Abstract:
A converting circuit for receiving an input voltage and generating an output current, including: a transistor, coupled to a supply voltage at a drain of the transistor, and a source of the transistor is coupled to a first voltage, and a gate of the transistor is coupled to the input voltage and a fixed voltage; and a resistor, coupled to the input voltage and the gate of the transistor, and the output current flows through the resistor, wherein the output current is related to the fixed voltage, the input voltage and the resistor.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This Application claims priority of China Patent Application No. CN 201110217009, filed on Jul. 29, 2011, the entirety of which is incorporated by reference herein. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates to a voltage to current converting circuit, and more particularly to a voltage to current converting circuit that is capable of operating at a low voltage. 
         [0004]    2. Description of the Related Art 
         [0005]    In analog circuits, a transconductance circuit is a voltage to current converting circuit, which converts an input voltage into an output current for subsequently other circuits. 
         [0006]      FIGS. 1A and 1B  show a single-end mode and a differential mode for a conventional transconductance circuit, respectively. In  FIG. 1A , a transistor M 1  is coupled to a ground GND via a resistor R. An input voltage V i  is used to control a gate of the transistor M 1 , to determine a current value of an output current i o  flowing through the transistor M 1 . In  FIG. 1B , a transistor M 1  is coupled to the ground GND via a first current source, and a transistor M 2  is coupled to the ground GND via a second current source, wherein the first and second current sources have the same current values I 0 . In addition, a resistor R is coupled between the drains of two transistors M 1  and M 2 . The input voltages V i+  and V i−  are a pair of differential signals, that are used to control the gates of the transistors M 1  and M 2 , to determine a current value of the output current i o+  flowing through the transistor M 1  and a current value of the output current i o−  flowing through the transistor M 2 . In the conventional transconductance circuit, the resistor R is much larger than the transconductance gm of each transistor, i.e. 
         [0000]    
       
         
           
             
               R 
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                 1 
                 gm 
               
             
             , 
           
         
       
     
         [0000]    so as to obtain better linearity. Furthermore, the conventional transconductance circuit needs to operate at an operating range having a good linearity as the input voltages V i+  and V i−  are applied to the gates of the transistors M 1  and M 2  directly. However, the operating range is decreased when a supply voltage is decreased. 
         [0007]      FIGS. 2A and 2B  show a single-end mode and a differential mode for another conventional transconductance circuit, respectively. In  FIG. 2A , a transistor M 1  is coupled to the ground GDN via a resistor R, wherein a gate of the transistor M 1  is coupled to an output terminal of an amplifier AMP 1 . By using a characteristic of virtual short between two input terminals of the amplifier AMP 1 , the voltages at two terminals of the resistor R are an input voltage V i  and the ground GND, thereby an output current i o  is obtained by applying the input voltage V i  into the resistor R, i.e. 
         [0000]    
       
         
           
             
               i 
               0 
             
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                   i 
                 
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         [0000]    In  FIG. 2B , a transistor M 1  is coupled to the ground GND via a first current source, and a transistor M 2  is coupled to the ground GND via a second current source, wherein the first and second current sources have the same current values I 0 . A gate of the transistor M 1  is coupled to an output terminal of an amplifier AMP 1 , and a gate of the transistor M 2  is coupled to an output terminal of an amplifier AMP 2 . Furthermore, a resistor R is coupled between the first terminals of the amplifiers AMP 1  and AMP 2 . The input voltages V i+  and V i−  are a pair of differential signals, wherein the input voltages V i+  and V i−  are applied to the second terminals of the amplifiers AMP 1  and AMP 2 , respectively. Similarly, by using a characteristic of virtual short between two input terminals of each of the amplifiers AMP 1  and AMP 2 , the output currents i 0+  and i 0−  are obtained by applying the input voltages V i+  and V i−  into the resistor R. Although the conventional transconductance circuits of  FIGS. 2A and 2B  use the amplifiers to overcome the problems of the conventional transconductance circuits of  FIGS. 1A and 1B , the amplifiers AMP 1  and AMP 2  must maintain in the virtual short status thereof, so as to maintain better linearity. However, the operating range of the virtual short status is decreased for an amplifier when a supply voltage of the amplifier is decreased, thus it is hard to maintain linearity. 
         [0008]    Following the advancement of process technology, integrated circuits (IC) can operate at a lower supply voltage, such as below 1.5V, so as to decrease power consumption for the IC. However, when the operating/supply voltage is decreased, the linearity of each conventional transconductance circuit of  FIGS. 1A ,  1 B,  2 A and  2 B is decreased, and can not meet operating requests. 
         [0009]    Therefore, a voltage to current converting circuit having better linearity is desired, that is capable of operating at a low voltage. 
       BRIEF SUMMARY OF THE INVENTION 
       [0010]    Converting circuits for converting input voltage into output current are provided. An embodiment of a converting circuit for receiving an input voltage and generating an output current is provided. The converting circuit comprises: a transistor, coupled to a supply voltage at a drain of the transistor, and a source of the transistor is coupled to a first voltage, and a gate of the transistor is coupled to the input voltage and a fixed voltage; and a resistor, coupled to the input voltage and the gate of the transistor, and the output current flows through the resistor, wherein the output current is related to the fixed voltage, the input voltage and the resistor. 
         [0011]    Furthermore, another embodiment of a converting circuit for receiving a plurality of input voltages and generating a plurality of output currents is provided. The converting circuit comprises a first transistor, coupled to a first supply voltage at a drain of the transistor, and a source the first transistor is coupled to a first voltage; a first amplifier, having a first input terminal for receiving a fixed voltage, a second input terminal coupled to a first input voltage, and an output terminal coupled to a gate of the first transistor; a first resistor, coupled to the first input voltage and the second input terminal of the first amplifier, and a first output current flows through the first resistor; a second transistor, coupled to a second supply voltage at a drain of the second transistor, and a source of the second transistor is coupled to a second voltage; a second amplifier, having a first input terminal for receiving the fixed voltage, a second input terminal coupled to a second input voltage, and an output terminal coupled to a gate of the second transistor; and a second resistor, coupled to the second input voltage and the second input terminal of the second amplifier, and a second output current flows through the second resistor. 
         [0012]    A detailed description is given in the following embodiments with reference to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0013]    The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
           [0014]      FIG. 1A  shows a conventional transconductance circuit that operates in a single-end mode; 
           [0015]      FIG. 1B  shows a conventional transconductance circuit that operates in a differential mode; 
           [0016]      FIG. 2A  shows another conventional transconductance circuit that operates in a single-end mode; 
           [0017]      FIG. 2B  shows another conventional transconductance circuit that operates in a differential mode; 
           [0018]      FIG. 3  shows a voltage to current converting circuit according to an embodiment of the invention, wherein the voltage to current converting circuit operates in a single-end mode; 
           [0019]      FIG. 4A  shows a diagram illustrating the relationships between the input voltage V i  and the output current i o  of various transconductance circuits; 
           [0020]      FIG. 4B  shows a diagram illustrating the relationships of all the output currents i o  of  FIG. 4A  differentiated with respect to the corresponding input voltages V i ; 
           [0021]      FIG. 5  shows a mixer according to an embodiment of the invention; 
           [0022]      FIG. 6  shows a voltage to current converting circuit according to an embodiment of the invention, wherein the voltage to current converting circuit operates in a differential mode; 
           [0023]      FIG. 7  shows a mixer according to another embodiment of the invention; 
           [0024]      FIG. 8  shows a voltage to current converting circuit according to another embodiment of the invention, wherein the voltage to current converting circuit operates in a single-end mode; and 
           [0025]      FIG. 9  shows a voltage to current converting circuit according to another embodiment of the invention, wherein the voltage to current converting circuit operates in a differential mode. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0026]    The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims. 
         [0027]      FIG. 3  shows a voltage to current converting circuit  100  according to an embodiment of the invention, wherein the voltage to current converting circuit  100  operates in a single-end mode. The voltage to current converting circuit  100  comprises a transistor M 1 , a resistor R, an amplifier  110  and a current source  120 , wherein the transistor M 1  is an NMOS transistor. The transistor M 1  being an NMOS transistor is an example and does not intend to limit the invention. The current source  120  is coupled between a ground GND and a node N 1 , wherein a current value of the current source  120  is I 0 . An output terminal of the amplifier  110  is coupled to a gate of the transistor M 1 . A first input terminal of the amplifier  110  is used to receive a fixed voltage V fix , and a second input terminal of the amplifier  110  is coupled to the node N 1 . One terminal of the resistor R is also coupled to the node N 1 , and an input voltage V i  is applied to another terminal of the resistor R. Thus, the input voltage V i  is not directly inputted to the gate of the transistor M 1 , thereby the problem of the conventional transconductance circuit of  FIG. 1A  that the operating range is decreased when a supply voltage is decreased is avoided. Furthermore, for the amplifier  110 , the input voltage V i  is directly applied to one terminal of the resistor R, and a voltage value of the fixed voltage V fix  is a predetermined fixed voltage. By using a characteristic of virtual short between two input terminals of the amplifier  110 , the voltages at two terminals of the resistor R are the input voltage V i  and the fixed voltage V fix , thereby a current i c  flowing through the resistor R is 
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                   i 
                 
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                   fix 
                 
               
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         [0000]    Therefore, an output current i o  is obtained according to the current value I 0  and the current i c  flowing through the resistor R, i.e. i o =I 0 −i c . It is to be noted that a direction of the current i c  is an example and does not intend to limit the invention. In actual applications, the direction of the current i c  is determined according to the input voltage V i  and the fixed voltage V fix . The fixed voltage V fix  is set according to actual requirements when the voltage to current converting circuit  100  is operating at a low supply voltage. Due to the fixed voltage V fix  being fixed and the amplifier  110  having a characteristic of virtual short between two input terminals thereof, a linearity of the amplifier  110  will not be influenced when a supply voltage of the amplifier  110  is decreased. Therefore, because the amplifier  110  may operate in a virtual short status, the voltage to current converting circuit of the invention still has better linearity even if the supply voltage is very low. So, in actual embodiments, the voltage value of the fixed voltage is determined to make the amplifier being operated in a virtual short status. 
         [0028]      FIG. 4A  shows a diagram illustrating the relationships between the input voltage V i  and the output current i o  of various transconductance circuits. In  FIG. 4A , the curve S 1  represents the conventional transconductance circuit of  FIG. 1A , the curve S 2  represents the conventional transconductance circuit of  FIG. 2A , and the curve S 3  represents the voltage to current converting circuit  100  of  FIG. 3 . Furthermore,  FIG. 4B  shows a diagram illustrating the relationships of all the output currents i o  of  FIG. 4A  differentiated with respect to the corresponding input voltages V i .  FIG. 4B  is drawn by taking 80 voltage sampling points. Therefore the abscissa of  FIG. 4B  represents the number of those 80 sampling points, and every point in  FIG. 4B  should have the same voltage value as the corresponding point in  FIG. 4A . In  FIG. 4B , the curve S 4  represents the conventional transconductance circuit of  FIG. 1A , the curve S 5  represents the conventional transconductance circuit of  FIG. 2A , and the curve S 6  represents the voltage to current converting circuit  100  of  FIG. 3 . Specifically, compared with the conventional transconductance circuits, the voltage to current converting circuit  100  of  FIG. 3  has better linearity. 
         [0029]      FIG. 5  shows a mixer  200  according to an embodiment of the invention. The mixer  200  comprises a differential voltage unit  250  and a voltage to current converting circuit  100 . In general, a mixer of a radio frequency (RF) circuit can convert an intermediate frequency signal V IF  from a digital to analog converter (DAC) into an RF signal V RF , and then provide the RF signal V RF  to a power amplifier (PA) (not shown). In the mixer  200 , the voltage to current converting circuit  100  obtains an output current i o  according to the received intermediate frequency signal V IF  (i.e. an input voltage V i ). The differential voltage unit  250  comprises the transistors M 2  and M 3  and the inductors L 1  and L 2 . The inductor L 1  is coupled between a supply voltage VDD and the transistor M 2 , and the inductor L 2  is coupled between the supply voltage VDD and the transistor M 3 . Furthermore, the transistor M 2  is coupled between the inductor L 1  and the voltage to current converting circuit  100 , and the transistor M 3  is coupled between the inductor L 2  and the voltage to current converting circuit  100 . The gates of the transistors M 2  and M 3  are used to receive the local oscillation signals LO_P and LO_N, wherein the local oscillation signals LO_P and LO_N are a pair of differential signals. Therefore, the differential voltage unit  250  generates the RF signal V RF  according to the local oscillation signals LO_P and LO_N and the output current i o . In the embodiment, a voltage level of the fixed voltage V fix  is between the supply voltage VDD and the ground GND. 
         [0030]      FIG. 6  shows a voltage to current converting circuit  300  according to an embodiment of the invention, wherein the voltage to current converting circuit  300  operates in a differential mode. The voltage to current converting circuit  300  comprises two voltage to current converting sub-circuits  310  and  320 . The voltage to current converting sub-circuit  310  comprises a transistor M 1 , a resistor R 1 , an amplifier  330  and a current source  340 , wherein the transistor M 1  is an NMOS transistor. The transistor M 1  being an NMOS transistor is an example and does not intend to limit the invention. The current source  340  is coupled between a ground GND and a node N 1 , wherein a current value of the current source  340  is I 0 . An output terminal of the amplifier  330  is coupled to a gate of the transistor M 1 . A first input terminal of the amplifier  330  is used to receive a voltage V fix , and a second input terminal of the amplifier  330  is coupled to the node N 1 . One terminal of the resistor R 1  is also coupled to the node N 1 , and an input voltage V i+  is applied to another terminal of the resistor R 1 . Thus, the input voltage V i+  is not directly inputted to the gate of the transistor M 1 . Moreover, the current i c+  flowing through the resistor R 1  is 
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         [0000]    Therefore, an output current i o+  is obtained according to the current value I 0  of the current source  340  and the current i c+  flowing through the resistor R 1 , i.e. i o+ =I 0 −i c+ . On the other hand, the voltage to current converting sub-circuit  320  comprises a transistor M 2 , a resistor R 2 , an amplifier  350  and a current source  360 , wherein the transistor M 2  is an NMOS transistor and the transistors M 1  and M 2  have the same parameters. The transistor M 2  being an NMOS transistor is an example and does not intend to limit the invention. The current source  360  is coupled between the ground GND and a node N 2 , wherein a current value of the current source  360  is identical to the current value of the current source  340 . An output terminal of the amplifier  350  is coupled to a gate of the transistor M 2 , thereby the problems of the conventional transconductance circuit of  FIG. 1B  are avoided. A first input terminal of the amplifier  350  is used to receive the fixed voltage V fix , and a second input terminal of the amplifier  350  is coupled to the node N 2 . One terminal of the resistor R 2  is also coupled to the node N 2 , and an input voltage V i−  is applied to the other terminal of the resistor R 2 . Thus, the input voltage V i−  is not directly inputted to the gate of the transistor M 2 . Moreover, the current i c−  flowing through the resistor R 2  is 
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         [0000]    Similarly, an output current i o−  is obtained according to the current value I 0  of the current source  360  and the current i c−  flowing through the resistor R 2 , i.e. i o− =I 0 −I c− . In the embodiment, the input voltages V i−  and V i+  are a pair of differential signals. Therefore, the output currents i o+  and i o−  are also a pair of differential signals. It is to be noted that a direction of the current i c+  or i c−  is an example and does not intend to limit the invention. In actual applications, the directions of the current i c+  and i c−  are determined according to the input voltages V i+  and V i−  and the fixed voltage V fix . Similar to the embodiment of  FIG. 3 , the fixed voltage V fix  is set according to actual requirements when the voltage to current converting circuit  300  is operating at a low supply voltage. Because of the fixed voltage V fix , each of the amplifiers  330  and  350  may be in a virtual short status even when the supply voltages of the amplifiers  330  and  350  are decreased. Therefore, because each of the amplifiers  330  and  350  would be operating in the virtual short status, the voltage to current converting circuit of the invention still has better linearity even if the supply voltage is very low. So, in actual embodiments, the voltage value of the fixed voltage is determined to make the first and second amplifiers being operated in a virtual short status. 
         [0031]      FIG. 7  shows a mixer  400  according to another embodiment of the invention. The mixer  400  comprises a differential voltage unit  450  and a voltage to current converting circuit  300 . In the mixer  400 , the voltage to current converting circuit  300  obtains the output currents i o+  and i o−  according to the received intermediate frequency signals V IF+  and V IF−  (i.e. the input voltages V i+  and V i− ). The differential voltage unit  450  comprises the transistors M 3 , M 4 , M 5  and M 6  and the inductors L 1  and L 2 . The inductors L 1  and L 2  are both coupled to the supply voltage VDD. The transistor M 3  is coupled between the inductor L 1  and the voltage to current converting sub-circuit  310 , and the transistor M 4  is coupled between the inductor L 2  and the voltage to current converting sub-circuit  310 . Furthermore, the transistor M 5  is coupled between the inductor L 1  and the voltage to current converting sub-circuit  320 , and the transistor M 6  is coupled between the inductor L 2  and the voltage to current converting sub-circuit  320 . The gates of the transistors M 3  and M 6  are used to receive a local oscillation signal LO_P, and the gates of the transistors M 4  and M 5  are used to receive a local oscillation signal LO_N, wherein the local oscillation signals LO_P and LO_N are a pair of differential signals. Therefore, the differential voltage unit  450  generates the RF signal V RF  according to the local oscillation signals LO_P and LO_N and the output currents i o+  and i o+ . In the embodiment, a voltage level of the fixed voltage V fix  is between the supply voltage VDD and the ground GND. 
         [0032]      FIG. 8  shows a voltage to current converting circuit  500  according to another embodiment of the invention, wherein the voltage to current converting circuit  500  operates in a single-end mode. Compared with the voltage to current converting circuit  100  of FIG.  3 , the voltage to current converting circuit  500  shows a circuit structure illustrating that the transistor M 1  is a PMOS transistor.  FIG. 9  shows a voltage to current converting circuit  600  according to another embodiment of the invention, wherein the voltage to current converting circuit  600  operates in a differential mode. Compared with the voltage to current converting circuit  300  of  FIG. 6 , the voltage to current converting circuit  600  shows a circuit structure illustrating that the transistors M 1  and M 2  are PMOS transistors. 
         [0033]    In the embodiments of the invention, the transistors (e.g. the transistors M 1  and M 2 ) of the voltage to current converting circuits are controlled by the amplifiers of the voltage to current converting circuits. Because the input voltage V i  is directly inputted to the resistor R and the voltage V fix  is a predetermined fixed voltage, the amplitude variable of the input voltage V i  can not affect the gain of the amplifier. Therefore, at a low operating/supply voltage, the voltage to current converting circuits of the invention has better linearity. 
         [0034]    While the invention has been described by way of example and in terms of the preferred embodiments, it is to be understood that the invention is not intend to limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.