Abstract:
A differential input circuit which can positively operate over a wide input range is provided. The differential input circuit includes a first constant current source of a current mirror type which generates a positive current and a second constant current source of a current mirror type which generates a negative current. The first and second constant current sources constitute a differential amplifier circuit. A current switch which is connected to a positive input and a negative input is also connected to said first and second constant current sources so that an operating point of the differential amplifier circuit can be changed.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a differential input circuit, and particularly to a differential input circuit having CMOS circuits for high-speed signal transmission. 
     2. Description of the Related Art 
     Small-amplitude signal transmission has been widely used in CMOS circuits with an increase in a speed of signal transmission. The small-amplitude signal transmission is a technique which transmits signals so that the transmitted signals are not saturated, that is, switching transistors are not saturated. In the small-amplitude signal transmission, since transmitted signals are easily affected by a noise, differential signals are generally used. 
     In an interface system called LVDS (Low Voltage Differential Signals), small-amplitude signals having an amplitude of 100 mV at minimum are inputted and a logic level of the inputted signals must be determined. Additionally, the inputted signals have a wide voltage range ranging from 0 V to 2.4 V. 
     FIG.1 shows an example of a conventional differential input circuit. Transistors Tr 11  and Tr 12  constitute a current mirror circuit with their gates being connected to each other, and supply currents having the same amplitude to transistors Tr 13  and Tr 14 . Precisely, the amplitudes of the currents supplied to the transistors Tr 13  and Tr 14  are proportional to widths of gates of the transistors Tr 11  and Tr 12 , respectively. Thus, if the transistors Tr 11  and Tr 12  are designed to have the same physical dimension, currents having the same amplitude are supplied to the transistors Tr 13  and Tr 14 . 
     An output voltage V out  at an output terminal Out is changed when ON-state resistances between a drain and a source of the transistors Tr 13  and Tr 14  are changed in accordance with gate voltages of the transistors Tr 13  and Tr 14 , which gate voltages correspond to input voltages of the circuit. When an input voltage V P  at a P −in  terminal connected to a gate of the transistor Tr 13  is larger than an input voltage V N  at an N −in  terminal connected to a gate of the transistor Tr 14 , the output voltage V out  is of a high level, and when V P  is smaller than V N , the output voltage V out  is of a low level. A transistor  15  which is connected to sources of the transistors Tr 13  and Tr 14  functions to define a total current flowing through the circuit. That is, the transistor  15  functions as a constant current source. 
     However, the above-mentioned conventional circuit can operate only when a common voltage (a center value of differential input signals) is within a limited range. FIG. 2 is a diagram for illustrating the common voltage. The common voltage is defined as a center value V ic  of the positive input voltage V P  and the negative input voltage V N  of a differential input circuit U. 
     For example, when both the input voltages V P  and V N  are so high that the transistor Tr 15  is saturated, both the transistors Tr 13 , Tr 14  are turned on. In this case, the output voltage V out  is fixed to be of the low level. On the other hand, if both the input voltages V P  and V N  are lower than a threshold voltage of the transistors Tr 13  and Tr 14 , both the transistors Tr 13  and Tr 14  are turned off. In this case, the output voltage V out  is indefinite. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a differential input circuit which can positively operate over a wide range of the common voltage. 
     FIG. 3 is a diagram showing a principle of the present invention. As shown in FIG. 3, a current switch  20  is connected to a positive signal input P −in  and a negative signal input N −in . A first constant current source  21  of a current mirror type generating a positive current and a second constant current source  22  of a current mirror type generating a negative current are connected symmetrically to each other to constitute a differential amplifier circuit. The current switch  20  is connected to the first and second constant current sources so that an operating point of the differential amplifier circuit can be changed. 
     According to the invention, since the current switch  20  is provided so that the operating point of the differential amplifier circuit can be changed, as mentioned above, the differential input circuit can positively operate over a wide range of the common voltage. 
     The current switch may comprise a first switching part which controls a current derived from said first constant current source and a second switch part which controls a current derived from the second constant current source. 
     In this case, each of the first and second switch parts may comprise a first switch element connected to the positive input and a second switch element connected to the negative input. Further, the first and second switch parts may be constructed so that the first switch elements of said first and second switch parts are complementary to each other and the second switch elements of said first and second switch parts are complementary to each other. 
     In this invention, two complementary switch elements are connected to each of the positive input P −in  and the negative input N −in . Here, the term “complementary switch elements” means that whenever one of the switch elements is not operating, the other switch element is operating. Thus, according to the invention, when the common voltage is high, at least one of the complementary switch elements can operate, and when the common voltage is low, at least the other switch element can operate. Thus, the differential input circuit can operate over a wide range of the common voltage. 
     Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG.1 is a circuit diagram of an example of a conventional differential input circuit; 
     FIG. 2 is a diagram for illustrating a common voltage; 
     FIG. 3 is a block diagram showing a principle of the present invention; 
     FIG. 4 is a circuit diagram showing a first embodiment of the present invention; and 
     FIG. 5 is a circuit diagram showing a second embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 4 is a circuit diagram showing a first embodiment of the present invention. In FIG. 4, P-channel transistors Tr 7  and Tr 8  constitute a P-channel constant current source, and N-channel transistors Tr 9  and Tr 10  constitute an N-channel constant current source. The circuit is designed so that currents flowing through the transistors Tr 7  to Tr 10  are the same when transistors Tr 1  to Tr 6  are not connected. In this case, a voltage (VDD−VSS)/2 (VDD is a higher power-supply voltage and VSS is a lower power-supply voltage) is outputted to an output terminal Out. 
     In the circuit shown in FIG. 4, since the P-channel constant current source is constituted by the P-channel transistors Tr 7 , Tr 8  and the N-channel constant current source is constituted by the N-channel transistors Tr 9 , Tr 10 , the constant current sources can be complementarily used. 
     The transistors Tr 1  to Tr 6  which constitute a current switch are connected to the constant current sources. If the voltages V P , V N  at the inputs P −in  and N −in , respectively, are the same, amplitudes of currents which are derived from the transistors Tr 7  and Tr 8  via the transistors Tr 5  and Tr 6 , respectively, are the same and amplitudes of currents which are supplied to the transistors Tr 9  and Tr 10  via the transistors Tr 2  and Tr 3 , respectively, are the same. In this case, the differential input circuit is balanced. 
     Now, a description will be given of operations of the circuit when the input voltage V P  and the input voltage V N  are changed. 
     (A) When V p &gt;V N  is established: 
     In this case, an ON-state resistance of the transistor Tr 5  becomes smaller than that of the transistor Tr 6  since a larger bias voltage is supplied to a gate of the transistor Tr 5  than to a gate of the transistor Tr 6 . Thus, a current supplied to the transistor Tr 5  from the transistor Tr 7  becomes larger than a current supplied to the transistor Tr 6  from the transistor Tr 8 . As a result, gate voltages of the transistors Tr 7 , Tr 8  become lower and thus the output voltage V out  becomes higher. 
     Additionally, an ON-state resistance of the transistor Tr 2  becomes larger than that of the transistor Tr 3 . Thus, a current supplied to the transistor Tr 9  from the transistor Tr 2  becomes smaller than a current supplied to the transistor Tr 10  from the transistor Tr 3 . As a result, gate voltages of the transistors Tr 9 , Tr 10  becomes lower and thus the output voltage V out  becomes higher. 
     (B) When V P &lt;V N  is established: 
     In this case, the circuit operates in an opposite way as compared to the case where V P &gt;V N  is established, and the output voltage V out  becomes lower. That is, when V P &lt;V N  is established, an ON-state resistance of the transistor Tr 5  becomes larger than that of the transistor Tr 6 . Thus, a current supplied to the transistor Tr 5  from the transistor Tr 7  becomes smaller than a current supplied to the transistor Tr 6  from the transistor Tr 8 . As a result, gate voltages of the transistors Tr 7 , Tr 8  become higher and thus the output voltage V out  becomes lower. 
     Additionally, an ON-state resistance of the transistor Tr 2  becomes smaller than that of the transistor Tr 3 . Thus, a current supplied to the transistor Tr 9  from the transistor Tr 2  becomes larger than a current supplied to the transistor Tr 10  from the transistor Tr 3 . As a result, the gate voltages of the transistors Tr 9 , Tr 10  become higher and thus the output voltage V out  becomes lower. 
     (C) When both the input voltages V P  and V N  are low: 
     If the voltages V P  and V N  become lower than a threshold voltage V th  of the transistors Tr 5 , Tr 6 , currents are no longer derived from the transistors Tr 7 , Tr 8  since the transistors Tr 5 , Tr 6  are turned off. In this case, however, the transistors Tr 2 , Tr 3  are operating. Therefore, the output voltage V out  is determined by operating states of the transistors Tr 9 , Tr 10  in accordance with a relationship between the voltages V P  and V N . 
     (D) When both the input voltages V P  and V N  are high: 
     If the voltage V P  and V N  are so high that the transistors Tr 2 , Tr 3  are reversely biased, the transistors Tr 2 , Tr 3  are turned off. In this case, however, the transistors Tr 5 , Tr 6  are operating. Thus, the output voltage V out  is determined by operating states of the transistors Tr 7 , Tr 8  in accordance with a relationship between the input voltages V P  and V N . 
     As mentioned above, since currents derived from each of the constant current sources are controlled by the current switch constituted by the transistors Tr 2 , Tr 3 , T 5 , Tr 6 , an operating point of the differential amplifier circuit constituted by constant current sources is changed in accordance with the operating states of the current switch. Thus, according to the present embodiment, it is possible to positively operate the differential input circuit in accordance with the input voltages V P  and V N . 
     Additionally, since two complementary transistors (namely, the N-channel transistor Tr 5  or Tr 6  and the P-channel transistor Tr 2  or Tr 3 ) are connected to each of the input terminals P −in  and N −in , at least one of the complementary transistors can always operate regardless of the input voltage level. Thus, according to the present embodiment, it is possible to positively operate the differential amplifier circuit in accordance with the operating states of the current switch. 
     Further, since CMOS FETs are used as the transistors Tr 2 , Tr 3 , Tr 5 , Tr 6  constituting the current switch and these CMOS FETs are operated in a linear operating region, it is possible to achieve a high-speed operation of the current switch. 
     FIG. 5 is a circuit diagram showing a second embodiment of the present invention. In FIG. 5, those parts which are the same as the parts shown in FIG. 4 are given the same reference numerals, and descriptions thereof will be omitted. In the present embodiment, a bypass resistor R is connected between the P-channel constant current source and the N-channel constant current source as shown in FIG.  5 . 
     In the above-mentioned first embodiment, when the transistor Tr 2  or Tr 5  is turned off, the transistor Tr 9  or Tr 7  is turned off accordingly, since no current is supplied to the transistor Tr 9  when the transistor Tr 2  is turned off and no current is derived from the transistor Tr 7  when the transistor Tr 5  is turned off. Thus, if the input voltages V P  and V N  are so high (or low) that the transistor Tr 2  (or Tr 5 ) is turned off, the output voltage V out  does not exhibit a change as expected when both the transistors Tr 2  and Tr 5  are operating. 
     In the present embodiment, the resistor R functions to prevent the transistors Tr 9 , Tr 7  from being turned off when the transistors Tr 2 , Tr 5  are turned off, respectively. This function is achieved by supplying a small current from the transistor Tr 7  to the transistor Tr 9 . Thus, the resistor R can be replaced by a transistor. 
     According to the present embodiment, a bias current, which is smaller than a current flowing through the current switch, flows through the resistor R between the P-channel constant current source and the N-channel constant current source. Thus, if one of the transistors Tr 2  and Tr 5  is turned off, the corresponding transistor Tr 6  or Tr 3  can control the output voltage V out  by changing a current derived from the transistor Tr 7  or a current supplied to the transistor Tr 9  in accordance with the negative input voltage V N . 
     As mentioned above, the differential input circuit of the present embodiment is achieved by simply providing the resistor R (or a transistor) in the differential input circuit of the above-mentioned first embodiment. Thus, according to the present embodiment, the above-mentioned function can be achieved by a simple structure. 
     The present invention is not limited to these embodiments, but variations and modifications may be made without departing from the scope of the present invention. 
     The present application is based on Japanese priority application No. 10-311228 filed on Oct. 30, 1998, the entire contents of which are hereby incorporated by reference.