differences in production, the gate voltage of the fourth NMOS transistor T 4  converges to a predetermined voltage by the feedback operation of the sixth NMOS transistor T 6 . Accordingly, it is possible to suppress the variation of the electric current flowing in the fourth NMOS transistor T 4 , and it is possible to realize a stable amplification operation. 
     The output circuit portion  20  is provided with the seventh NMOS transistor T 7 , and the output voltage of the second differential amplification circuit portion  10 - 2  is applied to the gate of the seventh NMOS transistor T 7 . Accordingly, when the gate voltage of the fourth NMOS transistor T 4  cannot obtain a low output, the electric current flows between the source and the drain of the seventh NMOS transistor T 7 . Therefore, the output can be taken out of the source of the eighth NMOS transistor T 8 . 
     Next, a verification test result of the current variation situation in the embodiment and the comparative example will be described. 
     In the comparative example, the fifth and sixth NMOS transistors T 5  and T 6  are removed from the operational amplification circuit shown in  FIG. 1 . 
       FIG. 2A  and  FIG. 2B  are graphs illustrating the current variation when the NMOS transistors produced from different lots are used as the fourth and seventh NMOS transistors of the output circuit portion  20 .  FIG. 2A  is a test result of the embodiment, and  FIG. 2B  is a test result of the comparative example. In the graphs, the vertical axis denotes a number (the number of samples) and the horizontal axis denotes an electric current (mA). The number of variation trial times used in the verification test is 100. As a result of the verification test, the operational amplification circuit was σES=0.34 [mA], but the comparative example was σTS=0.73 [mA]. In the current variation between lots, it was confirmed that the value of the comparative example is unsatisfactory. In addition, σTS(σES) is the standard deviation which can be obtained by the following formula.
 
Standard Deviation: σ2=Σ( Ii−Iave )2 /n  (Iave: average value of electric current, Ii: sample value, n: number of trial times)
 
       FIG. 3A  and  FIG. 3B  are graphs illustrating the current variation when the NMOS transistors produced from the same wafer are used as the fourth and seventh transistors of the output circuit.  FIG. 3A  is a test result of the operational amplification circuit of the embodiment, and  FIG. 3B  is a test result of the comparative example. In the graphs, the vertical axis denotes a number, and the horizontal axis denotes an electric current (mA). The number of variation trial times used in the verification test is 100. As a result of the verification test, the operational amplification circuit of the embodiment is σES=0.51 [mA], but the comparative example was σTS=1.9 [mA]. In the current variation in the wafer, it was confirmed that the value of the comparative example is unsatisfactory. 
     From the test result, clearly, in the operational amplification circuit of the invention, the current variation is reduced. 
     In the above description, the PMOS transistors are used as the transistors constituting the differential amplification circuit portions  10 - 1  and  10 - 2  of the first and second differential amplification circuit portion and the output circuit portion  20  of the operational amplification circuit, but NMOS transistors may be used instead of the PMOS transistors. When the NMOS transistors are used, the connection position of the constant current source  12  varies, but the basic configuration does not vary. 
     It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims of the equivalents thereof.