Abstract:
An analog amplifier for recovering an abnormal operation of a common-mode feedback is provided. An analog variable amplifier includes a first input transistor and a second input transistor, a first output transistor and a second output transistor, a third transistor and a fourth transistor, a first current source, a fifth transistor and a sixth transistor, and a second current source. The first input transistor and the second input transistor amplify a bias current depending on a magnitude of a first input voltage and a second input voltage. The first output transistor and the second output transistor output the amplified bias current. The third transistor and the fourth transistor receive an output voltage of the first output transistor as an input and amplifying the received output voltage. The first current source provides a predetermined current between the first output transistor and the third transistor.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
       [0001]    This application claims the benefit under 35 U.S.C. §119(a) of a Korean patent application filed on Sep. 3, 2012 in the Korean Intellectual Property Office and assigned Serial No. 10-2012-0097396, the entire disclosure of which is hereby incorporated by reference. 
       TECHNICAL FIELD 
       [0002]    The present disclosure relates to an analog amplifier for amplifying or attenuating an analog signal and an electronic device thereof 
       BACKGROUND 
       [0003]    As a result of a continuous increase in the integration degree and reliability of an integrated circuit, an analog circuit and a digital circuit are integrated and developed in one chip. Also, an analog circuit and a system play an important role in realization and application of a high-density Integrated Circuit (IC) technology. For example, nearly all high-density IC systems employ an amplifier, a filter, a detector, a comparator, etc. 
         [0004]      FIG. 1  illustrates an analog filter structure where analog variable amplifiers are configured in two stages according to the related art. 
         [0005]    Referring to  FIG. 1 , an analog variable amplifier  100  of a first stage includes an operational amplifier and two variable resistors R 1  and R 2 . Depending on the implementation, the analog variable amplifier  100  of the first stage may include one variable resistor and one variable capacitor instead of the operational amplifier and the two variable resistors R 1  and R 2 . 
         [0006]    Likewise, an analog variable amplifier  102  of a second stage includes an operational amplifier and two variable resistors R 3  and R 4 . Depending on the implementation, the analog variable amplifier  102  of the second stage may include one variable resistor and one variable capacitor instead of the operational amplifier and the two variable resistors R 3  and R 4 . 
         [0007]    In each analog variable amplifier stage, a gain is determined by a ratio of the input resistors R 1  and R 3  to the feedback resistors R 2  and R 4 . Assuming a transfer function of the operational amplifier is A(s), the gain is expressed below by Equation (1). In an ideal operational amplifier, a gain has a value of R 2 /R 1  or R 4 /R 3  in its infinity. Also, the gain of the entire circuit is represented as product (G1×G2) of gains at respective stages. Here, G1 is the gain of the analog variable amplifier  100  of the first stage, and G2 is the gain of the analog variable amplifier  102  of the second stage. 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       G 
                       1 
                     
                     = 
                     
                       
                         
                           R 
                           2 
                         
                          
                         
                           A 
                            
                           
                             ( 
                             s 
                             ) 
                           
                         
                       
                       
                         
                           R 
                           1 
                         
                         + 
                         
                           R 
                           2 
                         
                         - 
                         
                           
                             R 
                             1 
                           
                            
                           
                             A 
                              
                             
                               ( 
                               s 
                               ) 
                             
                           
                         
                       
                     
                   
                    
                   
                     
 
                   
                    
                   
                     
                       G 
                       2 
                     
                     = 
                     
                       
                         
                           R 
                           4 
                         
                          
                         
                           A 
                            
                           
                             ( 
                             s 
                             ) 
                           
                         
                       
                       
                         
                           R 
                           3 
                         
                         + 
                         
                           R 
                           4 
                         
                         - 
                         
                           
                             R 
                             3 
                           
                            
                           
                             A 
                              
                             
                               ( 
                               s 
                               ) 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   
                     ( 
                     1 
                     ) 
                   
                 
               
             
           
         
       
     
         [0008]    Here, the variable gain amplifier is configured by controlling the values of the input resistors R 1  and R 3  or the feedback resistors R 2  and R 4 . In the analog variable amplifier  100  of the first stage, when R 1  is greater than R 2 , the variable gain amplifier performs attenuation. When R 1  is less than R 2 , the variable gain amplifier performs amplification. Likewise, in the analog variable amplifier  102  of the second stage, when R 3  is greater than R 4 , the variable gain amplifier performs attenuation. When R 3  is less than R 4 , the variable gain amplifier performs amplification. 
         [0009]    Meanwhile, in a case of configuring an analog amplifier requiring a range from −40 dB or less to 0 dB or more, an analog filter processes a wide operation range via two or more analog amplifier stages. For example, a first analog amplifier stage processes −20 dB or less ˜0 dB or more, and a second analog amplifier stage processes −20 dB or less ˜0 dB or more, so that the entire gain becomes −40 dB or less ˜0 dB or more. 
         [0010]    However, an analog filter including a plurality of amplifier stages consumes twice more power compared to the case where one amplifier stage amplifies an analog signal, and requires twice more area, so that manufacturing costs increase. 
         [0011]    Therefore, there is a need for an analog amplifier including a single amplifier stage. 
         [0012]    The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present disclosure. 
       SUMMARY 
       [0013]    Aspects of the present disclosure are to address at least the above-mentioned problems and/or disadvantages and to provide at least the advantages described below. Accordingly, an aspect of the present disclosure is to provide an operational amplifier circuit that can realize a wide operation range required by an analog amplifier using only one stage. 
         [0014]    Another aspect of the present disclosure is to provide an analog circuit having low power consumption and a small area by preventing a phenomenon that a ratio of input resistance to feedback resistance becomes very large and so an operation of an input transistor is blocked, to widen an operation range. 
         [0015]    Still another aspect of the present disclosure is to provide an analog gain amplifier having a high efficiency and an improved yield. 
         [0016]    In accordance with an aspect of the present disclosure, an analog variable amplifier is provided. The analog variable amplifier includes a first input transistor and a second input transistor configured to amplify a bias current depending on a magnitude of a first input voltage and a second input voltage, a first output transistor and a second output transistor configured to output the amplified bias current, a third transistor and a fourth transistor configured to receive an output voltage of the first output transistor as an input and to amplify the received output voltage, a first current source configured to provide a predetermined current between the first output transistor and the third transistor, a fifth transistor and a sixth transistor configured to receive an output voltage of the second output transistor as an input and to amplify the received output voltage, and a second current source configured to provide a predetermined current between the second output transistor and the fifth transistor. 
         [0017]    In accordance with another aspect of the present disclosure, an operational amplifier is provided. The operational amplifier includes a first stage and a second stage of the operational amplifier, and a current source between the first stage and the second stage to prevent a voltage of an input transistor of the operational amplifier from dropping down a threshold or less. 
         [0018]    Other aspects, advantages and salient features of the disclosure will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses various embodiments of the present disclosure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The above and other aspects, features, and advantages of certain embodiments of the present disclosure will be more apparent from the following description taken in conjunction with the accompanying drawings, in which: 
           [0020]      FIG. 1  is a view illustrating an analog filter structure where an analog variable amplifier is configured in two stages according to the related art; 
           [0021]      FIG. 2  is a view illustrating an analog variable amplifier according to an embodiment of the present disclosure; 
           [0022]      FIG. 3  is a view illustrating a circuit of an operational amplifier according to an embodiment of the present disclosure; and 
           [0023]      FIG. 4  is a view illustrating a cut-off circuit of an operational amplifier according to an embodiment of the present disclosure. 
       
    
    
       [0024]    Throughout the drawings, like reference numerals will be understood to refer to like parts, components and structures. 
       DETAILED DESCRIPTION 
       [0025]    The following description with reference to the accompanying drawings is provided to assist in a comprehensive understanding of various embodiments of the present disclosure as defined by the claims and their equivalents. It includes various specific details to assist in that understanding but these are to be regarded as merely exemplary. Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the various embodiments described herein can be made without departing from the scope and spirit of the present disclosure. In addition, descriptions of well-known functions and constructions may be omitted for clarity and conciseness. 
         [0026]    The terms and words used in the following description and claims are not limited to the bibliographical meanings, but, are merely used by the inventor to enable a clear and consistent understanding of the present disclosure. Accordingly, it should be apparent to those skilled in the art that the following description of various embodiments of the present disclosure is provided for illustration purpose only and not for the purpose of limiting the present disclosure as defined by the appended claims and their equivalents. 
         [0027]    It is to be understood that the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a component surface” includes reference to one or more of such surfaces. 
         [0028]    The present disclosure relates to an analog amplifier having a wide gain range using one amplification stage by preventing a phenomenon that an input transistor does not operate in the case where it has a very low gain. 
         [0029]    Various embodiments of the present disclosure provide an analog amplifier for recovering an abnormal operation of a common-mode feedback. 
         [0030]      FIG. 2  illustrates an analog variable amplifier according to an embodiment of the present disclosure. 
         [0031]    Referring to  FIG. 2 , the analog variable amplifier has an analog filter structure configured in a first stage. The analog variable amplifier includes an operational amplifier  200 , an input resistor R 1    201 , and a feedback resistor R 2    202 . Here, the input resistor R 1    201  and the feedback resistor R 2    202  may be at least one variable resistor. 
         [0032]    Depending on the implementation, the analog variable amplifier may include one variable resistor and one variable capacitor instead of the operational amplifier and the two variable resistors R 1  and R 2  ( 201 ,  202 ). 
         [0033]    In an analog variable amplifier configured in the first stage, a gain is determined by a ratio of the input resistor R 1  to the feedback resistor R 2 . Assuming a transfer function of the operational amplifier  200  is A(s), the gain is given below by Equation (2). In an ideal operational amplifier, a gain has a value of R 2 /R 1  in its infinity. 
         [0000]    
       
         
           
             
               
                 
                   Gain 
                   = 
                   
                     
                       
                         R 
                         2 
                       
                        
                       
                         A 
                          
                         
                           ( 
                           s 
                           ) 
                         
                       
                     
                     
                       
                         R 
                         1 
                       
                       + 
                       
                         R 
                         2 
                       
                       - 
                       
                         
                           R 
                           1 
                         
                          
                         
                           A 
                            
                           
                             ( 
                             s 
                             ) 
                           
                         
                       
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   
                     ( 
                     2 
                     ) 
                   
                 
               
             
           
         
       
     
         [0034]    In Equation (2), a variable gain amplifier is configured by controlling a value of the input resistor R 1  or the feedback resistor R 2 . In the analog variable amplifier  200 , when R 1  is greater than R 2 , the variable gain amplifier performs attenuation. When R 1  is less than R 2 , the variable gain amplifier performs amplification. 
         [0035]    The operational amplifier  200  used inside the analog variable amplifier may have a Complementary Metal-Oxide Semiconductor (CMOS) structure as illustrated in  FIG. 3 . 
         [0036]      FIG. 3  is a view illustrating a circuit of an operational amplifier according to an embodiment of the present disclosure. 
         [0037]    Referring to  FIG. 3 , the operational amplifier  200  is a differential amplifier and includes a common source amplifier configured in two stages. For ease in explanation, the operational amplifier may be divided into half circuits  320  and  330  having a symmetric structure. 
         [0038]    In the first common source amplifier circuit  320  and the second common source amplifier circuit  330 , a P-type Metal-Oxide Semiconductor (PMOS) bias voltage and an N-type Metal-Oxide Semiconductor (NMOS) bias voltage are supplied to VP ( 331 ,  321 ) and VN ( 324 ,  334 ), respectively. For convenience in description, a common-mode feedback circuit and compensation elements have been omitted. 
         [0039]    The differential amplifier includes a first input terminal (i.e., gate of CMOS transistor  302 ) and a second input terminal (i.e., gate of CMOS transistor  303 ) to receive two voltage signals, that is, a first voltage and a second voltage. Here, the first voltage and the second voltage may be a positive voltage and a negative voltage, respectively. A bias current input from a current source  301  is amplified depending on each input voltage magnitude and forms an amplified voltage at output resistor terminals  324  and  334 , and is input to an input terminal (i.e., gate of CMOS transistors  322  and  332 ) of the second common-source amplifier. Also, this voltage is amplified by transistors  321  and  322 , or transistors  331  and  332  forming the second amplifier, and then is output via output terminals  323  and  333 . 
         [0040]    In the general case, a voltage differentially input to the input terminals  302  and  303  is amplified by a designed gain and output as a voltage form at the output terminals  323  and  333 . A common component of this signal always maintains a constant voltage via a common-mode feedback circuit. That is, a common voltage which is an average of the voltages of the positive output terminal  323  and the negative output terminal  333 , and may operate in the normal range. For example, in detail, when the common voltage of the output terminals  323  and  333  is raised simultaneously by an environmental factor (e.g., temperature, operation voltage, etc.) and becomes a voltage close to a voltage source VDD, a VN value (that is, bias voltage provided to gate of CMOS transistor  324  and gate of CMOS transistor  334 ) is lowered by the common-mode feedback circuit, which raises the voltage of a node A whose phase is opposite to that of the VN, so that the voltage of output terminals  323  and  333  whose phase is opposite to that of the node A is lowered consequently and balance is maintained. In contrast, when the common voltage of the output terminals lowers simultaneously and becomes a voltage close to the ground, a VN voltage is raised by the common-mode feedback circuit, which lowers the voltage of the node A, so that the voltage of the output terminals  323  and  333  is raised and balance is restored. 
         [0041]      FIG. 4  is a view illustrating a cut-off circuit of an operational amplifier according to an embodiment of the present disclosure. 
         [0042]      FIG. 4  illustrates the half of the differential amplifier  300  and the second common source amplifier circuit  330  in the operational amplifier of  FIG. 3 . Also,  FIG. 4  additionally illustrates the input resistor R 1  and the feedback resistor R 2  of  FIG. 2 . 
         [0043]    In the case where set attenuation of the differential amplifier is large and so the input resistor R 1  is considerably greater than the feedback resistor R 2 , a phenomenon that the input transistor  303  of the differential amplifier  300  does not operate due to a resistance ratio when one amplification stage processes a signal may occur. For example, in the case where a gain is −20 dB, a signal is attenuated by 1/10, and R 1  should be ten times greater than R 2 . Likewise, in the case where a gain is −40 dB, R 1  becomes a hundred times greater than R 2 . That is, when an attenuation range is widened, a ratio of R 1  to R 2  increases very much. In the case where R 1  becomes excessively larger than R 2  as described above, a possibility occurs that the input transistor  303  of the differential amplifier  300  in the analog variable amplifier does not operate. 
         [0044]    In a case of the general operation, a balance point is restored by controlling the voltage of the VN via the common-mode feedback circuit as described above. 
         [0045]    For example, when the VN which is a bias voltage of the first stage is raised ( 410 ) by an environmental factor (e.g., temperature, operation voltage, etc.) and becomes a voltage close to a voltage source VDD, the voltage of the node A is lowered by the common-mode feedback circuit ( 400 ,  412 ). When the voltage of the node A is lowered, an output voltage Vo− may be raised ( 414 ). To maintain a balance, the voltage of the node A whose phase is opposite to that of the VN is raised ( 402 ), and consequently the voltage of the output terminal  333  whose phase is opposite to that of the node A is lowered and a balance may be maintained ( 404 ). In contrast, when the common voltage of the output terminal is lowered simultaneously ( 404 ) and becomes a voltage close to the ground, the VN value is raised by the common-mode feedback circuit ( 410 ), which lowers the voltage of the node A ( 412 ), so that the voltage of the output terminal  333  is raised and a balance is restored. 
         [0046]    However, in the case where the input resistor R 1  is considerably larger than the feedback resistor R 2 , compensation by the common-mode feedback circuit does not operate, which will be described below. 
         [0047]    A voltage supplied to a node VG which is the virtual ground is given below by Equation (3) via a resistance divider. When R 1  becomes excessively larger than R 2 , when V 0−  becomes a voltage close to VDD, the voltage close to the voltage source VDD is applied to the node VG which is a gate of the input transistors  302  and  303  of the differential amplifier. 
         [0000]    
       
         
           
             
               
                 
                   
                     V 
                     VG 
                   
                   = 
                   
                     
                       
                         
                           R 
                           1 
                         
                          
                         
                           V 
                           
                             0 
                             - 
                           
                         
                       
                       + 
                       
                         
                           R 
                           2 
                         
                          
                         
                           V 
                           in 
                         
                       
                     
                     
                       
                         R 
                         1 
                       
                       + 
                       
                         R 
                         2 
                       
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   
                     ( 
                     3 
                     ) 
                   
                 
               
             
           
         
       
     
         [0048]    At this point, a voltage Vgs applied to the gate and the source of the input NMOS transistor  303  of the differential amplifier  300  becomes lower than a threshold voltage of the input NMOS transistor  303 , so that the input transistor is turned off. Accordingly, a current does not flow through the first PMOS transistor  332 , so that even when the VN voltage of the second PMOS transistor  334  is lowered, the voltage of the node A is not restored. This phenomenon conspicuously occurs in the case where a supply voltage is low and temperature is low, and direct relation to yield occurs. Therefore, to avoid this phenomenon, the related-art connects amplifier stages having a gain whose operation range is small in series and uses the same as illustrated in  FIG. 1 . As described above, the related-art has disadvantages of consuming much power and occupying a large area. 
         [0049]    Therefore, current sources  340  and  350  allowing a fine current to flow through the node A are provided inside the operational amplifier. 
         [0050]    The current sources I leak  supplied to the node A are added to the first common-source amplifier circuit  320  and the second common-source amplifier circuit  330 , and 1/10˜ 1/100 of the bias current I B    301  supplied to the main amplifier is appropriate for the magnitude of the current. When the added fine current source is too large, a mismatch increases at both terminals and the entire power consumption may increase. However, in the present disclosure, the magnitude of the current source supplied to the node A is not limited. 
         [0051]    As described above, under an environment where the output voltage V 0−  is raised, even in the state where a node VG approaches VDD due to a resistance divider and the input transistor  303  of the differential amplifier does not operate, a current is supplied to the node A via the current source I leak  and the voltage of the node A is raised again under control of the second PMOS transistor  334  and consequently the operation range of the output voltage V 0−  is lowered. 
         [0052]    Though the present disclosure has exemplarily described a common-source amplifier using a CMOS transistor, it is applicable to a common-gate amplifier and a common-drain amplifier, and the analog amplifier of  FIG. 4  may be configured using a Bi-polar Junction Transistor (BJT) instead of a CMOS transistor. 
         [0053]    As described above, the present disclosure processes an analog signal using a single stage or a smaller number of stages via an operational amplifier circuit that may apply amplification and attenuation of an analog signal over a wide range simultaneously, so that power consumption may be reduced, a circuit area may be reduced, and so manufacturing costs may be saved. 
         [0054]    Also, the present disclosure may increase reliability of a circuit operation and also expect yield improvement by including a current source supplying a fine current inside the circuit to prevent the operation of an input transistor from being cut-off. Particularly, the present disclosure has an effect of securing operation reliability of an operational amplifier under a poor environment such as a low voltage and low temperature. 
         [0055]    While the present disclosure has been shown and described with reference to various embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present disclosure as defined by the appended claims and their equivalents.