Abstract:
A current/voltage converter includes a current source connected to a first node; a first transistor element connected between the first node and a power supply or ground; a second transistor element which is connected between a second node and the power supply or ground, whose control terminal is connected to that of the first transistor element; a first control circuit which controls the voltages of control terminals of the first and second transistor elements so that the voltage of the first node becomes substantially equal to that of a bias voltage as one of inputs to the first control circuit; a second control circuit controlling the voltage of the second node so as to become substantially equal to that of the first node; and a resistance element one end of which is connected to the second node, and which converts the current flowing through the second transistor element into a voltage.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a current/voltage converter which converts a current supplied from a current source into a voltage, a current/voltage converter which converts the total current supplied from the current source supplying the current value corresponding to a digital signal, into a voltage, and relates to a D/A converter using the same.  
           [0003]    2. Description of the Related Art  
           [0004]    [0004]FIG. 3A is a simplified circuit diagram of an example of a current-cell type D/A converter  30 .  
           [0005]    Since an example of two-bit D/A converter will suffice for explaining the principle of current/voltage conversion, a two-bit D/A converter will be described hereinafter for simplification. However, the same goes with a n-bit converter.  
           [0006]    The current-cell type D/A converter (hereinafter abbreviated as a “DAC”)  30  shown in FIG. 3A comprises three current sources  32 ,  34 , and  36  which feed respective currents Is 1 , Is 2 , and Is 3 ; three changeover switches  38 ,  40 , and  42  provided corresponding to these current sources  32 ,  34 , and  36 , respectively; and a resistance element  44  (resistance value: R) for current/voltage (or referred to as I/V, hereinafter) conversion. Herein, Is 1 =Is 2 =Is 3 .  
           [0007]    The current source  32  is connected between the power supply and the changeover switch  38 , and likewise, the current source  34  is connected between the power supply and the changeover switch  40 , and the current source  36  is connected between the power supply and the changeover switch  42 . Each of the changeover switches  38 ,  40 , and  42  is connected so as to switch between the current source and the ground or an analog signal node Vout. The resistance element  44  is connected between the analog signal node Vout and the ground.  
           [0008]    In the illustrated DAC  30 , each of the changeover switches  38 ,  40 , and  42  is connected to either the node Vout side or the ground side, in response to a digital signal (not shown) inputted to the DAC  30 .  
           [0009]    For example, when the digital signal is “00”, all changeover switches  38 ,  40 , and  42  are connected to the ground side, and when the digital signal is “01”, the changeover switch  38  is connected to the node Vout side while the changeover switches  40  and  42  are connected to the ground side. Also, when the digital signal is “10”, the changeover switches  38  and  40  are connected to the node Vout side while the changeover switch  42  is connected to the ground side, and when the digital signal is “11”, all changeover switches  38 ,  40 , and  42  are connected to the node Vout side.  
           [0010]    Each of the current Is 1 , Is 2 , and Is 3  supplied from the respective current sources  32 ,  34 , and  36  flows to either the node Vout side or the ground side in accordance with the setting of the changeover switches, as described above. The total current Isig composed of the currents flowing from the current sources  32 ,  34 , and  36  to the node Vout side via the respective changeover switches  38 ,  40 , and  42  is I/V converted by the resistance element  44 , and outputted as an analog signal Vout=R·Isig, as shown in FIG. 3A.  
           [0011]    As shown in FIG. 3A, each of the current sources  32 ,  34 , and  36  is constituted of, for example, a P-type MOS transistor (hereinafter abbreviated as PMOS) or the like. As shown in FIG. 3C, however, the current Is supplied via a PMOS gradually decreases as the voltage Vds between the source and drain of the PMOS decreases. As a result, in FIG. 3B, the voltage of the analog signal Vout increases in a manner such that the voltage b 1 &gt;b 2 &gt;b 3 , although the voltage of the analog signal Vout should essentially increase in a manner such that b 1 =b 2 =b 3 . This raises a problem in that linearity failure of the DAC occurs.  
           [0012]    Meanwhile, the minimum value of the analog signal Vout outputted from the DAC  30  shown in FIG. 3A is 0 V. However, unless the output of the analog signal Vout is shifted in response to the input-output characteristic of the poststage circuit utilizing this analog signal Vout, the analog signal Vout cannot be used for the poststage circuit. It is therefore necessary to level-shift the analog signal Vout of the DAC  30  into the range of the optimum input voltages of the poststage circuit, the range being indicated by an double-headed arrow in FIG. 4A.  
           [0013]    [0013]FIG. 4B, 4C and  4 D are each circuit diagrams of examples of conventional level shift circuits.  
           [0014]    First, a level shift circuit  50  in FIG. 4B is arranged to utilize a source follower configuration, and comprises two PMOSs  52  and  54 . The PMOS  52  is connected between a power supply and an analog signal node Vout, and a bias voltage Vb is inputted to the gate thereof. On the other hand, the PMOS  54  is connected between the analog signal node Vout and the ground, and a signal IN is inputted to the gate thereof. As the signal IN, for example, the analog signal node Vout of the DAC  30  shown in FIG. 3A is inputted.  
           [0015]    In the illustrated level shift circuit  50 , the PMOS  52  supplies a current in response to the bias voltage Vb to the analog signal node Vout side, while the PMOS  54  feeds the current in response to the voltage of the signal IN to the ground side. Thereby, as the voltage of the signal IN increases, the voltage of the analog signal node Vout rises.  
           [0016]    However, the level shift circuit  50  utilizing the source follower configuration involve a problem of inherently having an inferior linearity.  
           [0017]    Next, a level shift circuit  56  shown in FIG. 4C is arranged to add a bias current Ib to the above-described total current Isig, and to I/V convert this summed current. The level shift circuit  56  comprises a current source  58  which feeds the current corresponding to the total current Isig which flows to the analog output node Vout side of the DAC  30 ; a current source  60  for use in the bias current Ib; and a resistance element  62 . Each of the current sources  58  and  60  is connected between the power supply and the analog signal node Vout, and the resistance element  62  is connected between the node Vout and the ground.  
           [0018]    In this level shift circuit  56 , the total current Isig and the bias current Ib are summed up, and this summed current (Isig+Ib) is I/V converted by the resistance element  62 , and outputted as an analog signal Vout=(Isig+Ib)·R.  
           [0019]    In this level shift circuit  56  utilizing the bias current Ib, however, when the potential of the analog signal Vout is increased by R·Ib, the amplitude of the Vds of the PMOSs of the current sources  32 ,  34 , and  36  in FIG. 3A is correspondingly reduced. This raises a problem in that the output amplitude decreases.  
           [0020]    Then, a level shift circuit  64  shown in FIG. 4D is arranged to utilize an operational amplifier, and comprises a current source  66  which feeds the current corresponding to the total current Isig which flows to the analog output node Vout side of the DAC  30 ; an operational amplifier  68 ; and a resistance element  70 . The current sources  66  is connected between the power supply and the negative input terminal of the operational amplifier  68 . The positive input terminal of the operational amplifier  68  is connected to the ground, and the resistance element  70  is connected between the negative input terminal of the operational amplifier  68  and the output terminal (analog signal Vout).  
           [0021]    In this level shift circuit  64 , the analog signal becomes Vout=−R·Isig. That is, the polarity of the analog signal Vout is inverted. This creates a problem in that a wide range of power supply voltage is required in order to secure the amplitude of analog signal Vout.  
         SUMMARY OF THE INVENTION  
         [0022]    Accordingly, it is an object of the present invention to solve the problems caused by the above-described conventional arts and to provide a current/voltage converter and a D/A converter using this which are capable of eliminating linearity failure, and which can level-shift the potential of an analog signal in response to the input-output characteristic of the poststage circuit.  
           [0023]    In order to achieve the above-described object, the present invention provides an I/V converter, comprising a current source connected to a first node; a first transistor element connected between the first node and a power supply or the ground; a second transistor element which is connected between a second node and the power supply or the ground, of which the control terminal is connected to that of the first transistor element, and which is of the same conductive type as the first transistor element; a first control circuit which uses a bias voltage as one of the inputs thereto, and which controls the voltages of the control terminals of the first and second transistor elements so that the voltage of the first node becomes substantially equal to the bias voltage; a second control circuit which uses the bias voltage as one of the inputs thereto, and which controls voltage of the second node so as to become substantially equal to that of the first node; and a resistance element one end of which is connected to the second node, and which converts the current flowing through the second transistor element into a voltage.  
           [0024]    Herein, since the control terminals of the first and second transistor elements are controlled so as to have a common potential, and one-side ends thereof are both connected to the power supply or the ground potential, mutually equal currents flow through these two transistor elements by the current mirror effect when the sizes thereof are equal.  
           [0025]    The transistor elements in the present invention may be MOS transistors or bipolar transistors. In order to generate the current mirror effect, however, it is necessary to use the same type of transistors. In a MOS transistor, the control terminal thereof is a gate terminal, while in a bipolar transistor, the control terminal is a base terminal. The size of a MOS transistor is determined by the gate length and gate width thereof, while that of bipolar transistor is determined by the junction area between the base and emitter thereof. In the present invention, however, the sizes of transistor elements are not limited.  
           [0026]    Herein, it is preferable that each of the first and second control circuits be an operational amplifier.  
           [0027]    Preferably, the above-described I/V converter further has means for changing the value of the aforementioned bias voltage.  
           [0028]    Moreover, the present invention provides a D/A converter comprising a current generating circuit which generates the total current corresponding to the digital signal to be converted into an analog signal; and the above-described I/V converter wherein a current is supplied from the above-mentioned current generating circuit to the first node thereof.  
           [0029]    Preferably, the above-described D/A converter further has a bias current supplying circuit capable of adjusting the current which is supplied from the above-described current generating circuit to the first node.  
           [0030]    The above and other objects, features, and advantages of the present invention will be clear from the following detailed description of the preferred embodiments of the invention in conjunction with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0031]    [0031]FIG. 1A is a circuit diagram of a current/voltage converter in accordance with an embodiment of the present invention;  
         [0032]    [0032]FIG. 1B is a circuit diagram of a current/voltage converter in accordance with another embodiment of the present invention;  
         [0033]    [0033]FIG. 2A is a circuit diagram of a current/voltage converter in accordance with still another embodiment of the present invention, and a D/A converter using this;  
         [0034]    [0034]FIG. 2B is diagram illustrating an example of output waveform of the D/A converter shown in FIG. 2A;  
         [0035]    [0035]FIG. 2C is a circuit diagram of a current/voltage converter in accordance with a further embodiment of the present invention, and a D/A converter using this;  
         [0036]    [0036]FIG. 3A is a simplified circuit diagram of an example of a conventional current-cell type D/A converter;  
         [0037]    [0037]FIG. 3B is a timing chart showing the operation of the current-cell type D/A converter shown in FIG. 3A;  
         [0038]    [0038]FIG. 3C is a diagram showing an example of the Is-Vds characteristic;  
         [0039]    [0039]FIG. 4A is an example of the relationship between an output of the D/A converter and the input of the next stage circuit; and  
         [0040]    [0040]FIG. 4B, 4C and  4 D are each circuit diagrams of examples of conventional level shift circuits. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0041]    [0041]FIG. 1 is a circuit diagram of an I/V converter  10  in accordance with an embodiment of the present invention.  
         [0042]    The I/V converter  10  shown in FIG. 1 comprises a N-type transistor (NMOS)  14  and an operational amplifier (hereinafter abbreviated as an “OP”)  16  provided on the left side in the figure; and a P-type transistor (PMOS)  18 , a resistance element  20  (resistance value: R), a NMOS  22 , and an OP  24  provided on the right side in the figure.  
         [0043]    Herein, the NMOSs  14  and  22  each constitute examples of the first and second transistor elements of the present invention. Likewise, the OPs  16  and  24  each constitute examples of the first and second control circuits, nodes A and B each constitute examples of the first and second nodes of the present invention, and the PMOS  18  constitutes an example of the third transistor element of the present invention.  
         [0044]    The current source  12  shown in the figure is connected between the power supply and the NMOS  14 , and supplies a current value Isig.  
         [0045]    The NMOS  14  is connected between the current source  12  and the ground, and an output signal from the OP  16  is inputted to the gate of this NMOS  14 . The output signal from the OP  16  is also inputted to the gate of the NMOS  22  on the right side in the figure. To the positive terminal of the OP  16 , the node A constituting the connection point between the current source  12  and the NMOS  14  is connected, while to the negative terminal of the OP  16 , a bias voltage Vb is inputted.  
         [0046]    On the other hand, the PMOS  18  on the right side in the figure is connected between the power supply and the analog signal node Vout, and an output signal from the OP  24  is inputted to the gate of this PMOS  18 . To the positive terminal of the OP  24 , the node B constituting the connection point between the resistance element  20  and the NMOS  22  is connected, while to the negative terminal of the OP  24 , a bias voltage Vb is inputted. The resistance element  20  is connected between the analog signal node Vout and the NMOS  22 , and the NMOS  22  is connected between the resistance element  20  and the ground.  
         [0047]    In the illustrated I/V converter  10 , therefore, the voltage Vg of the output signal of the OP  16  varies so that the voltages of the positive and negative input terminals of the OP  16  agree with each other, or in other words, so that the voltage of the node A becomes equal to the bias voltage Vb irrespective of the current Isig.  
         [0048]    In this manner, in the I/V conversion circuitry  10  in accordance with the present invention, the voltage of the node A, that is, the voltage Vds between the source and drain of the PMOS which is a current source in the I/V converter of the present invention, is controlled so as to be always constant, so that linearity failure of the DAC can be eliminated.  
         [0049]    In the illustrated I/V converter  10 , the NMOSs  14  and  22  constitute a pair of current mirror transistors. The node B, therefore, is controlled by the OP  24  so as to have a voltage equal to that of the node A, that is, so as to have the constant voltage equal to the bias voltage Vb. The node B is supplied with the current Isig by current-mirroring, then the current Isig is I/V converted by the resistance element  20 , and when the sizes of the NMOSs  14  and  22  are equal, the analog signal Vout is outputted as Vout=R·Isig+Vb. When the NMOS  22  is n-times the size of the NMOS  14 , the analog signal Vout is outputted as Vout n·R·Isig+Vb.  
         [0050]    Next, an I/V converter  10 ′ in accordance with another embodiment of the present invention will be described with reference to FIG. 1B. The I/V converter  10 ′ shown in the figure is constituted by replacing the NMOSs in the I/V converter in FIG. 1A with PMOSs, then by replacing the PMOSs therein with a NMOSs, and by interchanging the positions of the power supply and the ground therein. Corresponding elements, therefore, are designated by the same reference characters with a prime affixed. With regard to circuit operation also, this I/V converter  10 ′ is similar to the above-described I/V converter  10 , and therefore, the circuit operation thereof will be omitted from description. In the I/V converter  10 ′, the sizes of the PMOSs  14 ′ and  22 ′ are equal, the analog signal Vout′ is outputted as Vout′=Vb′−R′·Isig′.  
         [0051]    Next, an I/V converter  15  in accordance with still another embodiment of the present invention and a D/A converter using this will be described with reference to FIG. 2A. The I/V converter  15  differs from the I/V converter  10  in FIG. 1A only in that the I/V converter  15  uses a current generating circuit which generates the current corresponding to the digital signal to be converted into an analog signal, instead of the current source in the I/V converter  10 , and that the I/V converter  15  has a current source  13  although it is not an essential condition therefor. Therefore, corresponding elements are designated by the same reference characters.  
         [0052]    The current generating circuit  12  shown in the figure, which is a current generating circuit which generates the current corresponding to the digital signal to be converted into an analog signal in the D/A converter in accordance with the present invention, is connected between the power supply and the NMOS  14 , and supplies the total current Isig. The current source  13  shown in the figure is a bias current supplying means which can adjust the current supplied from the current generating circuit  12  to the node A, and connected between the power supply and the NMOS  14  as in the case of the current generating circuit  12 . This current source  13  supplies a bias current Ib to the node A, and adjusts an overall current supplied to the node A. Herein, it is not an essential condition for the D/A converter in accordance with the present invention to have the current source  13 , but it is preferable to have it like the embodiment shown in FIG. 2A.  
         [0053]    The total current Isig supplied from the current source  12  varies in response to the digital signal to be converted into an analog signal by the D/A converter in accordance with the present invention.  
         [0054]    In the I/V conversion circuitry  15 , the voltage Vg of the output signal of the OP  16  varies so that the voltages of the positive input terminal and the negative input terminal of the OP  16  agree with each other, or in other words, so that the voltage of the node A becomes equal to the bias voltage Vb irrespective of the current Isig.  
         [0055]    In this manner, in the I/V conversion circuitry  15  in accordance with the present invention, the voltage of the node A, that is, the voltage Vds between the source and drain of the current source  12  which supplies the total current in response to a digital signal in the D/A converter of the present invention, for example, the voltage Vds between the source and drain of the PMOS which is a current source in the D/A converter  30  in FIG. 3A, is controlled so as to be always constant. This allows linearity failure of the DAC to be eliminated.  
         [0056]    In the I/V conversion circuitry  15 , the NMOSs  14  and  22  constitute a pair of current mirror transistors. The node B, therefore, is controlled by the OP  24  so as to have a voltage equal to that of the node A, that is, the constant voltage equal to the bias voltage Vb. The node B is supplied with the total current Isig by current-mirroring, then the total current Isig is I/V converted by the resistance element  20 , and when the sizes of the NMOSs  14  and  22  are equal, and the current source  13  is provided as shown in FIG. 2A, the analog signal is outputted as Vout=R·(Isig+Ib)+Vb.  
         [0057]    That is, in the I/V conversion circuitry  15 , the voltage level of the analog signal Vout is clamped to the voltage of the bias voltage Vb. Therefore, by appropriately setting the bias voltage Vb in response to the input-output characteristic of the poststage circuit utilizing the analog signal Vout of the D/A converter, the output level of the analog signal Vout can be shifted, and thereby facilitating the transmission of the analog signal Vout to the poststage circuit.  
         [0058]    In the embodiment shown in FIG. 2A, a specific example is illustrated for implementing the I/V conversion circuitry  15  in accordance with the present invention and the D/A converter using this, by employing the NMOSs  14  and  22  as the current mirror circuit, and the OPs  16  and  24  as the first and second control circuits. However, the present invention is not limited to this embodiment, but the I/V converter in accordance with the present invention and the D/A converter using this may be realized by using other means for implementing the same function.  
         [0059]    As another embodiment, one wherein, in the circuit in FIG. 2A, NMOSs are replaced with PMOSs, PMOSs are replaced with NMOSs, and wherein the positions of the power supply and the ground is interchanged, is shown in FIG. 2C. Corresponding elements are designated by the same reference characters with a prime affixed. The operation of this embodiment is similar to that of the embodiment shown in FIG. 2A, the operation thereof will be omitted from description.  
         [0060]    As a further embodiment, a configuration may be adopted wherein, in the I/V converter in FIGS. 1A and 2A, the connection position between the PMOS  18  and the resistance  20  is changed so that the resistance element  20  is connected between the power supply and the analog output node Vout, and that the PMOS  18  is connected between the node Vout and the second node B.  
         [0061]    Alternatively, a configuration may be adopted wherein, in the I/V converter in FIGS. 1B and 2C, the connection position between the NMOS  18 ′ and the resistance element  20 ′ is changed so that the resistance element  20 ′ is connected between the ground and the analog output node Vout′, and that the NMOS  18 ′ is connected between the node Vout′ and the second node B′.  
         [0062]    In the present invention, the transistor elements which constitute a pair of current mirror transistors are not limited to MOS transistors, but bipolar transistors can also be used as the transistor elements which constitute a pair of current mirror transistors.  
         [0063]    The D/A converter in accordance with the present invention is arranged so that the I/V converters of the present invention shown in FIG. 2A and 2C are each used for the output stage of the D/A converter. More specifically, the D/A converter in accordance with the present invention is one wherein, in the conventional current-cell type DAC shown in FIG. 3A, each of the I/V converters  15  and  15 ′ shown in FIGS. 2A and 2C, respectively, in accordance with the present invention, is used in place of the resistance element  44 . As a current generating circuit, any that can generate the total current corresponding to the digital signal to be converted into an analog signal can be employed, even if it is a known current generating circuit.  
         [0064]    While the I/V converter in accordance with the present invention and the D/A converter using this have been described in detail, the present invention is not restricted to the above-described embodiments, but various changes and modifications may be made thereto without departing from the true spirit and scope of the invention.  
         [0065]    As is evident from the foregoing, in accordance with the present invention, since the voltage of the first node, for example, the voltage between the source and drain of the MOS transistor constituting the current source of the DAC is fixed at a fixed voltage, linearity failure of the DAC can be eliminated. In addition, by appropriately changing the setting of a bias voltage, and by shifting the output level of an analog signal in response to the input-output characteristic of the poststage circuit which utilizes the analog signal of the DAC, the transmission of the analog signal to the poststage circuit can be facilitated.