Abstract:
A logic circuit, comprising a plurality of driver MOSFETs and one load MOSFET, produces an output signal responsive to input signals. The driver MOSFETs are each controlled by an input signal and the load MOSFET is controlled by one of input signals to conduct at opposite times to the driver MOSFET controlled by the same input signal. Therefore, a logic circuit with low power consumption and a small area is provided.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to logic circuits such as NOR and NAND gates for producing an output signal responsive to a plurality of input signals. 
     Referring now to FIG. 1, there is shown a prior art NOR circuit having four inputs. N-channel enhancement type MOSFETs Q1, Q2, Q3 and Q4 are connected in parallel between an output terminal and a ground terminal. The gate electrodes of MOSFETs Q1, Q2, Q3 and Q4 are connected to input terminals IN1, IN2, IN3 and IN4 respectively. A load element 2 is connected between a power supply terminal V DD  and the output terminal and usually consists of a resistor or a MOSFET, the gate and source electrodes of which are connected to each other. Note that even though only one of MOSFETs Q1 through Q4 becomes conductive, a current path between the power supply and ground terminals is formed and current flows through the path. Power consumption of such a NOR circuit with a load element is very large. 
     Referring to FIG. 2, there is shown another prior art NOR circuit of complementary MOSFETs (CMOS FETs). N-channel enhancement type MOSFETs Q1 through Q4 are connected in parallel between the output terminal and the ground terminal. P-channel enhancement type MOSFETs Q5, Q6, Q7 and Q8 are connected in series between the power supply terminal and the output terminal. MOSFETs Q1 and Q5, Q2 and Q6, Q3 and Q7 and Q4 and Q8 are connected to input terminals IN1, IN2, IN3 and IN4 through the gate electrodes respectively. When MOSFET Q1 is conductive, MOSFET Q5 is not conductive and no current path between the power supply terminal and the ground-terminal is formed. In this way, power consumption of the NOR circuit of CMOSFETs is very small. However, the number of transistors in such a circuit is twice that of input terminals. This results in complex wiring and a larger area than that of a NOR circuit which uses a load element. Moreover, when the logic level of the output signal responsive to input signals changes to a logic &#34;1&#34;, the voltage of the power supply terminal is transmitted through MOSFETs Q5, Q6, Q7 and Q8. Therefore it takes longer to change its output to a logic &#34;1&#34; than a circuit with a load element. 
     SUMMARY OF THE INVENTION 
     It is an object of this invention to provide a logic circuit with low power consumption and a small area. 
     According to the invention, a logic circuit for producing an output signal responsive to input signals comprises first and second power source terminals, an output terminal, plurality of MOSFETs of first conductivity type connected between the first power source terminal and the output terminal in parallel and controlled by input signals respectively and a MOSFET of second conductivity type connected between the second power source terminal and the output terminal and controlled by one of the input signals. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a prior art NOR circuit, 
     FIG. 2 shows another prior art NOR circuit, 
     FIG. 3 shows a NOR circuit of a preferred embodiment of the invention, 
     FIG. 4 shows a NOR circuit of another embodiment of the invention, and 
     FIG. 5 shows a NAND circuit of yet another embodiment of the invention. 
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawings, there is shown in FIG. 3 a NOR circuit in accordance with the present invention. N-channel enhancement type MOSFETs Q11, Q12, Q13 and Q14 are connected in parallel between an output terminal and a ground terminal. The gate electrodes of MOSFETs Q11, Q12, Q13 and Q14 are connected to input terminals IN1, IN2, IN3 and IN4, respectively, which are, in turn, connected to signal generator SG. Signal generator SG may be any logic circuit. A P-channel enhancement type MOSFET type Q15 is connected between a power supply terminal V DD  and the output terminal. The gate electrode of MOSFET Q15 is connected to input terminal IN1. The channel resistance of MOSFET Q15 is set larger than that of each one of MOSFETs Q11 through Q14. 
     When the logic level of input terminal IN1 is a logic &#34;1&#34;, MOSFET Q11 is conductive and MOSFET Q15 is nonconductive. Therefore, the logic level of the output terminal is a logic &#34;0&#34; without reference to the logic levels of input terminals IN2, IN3 and IN4. In this state, no current path between the power supply terminal and the ground terminal is formed and current does not flow. 
     When the logic level of input terminal IN2 is a logic &#34;0&#34;, MOSFET Q11 is nonconductive and MOSFET Q15 is conductive. Additionally, if the logic level of at least one of input terminals IN2, IN3 and IN4, for example IN2, is a logic &#34;1&#34;, MOSFET Q12 is conductive and a current path through MOSFETs Q15 and Q12 between the power supply terminal and the ground terminal is formed. Therefore, the logic level of the output terminal is a logic &#34;0&#34;. It is effective for low power consumption to make the channel resistance of MOSFET Q15 big. 
     When all logic levels of input terminals IN1 through IN4 are a logic &#34;0&#34;, only MOSFET Q15 is conductive. Therefore, no current path is formed and the logic level of the output terminal is a logic &#34;1&#34;. 
     The number of elements of this circuit is the same as that of the prior art circuit shown in FIG. 1 and is less than that of the prior art circuit shown in FIG. 2. Therefore, this circuit is suitable for a large scale integrated circuit. 
     Since current does not flow in this circuit when the logic level of the input terminal IN1 is a logic &#34;1&#34;, power consumption of this circuit is smaller than that of the circuit which uses a load element like the prior art circuit shown in FIG. 1. 
     Usually, signal generator SG generates different input signals to be applied to input terminals IN1 through IN4 and the period during which the logic levels of the input signals are a logic &#34;1&#34; are different from each other. Keep in mind that the current path between the power supply terminal and the ground terminal is not formed when the logic level of the input terminal IN1 is a logic &#34;1&#34; independent of the logic levels of input terminals IN2, IN3 and IN4. Therefore, power consumption of this circuit is made smallest if the input signal from signal generator SG that has the longest period during which the logic level is a logic &#34;1&#34; is applied to the input terminal IN1. 
     Moreover, the voltage of the power supply terminal is transmitted only through MOSFET Q15 to the output terminal when all logic levels of input terminals IN1 through IN4 are a logic &#34;0&#34;. Therefore, the time required to output a logic &#34;1&#34; is shorter than that of the prior art circuit shown in FIG. 2. 
     Referring to FIG. 4, there is shown another embodiment of this invention. This NOR circuit is identical to that of FIG. 3 with the addition of a P-channel MOSFET Q16 connected between the power supply terminal V DD  and MOSFET Q15. The gate electrode of MOSFET Q16 is connected to input terminal IN2. The total resistance of the channel resistances of MOSFETs Q15 and Q16 is set larger than that of each one of MOSFETs Q11 through Q14. 
     When the logic level of at least one of input terminals IN1 and IN2 is a logic &#34;1&#34;, no current path between the power supply terminal and the ground terminal is formed. Therefore, power consumption of this circuit is smaller than that of the circuit shown in FIG. 3. 
     In the embodiments shown in FIGS. 3 and 4, MOSFETs Q15 and Q16 are enhancement type. However, MOSFETs Q15 and Q16 may be depletion type. 
     Referring now to FIG. 5, there is shown a NAND circuit in accordance with the present invention. P-channel enhancement type MOSFETs Q21, Q22, Q23 and Q24 are connected in parallel between the power supply terminal V DD  and the output terminal. The gate electrodes of MOSFETs Q21, Q22, Q23 and Q24 are connected to input terminals IN1, IN2, IN3 and IN4, respectively, which are, in turn, connected to signal generator SG. A N-channel MOSFET of enhancement type Q25 is connected between the output terminal and the ground terminal. The gate electrode of MOSFET Q25 is connected to input terminal IN1. The channel resistance of MOSFET Q25 is larger than that of each one of MOSFETs Q21 through Q24. 
     When the logic level of input terminal IN1 is a logic &#34;0&#34;, MOSFET Q21 is conductive and MOSFET Q25 is nonconductive. Therefore, no current path between the power supply terminal and the ground terminal is formed and the logic level of the output terminal is a logic &#34;1&#34;. 
     When the logic level of input terminal IN1 is a logic &#34;1&#34; and the logic level of at least one of input terminals IN2, IN3 and IN4, for example IN2, is a logic &#34;0&#34;, MOSFETs Q22 and q25 are conductive. Therefore, a current path between the power supply terminal and the ground terminal is formed and the logic level of the output terminal is a logic &#34;1&#34;. 
     When all logic levels of input terminals IN1 through IN4 are a logic &#34;1&#34;, only MOSFET Q25 is conductive. Therefore, no current path between the power supply terminal and the ground terminal is formed and the logic level of the output terminal is a logic &#34;0&#34;. 
     This NAND circuit also achieves low power consumption, small area and high speed. 
     While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications of the embodiments will be apparent to persons skilled in the art upon reference to this description. It is, therefore, contemplated that the appended claims will cover any such modifications or embodiments as fall within the true scope of the invention.