Abstract:
In a Bi-CMOS logic circuit in which a bipolar transistor and a CMOS element are formed on the same semiconductor substrate, a pull-down current of an output circuit is discharged through a MOS transistor having a large transconductance gm and a constant current source having a large current value. Thus, the speed of a pull-down operation is increased. Moreover, a constant current source need not be provided in an output circuit, and the MOS transistor is operated when it is required during a pull-down operation, thereby reducing an extra current. Therefore, a circuit having low power consumption can be obtained.

Description:
This application is a continuation of application Ser. No. 07/735,043, filed Jul. 24, 1991, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a logic circuit using a bipolar transistor and a MOS transistor and, more particularly, to a Bi-CMOS logic circuit in which a bipolar transistor and a CMOS element are formed on the same semiconductor substrate. 
     2. Description of the Related Art 
     The basic arrangement of an ECL logic circuit using a conventional bipolar transistor is shown in FIG. 1. Complementary inputs IN and IN are input to an emitter-coupled differential circuit constituted by npn bipolar transistors Q 1  and Q 2 . In this case, when the input IN is a high-potential signal, the transistor Q 1  is turned on, and the transistor Q 2  is turned off, thereby flowing a current I 1  through the transistor Q 1 . Therefore, the output potential of the transistor Q 1  becomes V CC  -I 1  R, where R is the resistance value of resistor R 1  and the output potential of the transistor Q 2  becomes VCC. It should be noted that R is also the resistance value of resistor R 2 . The output potentials V CC  -I 1  R and V CC  are level-shifted to V CC  -I 1  R -V f  and V CC  -V f , respectively, by an emitter follower circuit constituted by npn transistors Q 3  and Q 4 . The voltage V f  is a base-emitter forward biasing voltage of each of the bipolar transistors Q 3  and Q 4 . 
     In the above ECL logic circuit, bipolar transistors are unsaturated. When inequality I 1  R&lt;V f  is satisfied due to the transistors Q 3  and Q 4  of a level shift circuit, the unsaturation of the bipolar transistors is ensured. Therefore, the ECL logic circuit is a logic circuit system having a high-speed operation and a large operation margin of bipolar logic transistors. 
     The above conventional ECL logic circuit has a merit of a high-speed operation. However, when the load capacitances of the outputs OUT and OUT are large, the ECL logic circuit has a problem. 
     That is, when the potential of the output OUT is pulled up from a low potential to a high potential, since the load is charged by the bipolar transistor Q4, this charging is performed at a sufficiently high speed. However, when the potential of the output OUT is pulled down from the high potential to the low potential, the bipolar transistor Q 4  is cut off, discharging is performed by only a constant current source I 2 . Therefore, when the load capacitance is large, the speed of the pull-up operation is higher than that of the pull-down operation, and the operation speed of the whole circuit is decreased. 
     When the value of constant current sources I 2  and I 3  is increased to compensate for this drawback, the operation speed is not decreased, but power consumption in unit gate is increased. Since the ECL logic circuit generally has power consumption larger than that of a CMOS element or other bipolar logic circuits, a countermeasure for increasing the capacity of the constant current source cannot easily be employed. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a logic circuit having an operation speed equal to that of an ECL logic circuit, small dependency of load capacitance, and low power consumption. 
     According to the present invention, there is provided a logic circuit comprising a bipolar differential circuit for obtaining first and second outputs which are complementary to each other, a first output circuit for obtaining a first output between a first bipolar transistor having the first output as a control input and a first MOS transistor having the second output as a control input, the first bipolar transistor and the first MOS transistor being connected in series with each other between a constant current source and a power source of the bipolar differential logic circuit, and a second output circuit for obtaining a second output complementary to the first output between a second bipolar transistor having the second output as a control input and a second MOS transistor having the first output as a control input, the second bipolar transistor and the second MOS transistor being connected in series with each other between the constant current source and the power source. 
     According to the present invention, since a pull-down current of an output circuit (emitter follower) is discharged through a MOS transistor having a large transconductance gm and a constant current source having a large current value, the speed of a pull-down operation is increased. In addition, a constant current source need not be arranged in an output circuit, and the MOS transistor is operated when it is required during a pull-down operation, thereby reducing an extra current. Therefore, a circuit having low power consumption can be obtained. 
     Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention. 
     FIG. 1 is a circuit diagram showing a conventional ECL bipolar logic circuit; 
     FIG. 2 is a circuit diagram showing a logic circuit according to an embodiment of the present invention; and 
     FIG. 3 is a circuit diagram showing a logic circuit according to another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiment of a logic circuit according to the present invention will bedescribed below with reference to the accompanying drawings. FIG. 2 is a circuit diagram showing a logic circuit according to an embodiment of the present invention, and the same reference numerals as in FIG. 1 denote thesame parts in FIG. 2. An emitter-coupled differential circuit 1 is constituted by npn bipolar transistors Q 1  and Q 2 , load resistorsR 1  and R 2 , and a constant current source I 4 . The base input to the transistor O 1  is an input IN, and the base input to the transistor Q 2  is an inverted input IN. A first output buffer 2 is constituted by a series circuit of an npn transistor Q 3  and an n-channel MOS transistor T 1 . A second output buffer 3 is constituted by a series circuit of an npn transistor Q 4  and an n-channel MOS transistor T 2 . The constant current source I 4  is used as not only the current source of the differential circuit 1, but the current sources of the first and second output buffers 2 and 3. A differential output O 1  is applied to the base of the transistor Q 3  and the gate of the transistor T 2 , and a differential output O 2  is applied to the gate of the transistor T 1  and the base of the transistor Q 4 . As the outputs of the output buffers 2 and 3, an output OUT (having the same phase as that of the input IN) and an output OUT (having the same phase as that of the input IN) which are complementary to each other are obtained from the junction between the transistors O 4  and T 2  and the junction between the transistors Q 3  and T 1 , respectively. When the value I 4  of the constant current source I 4  in FIG. 2 is compared with the value in FIG. 1, thevalue I 4  can be set to I 4  ≈I 1  +I 2  or I 4  ≈I 1  +I 3 . I 2  and I 3  have substantially the samevalue. In the circuit in FIG. 2, R (the resistance value of each one of resistor R 1  and resistor R 2 ) and I (the current flowing across one of the resistors having the resistance value of R) are set such that the output amplitude is given by IR=V f . It can be considered that power sources are given by V CC  =0 [V] and V EE  =-5 [V]. 
     In FIG. 2, when the input IN of the complementary inputs IN and IN of the differential circuit 1 is set to be a high potential and the input IN is set to be a low potential, a current flows through a branch circuit on thehigh-potential side, i.e., the resistor R1 and the transistor Q1, and a voltage drop IR (where R=the resistance of each one of resistor R 1  and resistor R 2 ) occurs across the resistor R 1 . The output terminal O 1  is set to be a low potential &#34;V CC  -I 4  R&#34; and the output terminal O 2  of a branch circuit on the low potential side,i.e., a circuit constituted by the resistor R 2  and the transistor Q 2 , is charged to be the high potential V CC . When the input terminal IN is set to be a high potential, the output OUT is set to be a low potential, and the output OUT is set to be a high potential. That is, the output OUT is pulled down, and the output OUT is pulled up. 
     In the pull-down operation, the transistor Q 3  is turned off, and the transistor T 1  is turned on. Therefore, the output OUT is discharged through the transistor T 1  and the constant current source I 4 . 
     In the pull-up operation, the transistor Q 4  is turned on, and the transistor T 2  is turned off. Therefore, the output OUT is charged through the transistor Q 4 . 
     In the circuit in FIG. 2, the MOS transistor of the output buffer circuit on the pull-up operation side e.g., the MOS transistor T 2  when the input IN is set to be a high potential is turned off. Thus, the output OUTis charged through an unsaturated transistor Q 4  at a high speed. The MOS transistor T 1  of the output buffer circuit on the pull-down side can be pulled-down at a high speed, by setting a large transconductance gmto the MOS transistor T 1 , and a large capacity to the constant currentsource I 4 . Constant current sources I 2  and I 3  of the output buffer used in the conventional circuit need not be provided in the circuit in FIG. 2, and the MOS transistor (T 1  or T 2 ) of the output buffer is turned on only when it is required. Therefore, a wastefulcurrent is small, and a circuit having low power consumption can be obtained. 
     The present invention is not limited to the above embodiment, and various modifications can be effected. For example, in this embodiment, the emitter-coupled differential circuit 1 is used as a bipolar differential logic circuit, and the complementary inputs IN and IN are used as the inputs to the circuit 1. As shown in FIG. 3, a reference input V REF  may be used as one of the inputs, e.g., the input IN. In addition, a logiccircuit having OR and NOR outputs, a logic circuit having AND and NAND outputs, or the like can be used in place of the differential circuit 1, thereby driving the bipolar transistor and a MOS transistor of an output circuit (output buffer). Although the logic amplitude is set to be a voltage V f  (base-emitter forward biasing voltage) in this embodiment,the logic amplitude may be set to be other logic amplitudes. For example, the output amplitude may be further increased. 
     As described above, according to the present invention, large-current discharging of an output in a circuit on the pull-down side can be performed. Since charging is performed in the circuit on the pull-up side at a high speed, a high-speed operation can be performed in the whole circuit. In addition, the number of constant current sources becomes small, and a current flows only when it is required. Therefore, a circuit having low power consumption can be obtained. 
     Additional advantages and modifications will readily occur to those skilledin the art. Therefore, the invention in its broader aspects is not limited to the specific details, and representative devices shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.