Abstract:
Digitally-operated analog buffer amplifiers which are characterized by small circuits, no offset voltage, high speed and low power consumption. Circuit means are provided so that when the value of the output voltage of the buffer amplifier nears the approximate value (set voltage value) of the input voltage value, the input terminal and output terminal short-circuit so that the output voltage and the input voltage are made equal, thereby preventing the generation of offset voltage. Power consumption is reduced because the circuit completes the operation in a digital operation state.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention provides positive and negative directional digitally operated buffer amplifiers. 
     2. Description of the Prior Art 
     When load is connected to an analog signal (large-amplitude low-frequency alternating current) source that has no driving capability, it has been conventional to use a buffer amplifier between the signal source and the load so that the load will not affect signals. However, if the buffer amplifier consists of an operational amplifier in which MOS elements are used, offset voltage is generated due to the poor consistency of the MOS elements, thus causing errors to occur in the input signal source voltage and the output voltage. 
     SUMMARY OF THE PRESENT INVENTION 
     The present invention provides digitally-operated analog buffer amplifiers which are characterized by small circuits, no offset voltage, high speed and low power consumption. 
     In accordance with the teachings of the present invention, circuit means are provided so that when the value of the output voltage of the buffer amplifier nears the approximate value (set voltage value) of the input voltage value, the input terminal and output terminal short-circuit so that the output voltage and the input voltage are made equal, thereby preventing the generation of offset voltage. Power consumption is reduced because the circuit completes the operation in a digital operation state. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     For better understanding of the present invention as well as other objects and further features thereof, reference is made to the following description which is to be read in conjunction with the accompanying drawing therein: 
     FIG.  1 ( a ) is a block diagram of a first embodiment of the present invention; and FIG.  1 ( b ) the circuit diagram thereof; 
     FIG.  2 ( a ) is a block diagram showing a second embodiment example of the present invention; and FIG.  2 ( b ) the circuit diagram thereof; 
     FIG. 3 is a block diagram showing a third embodiment of the present invention; 
     FIG. 4 is a circuit diagram showing a fourth embodiment of the present invention; and 
     FIG. 5 is a circuit diagram showing a fifth embodiment of the present invention which incorporates overshooting or undershooting characteristics. 
    
    
     DESCRIPTION OF THE INVENTION 
     Referring now to FIG. 1, a block diagram of the positive directional digitally operated analog buffer amplifier embodiment of the present invention is illustrated. 
     Comparator CMPT 1  is connected to input terminal IN and output terminal OUT so as to compare the input voltage and the output voltage. 
     Level shifter LVSF 1  is connected to the output section of CMPT 1  so as to amplify and convert the output voltage level of CMPT 1  to the power source and earth voltage levels; switch SW 1  is connected so as to short-circuit and release power source S and OUT; also, SW 1  is controlled by the inversion output of LVSF 1 . Switch SW 2  is connected so as to short-circuit and release IN and OUT; and the input of SW 2  is controlled by the positive output of LVSF 1 . 
     If the input terminal voltage is higher than the output terminal voltage, SW 2  is controlled so as to release IN and OUT; at the same time, SW 1  short-circuits S and OUT; and if the output terminal voltage rises in the direction of the power source voltage level and reaches a pre-set voltage, which is lower than the input terminal voltage, SW 1  is released so that the rise in the output terminal voltage halts, SW 2  short-circuits at the same time and the input terminal voltage and output terminal voltage become equal. 
     Referring now to the negative-directional digitally-operated analog buffer amplifier embodiment shown in FIG. 2, Comparator CMPT 2  is connected to input terminal IN and output terminal OUT so as to compare the input voltage and the output voltage. Level shifter LVSF 2  is connected to the output section of CMPT 2  so as to amplify and convert the output voltage level of CMPT 2  to the power source and earth voltage levels. Switch SW 3  is connected so as to short-circuit and release earth E and OUT; also, SW 3  is controlled by the positive output of LVSF 2 . 
     Switch SW 4  is connected so as to short-circuit and release IN and OUT; and the input of SW 4  is controlled by the inversion output of LVSF 2 . 
     If the input terminal voltage is lower than the output terminal voltage, SW 4  is controlled so as to release IN and OUT; at the same time, SW 3  short-circuits E and OUT; and when the output terminal voltage drops in the direction of the earth voltage level and reaches a pre-set voltage, which is higher than the input terminal voltage, SW 3  is released so that the drop in the output terminal voltage halts, SW 4  short-circuits at the same time and the input terminal voltage and output terminal voltage become equal. 
     Referring to the digitally-operated analog buffer amplifier embodiment shown in FIG. 3, the output sections of LVSF 1  and LVSF 2  of the positive- and negative-directional digitally-operated analog buffer amplifiers are connected to the input section of Exclusive OR (logic seen) circuit EXOR; the output section of EXOR is connected to SW 4  of the negative-directional digitally-operated analog buffer amplifier; also, SW 4  is controlled by the output of EXOR. 
     The input section of inversion logic circuit INV is connected to the output section of EXOR; the output on INV is connected to SW 2  of the positive-directional digitally-operated analog buffer amplifier; and SW 2  is controlled by the output of INV; and if the input terminal voltage is higher than the output terminal voltage, SW 2  and SW 4  of the positive- and negative-directional digitally-operated analog buffer are controlled so as to release IN and OUT; at the same time, SW 1  of the positive-directional digitally-operated analog buffer amplifier short-circuits; the output terminal voltage rises in the direction of the power source voltage level; and, at the same time, SW 3  of the negative-directional digitally-operated analog buffer amplifier is in a released state; if the input terminal voltage is lower than the output terminal voltage, SW 2  and SW 4  of the positive- and negative-directional digitally-operated analog amplifiers are controlled so as to release IN and OUT; at the same time, SW 3  of the negative-directional digitally-operated analog buffer amplifier short-circuits; the output terminal voltage drops in the direction of the earth voltage level; and, at the same time, SW 1  of the positive-directional digitally-operated analog buffer amplifier is in a released state. 
     If the input terminal voltage approximates the output terminal voltage, SW 2  and SW 4  of the positive- and negative-directional digitally-operated buffer amplifiers are controlled so as to short-circuit IN and OUT; and, at the same time, SW 1  and SW 3  of the positive- and negative-directional digitally-operated analog buffer amplifiers are released. 
     The digitally-operated analog buffer amplifiers embodiments mentioned hereinabove are capable of adjusting the set voltage by varying the size of the elements of CMPT 1  and CMPT 2 . 
     In addition, the comparators of the digitally-operated analog buffer amplifiers are structured so that the size of the elements of CMPT 1  and CMPT 2  are controlled by a switch, thereby varying the set of voltage so as to provide the hysterisis characteristics. 
     Operations of digitally-operated analog buffer amplifiers of the present invention will be explained below by referring to the block diagram shown in FIG.  3 . CMPT 1 , LVSF 1 , SW 1  and SW 2  serve as a buffer amplifier in the positive direction (the direction toward the power source) and CMPT 2 , LVSF 2 , SW 3  and SW 4  serve as a buffer amplifier in the negative direction (the direction toward earth voltage). The control input terminals of SW 1  and SW 2  show that they are closed “L” level signal. The exclusive logical sum treatment is applied to the outputs of LVSF 1  and LVSF 2  and the summed outputs and its inversion signal control SW 2  and SW 4 . 
     When the input voltage becomes higher than the output voltage, CMPT 1  detects this and turns the output of LVSF 1  to “L”. Here, the output of CMPT 2  does not change and the output of LVSF 2  remains in “L”; thus, the output of EXOR becomes “L” and the output of INV becomes “H” so that SW 2  and SW 4  are released. At the same time, SW 3  is released and SW 1  is short-circuited so that the output voltage rises in the direction of the power source voltage. 
     When the output voltage reaches the set voltage, which is lower than the input voltage, CMPT 1  detects this and the output of LVSF 1  changes from “L”to “H”. Also, the output of EXOR changes from “L” to “H”, and the output of INV changes from “H”to “L”. Thus, SW 1  is released and SW 2  and SW 4  are short-circuited so that the output voltage rises to a voltage level equal to the input voltage and halts there. As the output voltage rises to a voltage level that is higher than the set voltage, the output of LVSF 1  becomes a complete “H”, and the operation completes in a stable digital state. 
     When the input voltage becomes lower than the output voltage, CMPT  2  detects this and turns the output of LVSF 2  to “H”. Here, the output of CMPT 1  does not change and the output of LVSF 1  remains in “H”; thus, the output of EXOR becomes “L”and the output of INV becomes “H”so that SW 2  and SW 4  are released. At the same time, SW 1  is released and SW 3  is short-circuited so that the output voltage drops in the direction of the earth voltage. When the output voltage reaches the set voltage, which is higher than the input voltage, CMPT 2  detects this and the output of LVSF 2  changes from “H”to “L”. Also, the output of EXOR changes from “L”and “H”, and the output of INV changes from “H”to “L”. Thus, SW 3  is released and SW 2  and SW 4  are short-circuited so that the output voltage drops to a voltage level equal to the input voltage and halts there. As the output voltage drops to a voltage level that is lower than the set voltage, the output of LVSF 2  becomes a complete “L”, and the operation completes in a stable digital state. 
     As the operation completes with SW 2  and SW 4  short-circuited as mentioned above, offset voltage, which is seen in buffer amplifiers using operational amplifiers, is not generated. Also, as the operation completes in stable digital state and direct current is consumed only by the comparator, power consumption is low. Further, the operation is very speedy and stable because it is digital. 
     FIG. 4 illustrates a further use of the concept of the invention. The ratios of gate width (W) and gate length (L) (W/L) are different in the transistors MN 1  and MN 2  in CMPT 1 ; and the difference provides the set voltage. In FIG. 4, the ratio W/L in MN 2  is larger than that in MN 1 , which decreases the threshold voltage so that MN 2  incurs the switching of current from MN 1  to MN 2  by way of a voltage that is lower than the gate voltage of MN 1 . As the result, the output of LVSF 1  is switched due to voltage fluctuations (drops) in resistors R 1  and R 2 . 
     Similarly, the ratios (W/L) of the gate width and the gate length of the transistors MP 5  and MP 6  in CMPT 2  are different. The threshold voltage decreases when W/L of MP 6  is larger than that of MP 5 . Thus, the switching of current occurs to MP 6  with: a gate voltage that is higher than that of MP 5 . 
     The set voltage can be freely changed by changing the ratio W/L. Also, high-speed operations corresponding to load conditions are possible by changing the set voltage temporarily. When a large time-constraint load is applied, it may take a while for the output voltage to reach the input voltage level with SW 2  and SW 4  short-circuited. The time can be shortened by temporarily overshooting or undershooting the output voltage so as to offset the overshoot (or undershoot) and the difference. 
     FIG. 5 shows a further version of the circuit shown in FIG.  4 . In particular, a transistor MNOV is added to MN 1  in CMPT 1  so as to temporarily change the ratio W/L (i.e. to have a large W/L ratio). The switch ASW 1  short-circuits only when the input voltage becomes higher than the output voltage so as to make W/L of MN 1  larger than that of MN 2  temporarily. 
     Thus, the set voltage value becomes higher than the target voltage value and overshooting is possible. When the output voltage nears the set voltage value, the current that has flowed only through MN 1  and MNOV will gradually flow through MN 2 . IF LVSF 1  is set so as to inverse with such a change of current, ASW 1  is released, the set voltage value fluctuates (drops) as if it drops downward sharply and the current flows only through MN 2 . 
     The above is the same as the application of positive feedback; thus, the comparator&#39;s threshold voltage characteristics show hysterisis characteristics. Thus, the output voltage temporarily overshoots and thereafter converges to the target value so that the circuit completes its operations in a stable digital operation state. 
     Similarly, undershooting is available by changing W/L of MP 5  in CMPT 2  temporarily. As mentioned above, it is possible to easily generate overshoot by adding a few elements so as to correspond to large time-constant loads. 
     This present invention thus prevents the generation of offset voltage; provides equal input signal source voltage and output voltage; drives at high speeds; and operates at a low power consumption level. 
     While the invention has been described with reference to its preferred embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the true spirit and scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from its essential teachings.