Abstract:
There is provided an operational amplifier capable of detecting that an input terminal has been open circuited without restricting the voltage range of an input signal. The operational amplifier includes a first comparator which detects that an inverting input terminal of an operational amplifier has been open circuited, a second comparator which detects that a non-inverting input terminal of the operational amplifier has been open circuited, a first resistor and a first switch which are controlled by output signals of the first comparator and the second comparator and which are connected in series between the non-inverting input terminal and a ground terminal of the operational amplifier, and a second resistor and a second switch which are connected in series between the inverting input terminal and a supply terminal of the operational amplifier.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2013-039652 filed on Feb. 28, 2013, the entire content of which is hereby incorporated by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to an operational amplifier and more particularly to an operational amplifier capable of preventing, without restricting an input signal voltage range, an output terminal from becoming inconstant when an input terminal is open circuited. 
         [0004]    2. Background Art 
         [0005]      FIG. 3  is a circuit diagram of a conventional operational amplifier. The conventional operational amplifier is comprised of comparators  301  and  303 , reference voltage circuits  302  and  304 , an OR circuit  305 , a constant current circuit  306 , an operational amplifier  106 , resistors  307  and  308 , input terminals  101  and  102 , an output terminal  103 , a supply terminal  104 , and a ground terminal  105 . 
         [0006]    The operation of the conventional operational amplifier will be described. 
         [0007]    The comparator  301  outputs a high signal when the voltage of a non-inverting input terminal exceeds the voltage of the reference voltage circuit  302 . The voltage of the reference voltage circuit  302  is set to a voltage that is lower than a voltage VDD of the supply terminal  104 . When the input terminal  101  is placed in an open state, the non-inverting input terminal of the comparator  301  is pulled up to the voltage VDD by the resistor  307 . This enables the comparator  301  to detect that the input terminal  101  has been open circuited. 
         [0008]    The comparator  303  outputs the High signal when the voltage of the inverting input terminal reduces to be lower than the voltage of the reference voltage circuit  304 . The voltage of the reference voltage circuit  304  is set to a voltage that is higher than a voltage VSS of the ground terminal  105 . If the input terminal  102  is open circuited, then the inverting input terminal of the comparator  303  is pulled down to the voltage VSS by the resistor  308 . This enables the comparator  303  to detect that the input terminal  102  has been open circuited. 
         [0009]    If an output signal of either one of the comparator  301  and the comparator  303  becomes a high signal, then the OR circuit  305  outputs a high signal. The constant current circuit  306  stops, so that the output terminal  103  of the operational amplifier  106  exhibits high impedance, making it possible to externally notify that the input terminal of the operational amplifier has been open circuited (refer to, for example, Patent Document 1). 
         [0010]    [Patent Document 1] Japanese Patent Application Laid-Open No. 2003-143239 
         [0011]    However, according to the conventional operational amplifier, there has been a problem in that an input terminal resistance cannot be increased in a normal operation mode, thus inconveniently restricting the voltage range of input signals that can be handled by the operational amplifier. 
         [0012]    There has been another problem in that the output of the output terminal becomes inconstant when the input terminal is open circuited, resulting in an unstable operation of a circuit connected to the output terminal of the operational amplifier. 
       SUMMARY OF THE INVENTION 
       [0013]    The present invention has been made to solve the problems described above and an object of the invention is to provide an operational amplifier that is capable of increasing an input terminal resistance in a normal operation mode and capable of detecting that an input terminal has been open circuited without restricting the voltage range of an input signal that can be handled by the operational amplifier. 
         [0014]    To this end, an operational amplifier in accordance with the present invention has the following configuration. 
         [0015]    The operational amplifier includes: a first comparator which has a non-inverting input terminal of the operational amplifier connected to a non-inverting input terminal thereof and which has an inverting input terminal of the operational amplifier connected to an inverting input terminal thereof having an input offset voltage; a second comparator which has an inverting input terminal of the operational amplifier connected to the non-inverting input terminal thereof and which has the non-inverting input terminal of the operational amplifier connected to an inverting input terminal thereof having an input offset voltage; an amplifier which has the non-inverting input terminal of the operational amplifier connected to a non-inverting input terminal thereof and which has the inverting input terminal of the operational amplifier connected to an inverting input terminal thereof; a logic circuit connected to an output terminal of the first comparator and an output terminal of the second comparator; a first resistor and a first switch which are provided between the non-inverting input terminal and a ground terminal of the operational amplifier and which are connected in series; and a second resistor and a second switch which are provided between the inverting input terminal and a supply terminal of the operational amplifier and which are connected in series. 
         [0016]    According to the present invention, the comparators detect that the input terminal is open circuited, and the input terminal resistance can be increased in the normal operation mode by pulling down or pulling up the input terminal only when the input terminal is open circuited. This makes it possible to detect that the input terminal has been open circuited without restricting the voltage range of an input signal that can be handled by the operational amplifier. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is a circuit diagram of an operational amplifier according to a first embodiment of the present invention; 
           [0018]      FIG. 2  is a circuit diagram of an operational amplifier according to a second embodiment of the present invention; and 
           [0019]      FIG. 3  is a circuit diagram of a conventional operational amplifier. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
       [0020]      FIG. 1  is a circuit diagram of an operational amplifier according to a first embodiment. The operational amplifier according to the first embodiment is comprised of comparators  107 ,  109 , an OR circuit  111 , switches  114 ,  116 , an operational amplifier  106 , resistors  113 ,  115 , input terminals  101 ,  102 , an output terminal  103 , a supply terminal  104 , and a ground terminal  105 . 
         [0021]    The comparator  107  has a non-inverting input terminal thereof connected to the input terminal  101 , a inverting input terminal thereof connected to the input terminal  102 , and an output terminal thereof connected to a first input terminal of the OR circuit  111 . The comparator  109  has an inverting input terminal thereof connected to the input terminal  101 , the non-inverting input terminal thereof connected to the input terminal  102 , and the output terminal thereof connected to the second input terminal of the OR circuit  111 . An output of the OR circuit  111  is connected to the switch  116  and the switch  114  to control the ON/OFF thereof. The operational amplifier  106  has the non-inverting input terminal thereof connected to the input terminal  101 , the inverting input terminal thereof connected to the input terminal  102 , and the output thereof connected to the output terminal  103 . The resistor  115  has one terminal thereof connected to the switch  116  and the other terminal thereof connected to the input terminal  102 . The other terminal of the switch  116  is connected to the supply terminal  104 . The resistor  113  has one terminal thereof connected to the switch  114  and the other terminal thereof connected to the input terminal  101 . The other terminal of the switch  114  is connected to the ground terminal  105 . 
         [0022]    The operation of the operational amplifier according to the first embodiment will be described. 
         [0023]    The comparator  107  has an offset  108  set at the inverting input terminal thereof Further, the comparator  109  has an offset  110  set at the inverting input terminal thereof. The voltage of the input terminal  101  will be denoted by V+, the voltage of the input terminal  102  will be denoted by V−, the voltage of the offset  108  will be denoted by VA, and the voltage of the offset  110  will be denoted by VB. 
         [0024]    If (V+)&gt;(V−)+(VA), then the comparator  107  outputs a high signal, while the comparator  109  outputs a low signal. This is expressed as follows. 
         [0000]      ( V +)−( V −)&gt;( VA )   (1)
 
         [0025]    If the potential difference between the input terminal  101  and the input terminal  102  exceeds the voltage of the offset  108 , then the comparator  107  outputs the high signal. 
         [0026]    If (V−)&gt;(V+)+(VB), then the comparator  109  outputs the high signal, while the comparator  107  outputs the low signal. This is expressed as follows. 
         [0000]      ( V +)−( V −)&lt;−( VB )   (2)
 
         [0027]    If the potential difference between the input terminal  101  and the input terminal  102  becomes smaller than the voltage obtained by multiplying the voltage of the offset  110  by −, then the comparator  109  outputs the high signal. 
         [0028]    Thus, the offset voltages VA and VB are set such that they are sufficiently large so as not to limit the voltage range of an input signal that can be handled by the operational amplifier and that the open circuiting of the input terminal can be detected. 
         [0029]    The outputs of the comparator  107  and the comparator  109  are input to the OR circuit  111 , and if either one of the comparator  107  and the comparator  109  outputs the high signal, then the OR circuit  111  outputs the high signal, thus turning the switches  116  and  114  on. Then, the input terminal  102  is pulled up to the voltage VDD of the supply terminal  104 , while the input terminal  101  is pulled down to the voltage VSS of the ground terminal. Then, the output of the operational amplifier  106  is fixed to a low level. 
         [0030]    The resistance values of the resistors  113  and  115  are set to be sufficiently large. Therefore, unless the input terminal  101  is open circuited, the input terminal  101  will not be pulled down to the voltage VSS even if the switch  114  is turned on. Similarly, even if the switch  116  is turned on, the input terminal  102  will not be pulled up to the voltage VDD. 
         [0031]    When the input terminal  101  is open circuited, expression (2) is satisfied, so that the comparator  109  outputs the high signal. Upon receipt of the high signal output from the comparator  109 , the OR circuit  111  outputs the high signal. Upon receipt of the high signal output from the OR circuit, the switch  114  is turned on, connecting the non-inverting input terminal of the operational amplifier  106  to the voltage VSS through the resistor  113 . Upon receipt of the high signal output from the OR circuit  111 , the switch  116  is turned on, connecting the inverting input terminal of the operational amplifier  106  to the voltage VDD through the resistor  115 . 
         [0032]    In this case, the input terminal  101  is open circuited, so that the non-inverting input terminal of the operational amplifier is pulled down to the voltage VSS. Hence, the operational amplifier  106  outputs the low signal to the output terminal  103 . Thus, the operational amplifier according to the first embodiment detects that the input terminal  101  has been open circuited and externally notifies to that effect by fixing the output of the operational amplifier  106  to the low level. 
         [0033]    When the input terminal  102  is open circuited, expression (1) is satisfied, so that the comparator  107  outputs the high signal. The OR circuit  111  outputs the high signal upon receipt of the high signal output from the comparator  107 . The switches  116  and  114  turn on upon the receipt of the high signal. 
         [0034]    In this case, the input terminal  102  is open circuited, so that the non-inverting input terminal of the operational amplifier  106  is pulled up to the voltage VDD. Hence, the operational amplifier  106  outputs the low signal to the output terminal  103 . Thus, the operational amplifier according to the first embodiment detects that the input terminal  102  has been open circuited and externally notifies to that effect by fixing the output of the operational amplifier  106  to the low level. 
         [0035]    In a normal operation state wherein a voltage signal is applied to the input terminal  101  and the input terminal  102 , neither expression (1) nor (2) is satisfied, so that the comparators  107  and  109  output the low signal. The OR circuit  111  outputs the low signal, causing the switches  116  and  114  to turn off. Thus, the lowering of the input terminal resistance due to the resistors  115  and  113  can be prevented, making it possible to prevent the input signal voltage range from being restricted. 
         [0036]    As described above, the operational amplifier according to the first embodiment allows the input terminal resistance to be increased during the normal operation state. This makes it possible to detect that an input terminal has been open circuited without restricting the voltage range of the input signals that can be handled by the operational amplifier. 
       Second Embodiment 
       [0037]      FIG. 2  is a circuit diagram of an operational amplifier according to a second embodiment. The difference from operational amplifier according to the first embodiment is the addition of a delay circuit  201 . The delay circuit  201  is connected between an OR circuit  111  and switches  116  and  114 . The rest of the circuit configuration is the same as that of the operational amplifier according to the first embodiment, so that the like reference numerals will be assigned and the description of the connections will be omitted. 
         [0038]    The operation of the operational amplifier according to the second embodiment will be described. 
         [0039]    A comparator  107  has an offset  108  set at an inverting input terminal. Further, a comparator  109  has an offset  110  set at an inverting input terminal. The voltage of an input terminal  101  will be denoted by V+, the voltage of an input terminal  102  will be denoted by V−, the voltage of the offset  108  will be denoted by VA, and the voltage of the offset  110  will be denoted by VB. 
         [0040]    If (V+)&gt;(V−)+(VA), then the comparator  107  outputs a high signal, while the comparator  109  outputs a low signal. 
         [0041]    If (V−)&gt;(V+)+(VB), then the comparator  109  outputs the high signal, while the comparator  107  outputs the low signal. 
         [0042]    The outputs of the comparator  107  and the comparator  109  are input to an OR circuit  111 , and if either of the comparators  107  and  109  outputs the high signal, then the OR circuit  111  outputs the high signal, causing the switches  116  and  114  to turn on through the delay circuit  201 . Then, the input terminal  102  is pulled up to the voltage VDD of a supply terminal  104  and the input terminal  101  is pulled down to the ground terminal  105 , fixing the output of the operational amplifier  106  to a low level. 
         [0043]    In this case, the delay circuit  112  operates not to output the high signal from the OR circuit  111  to a circuit in a following stage for a certain period of time or less thereby to prevent a malfunction from taking place due to a transient voltage fluctuation of the input terminal  101  or  102 . 
         [0044]    As described above, the operational amplifier according to the second embodiment allows an input terminal resistance to be increased in a normal operation state, making it possible to detect that the input terminal has been open circuited without restricting the voltage range of input signals that can be handled by the operational amplifier. In addition, even if a transient voltage fluctuation takes place at an input terminal in the normal operation state, a malfunction of the circuit can be prevented.