Abstract:
In a memory system where certain memory systems a signal detector is provided which begins to detect the voltage of a data line to which a plurality of memory cells are connected when the voltage of a latch node of the detector is shifted from a first to a second level by discharging the node through a switch in response to an input signal provided by a pulse circuit. To avoid discharging the node at an improper time, a signal transformation circuit is interposed between the pulse circuit and the switch to provide the switch with a signal to turn it on only when the level of the signal provided by the pulse circuit is high enough. In this manner, improper discharging due to shifts in the pulse circuit can be avoided.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a memory system having a fast and stable sense amplifier with a high sensitivity, and, more particularly, to a monolithic memory system which makes use of field effect transistors as circuit elements. 
     A recently developed monolithic memory comprises a plurality of memory cells, each including a field effect transistor and a capacitance, and a differential sense amplifier having a flip-flop circuit constructed with field effect transistors. This sense amplifier differentially amplifies the voltages of a pair of data lines and detects the signal stored in a selected one of the memory cells. The flip-flop circuit has a latch node which is precharged to a predetermined voltage before the flip-flop circuit begins its amplifying operation. The data lines are also precharged to nearly the same voltage. Before the amplifying operation, the transistors which constitute the flip-flop circuit are in off states. Under that condition, the signal stored in one of the memory cells is read out, and the voltage of one of the data lines changes in response to the read out signal. Then, a discharging field effect transistor connected to the latch node of the flip-flop circuit is turned on. 
     As a result, the node voltage is lowered to 0 volts and the flip-flop circuit begins its amplifying operation. The switching of the discharging field-effect transistor is controlled by a voltage pulse supplied by a pulse circuit located at the periphery of the memory cell area. When the voltage pulse becomes higher than the threshold voltage V T  of the field-effect transistor, the transistor becomes conductive. The transistor must become conductive after the signal stored in a memory cell is read out onto a corresponding data line. 
     One problem with this prior art memory system is that it often fails to operate normally when the threshold voltage V T  is small for reasons discussed below. 
     In order to develop a memory with a higher packing density, it is necessary to reduce the sizes of the transistors used in the memory. It is also necessary to reduce the power supply voltage V DD  which is provided to the transistors, in order to avoid the dielectric breakdown of the transistors due to scaling down the component size to achieve the higher packing density. At the same time, it is necessary to reduce the threshold voltage V T  of the transistors in order to maintain a high speed operation. The switching speed of a field effect transistor is roughly proportional to (V G  -V T ) n , where V G  is a gate voltage and n is a an experimentally determined number among 1.0 and 2.0. If the gate voltage V G  is assumed to be nearly equal to the power supply voltage V DD , a lower threshold voltage V T  is desirable when a lower supply voltage is used, in order to maintain a high speed operation. 
     If the discharging field effect transistor connected to the latch node of flip-flop circuit as discussed above has a lower threshold voltage V T , the transistor is apt to become conductive even when it should be non-conductive, due to voltage drift of the voltage pulse provided to the gate thereof. As a result, the latch node of the flip-flop circuit begins to discharge during a period when it is not supposed to, and the circuit performs its amplifying operation, discharging the data lines. The output signal of the flip-flop circuit at the period when the amplified signal should be detected is then lowered because of the above-mentioned malfunction. 
     The voltage drift of the voltage pulse which causes this malfunction may be introduced because of several reasons. One of these reasons stems from the fact that pulse circuit which provides the voltage pulse is connected to a common ground line and provides a voltage pulse the level of which is dependent on the voltage of the common ground line at the point where the pulse circuit is connected thereto. The resistance value of the ground line is not negligible and several other peripheral circuits are also connected to this ground line. Each of these peripheral circuits introduces a current flow through the ground line while the circuit operates. As a result, the voltage of the ground line drifts depending on how many of the peripheral circuits are operating. Therefore, the discharging transistor begins to discharge during an undesirable period due to this voltage drift, if the threshold value V T  is low. 
     LIST OF THE PRIOR ART 
     The following references are cited to show examples of the present state of the art: 
     (1) U.S. Pat. No. 4,061,999 issued to Proebsting et al., on Dec. 6, 1977 and 
     (2) U.S. Pat. No. 3,810,124 issued to Hoffman et al., May 7, 1974. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an object of the present invention to provide a memory system which maintains a stable operation of a signal detection amplifier in spite of the voltage drift of a control signal provided thereto. 
     It is another object of the present invention to provide a memory system which maintains a stable operation of a signal detection amplifier which amplifier includes a switching element with a low threshold voltage in the discharging circuit. 
     It is another object of the present invention to provide a memory system which maintains a stable operation of a discharging circuit in spite of the voltage drift of a control signal provided thereto. 
     According to the present invention, a memory system comprises a pulse circuit for generating a first pulse signal which can take two different levels and a signal transformation circuit for transforming this control pulse into a second pulse signal which is provided to the discharging circuit of a sense amplifier. The transformation circuit outputs a pulse signal with a level high enough to allow the discharging by the discharging circuit only when the first pulse signal has a level sufficiently greater than the threshold voltage of the discharge circuit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a schematic circuit diagram of a memory system according to the present invention. 
     FIG. 2 shows a memory cell used in a memory system of FIG. 1. 
     FIG. 3 shows a dummy cell used in a memory system of FIG. 1. 
     FIG. 4 shows a time diagram which serves to illustrate the operation of the system of FIG. 1. 
     FIG. 5 shows a latch circuit of another embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1 is shown a memory system which uses N channel field effect transistors, wherein a memory cell MC is located at each of the intersections of a plurality of data lines DL and DL and a plurality of word lines W, and a dummy cell DC is located at each of the intersections of the data lines DL and DL and dummy word lines DW. Circuits for enabling the word lines W and dummy word lines DW are not shown for the sake of simplicity. 
     Each memory cell MC comprises, as shown in FIG. 2, a field effect transistor Q 1  and a capacitance C S  which are series-connected between a corresponding data line DL and a power supply voltage V DD  of, for example, 5 volts. The gate electrode of the transistor Q 1  is connected to a corresponding word line W. 
     Each dummy cell DC comprises, as shown in FIG. 3, a field effect transistor Q 2  and a capacitance C DD  which are series-connected between a corresponding data line DL and the power supply voltage V DD , as well as a field effect transistor Q 3  which is parallel-connected to the capacitance C DD . The gate electrodes of the transistors Q 2  and Q 3  are connected to a corresponding dummy word line DW and a precharge line POL, respectively. The source electrode of the transistor Q 3  is connected to the ground level. 
     Referring to FIG. 1 again it can be seen that a plurality of precharging field-effect transistors Q P0  are provided. The source and drain of each of precharging field effect transistors Q P0  are respectively connected to a corresponding data line DL or DL and a common supply line SL to which the supply voltage V DD  is provided from a pulse circuit PC 1 . A precharge signal CE 0  is provided to each of the gate electrodes of the precharging transistors Q P0  by way of a common precharge line POL. If the high level of the precharge signal CE 0   is greater than (V DD  +V T ), for example, equal if it is to (V DD  +2 V T ) as shown in FIG. 4, the data lines are precharged to the level V DD . FIG. 1 also shows that a plurality of preamplifiers PA are also provided in the memory system. 
     Each of these preamplifiers PA comprises a flip-flop comprising a pair of cross-coupled field effect transistors Q 1  and Q 1 . The drains of these transistors Q 1  and Q 1  are respectively connected to data lines DL and DL, and the gates of the transistors Q 1  and Q 1  are respectively connected to the drains of the transistors Q 1  and Q 1 . The sources of the transistors Q 1  and Q 1  are mutually coupled at connecting points 70 which are termed latch nodes. These latch nodes 70 are connected to a preamplifier line PAL to allow common connection of the amplifiers PA to this preamplifier line PAL. 
     The preamplifier line PAL is connected to a source of a field effect transistor Q P1  which has its drain connected the power supply voltage V DD . The transistor Q P1  precharges the line PAL, when a precharge signal CE 1  provided to the gate thereof by way of a line PIL from a pulse circuit PC 3  is high. The signal CE 1  is designed so that the difference between the precharge level of the line PAL and that of the data lines DL and DL is less than the threshold voltage V T  of the transistors Q 1  and Q 1 . For example, the high level of the signal CE 1  is set, as shown in FIG. 4, to be equal to (V DD  +2 V T ), as is the case with the signal CE 0 . The precharge level of the line PAL is V DD  volts, and the transistors Q 1  and Q 1  are non-conductive. 
     The line PAL is connected to a ground voltage line GL by way of a discharge field effect transistor Q D1 . The gate of the transistor Q D1  is provided with a voltage pulse VP provided by a pulse circuit PC 2  by way of a discharge control line DCL. During the precharge period, the level of the voltage pulse VP is low, as shown in FIG. 4, and, therefore, the transistor Q D1  is non-conductive. 
     When a memory cell MC connected to a selected one of the data lines DL is to be read out, the word line W connected to the memory ell and the dummy word line DW connected to the data lines DL are selectively enabled. When a memory cell MC connected to a selected one of the data lines DL is to be read out, the word line W connected to the memory cell MC and the dummy word line DW connected to the data lines DL are selectively enabled. The voltages of the selected word line W and dummy word line DW becomes high, as shown in FIG. 4. In either of the two cases, a memory cell MC and a dummy cell DC are read out simultaneously. 
     A memory cell MC, when it is read out, lowers the voltage of a corresponding data line to one of two possible voltages which depend on the stored signals of the memory cell. On the other hand, a dummy cell DC, when it is read out, lowers the voltage of a corresponding data line to a level midway between the two possible voltages. 
     In order to initiate the operation of the preamplifiers PA, it is necessary to make the difference between the voltage of the line PAL and that of the data lines DL and DL greater than the threshold voltage V T  of transistors Q 1  and Q 1 . 
     The level of the voltage pulse VP is raised after a word line W and a dummy word line DW are selected, as shown in FIG. 4. Until the voltage pulse VP becomes much higher than the threshold voltage V T , the transistor Q D1  does not become conductive because of the reason explained below. 
     A latch circuit LCH effectively connects a low resistance (R L ) and a high resistance (R H ) selectively between the gate of the transistor Q D1  and the ground voltage line GL. The latch circuit LCH comprises mutually cross-coupled transistors Q S  and Q D2 . The drain of the transistor Q S  is connected to the discharge control line DCL at a point 60 near the gate of the transistor Q D1 . The sources of the transistors Q S  and Q D2  are connected to the ground voltage line GL at a point 50 which is near the point 40 where the source of the transistor Q D1  is connected. The gate of the transistor Q D2  is connected to the drain of the transistor Q S . The gate of the transistor Q S  and the drain of the transistor Q D2  are both connected to the source of the transistor Q P2  at a point 61. The drain of the transistor Q P2  is provided with the supply voltage V DD  and the gate thereof is provided with a precharge signal CE 2  by a pulse circuit PC 4 . The precharge signal CE 2  is high during the precharge period, as with the precharge signals CE 0  and CE 1 , and precharges the point 61 to a voltage of (V DD  -V T ). Therefore, during the precharge period, the gate of the transistor Q S  is at a high level, and the transistor Q S  is conductive and has a low resistance value R L  of about 1 KΩ. As a result, the discharge control line DCL and the ground voltage line GL is shunted by the low resistance. 
     A resistance R 6  interposed into the line DCL represents an equivalent lump resistance of the distributed resistance of the line DCL, and is nearly equal to 1 KΩ. Therefore, during the precharge period, the gate of the transistor Q D1  is provided with a voltage of about 1/2 V L , where V L  is the low level voltage of the pulse VP. As a result, the transistor Q D1  does not become conductive when the low level V L  of the pulse VP is less than 1 volt. 
     When the voltage pulse VP begins to raise to the high level of V DD  volts, the voltage of the point 60 raises in proportion to the voltage of the pulse VP. When the voltage of the point 60 becomes higher than one volt, the transistor Q D1  becomes conductive. At the same time, the transistor Q D2  also becomes conductive. As a result, the voltage of the point 61 decreases rapidly to the ground level, and, therefore, the transistor Q S  becomes non-conductive. The transistor Q S  then has a high resistance R H . Therefore, the latch circuit LCH is effectively decoupled from the gate of the transistor Q D1 . Therefore, the voltage of the gate of the transistor Q D1  becomes rapidly equal to the voltage of the pulse VP. When pulse VP raises further, the voltage of the point 60 raises also. 
     When the preamplifier line PAL is discharged, the preamplifiers PA amplify the voltage differences between two corresponding data lines. After amplification is finished, all control signals are returned to their levels during the precharge period. 
     As explained above, the transistor Q D1  is effectively non-conductive until the voltage level of the pulse VP is greater than 1 volt. This means that the threshold voltage of the transistor Q D1  is effectively raised by virtue of the latch circuit. 
     In order to assure the above-mentioned operation, the two levels V L  and V H  of the pulse VP and threshold voltage V T  are set so that they satisfy the following inequality: ##EQU1## 
     The circuits PC 1  to PC 4  are connected to the ground voltage line GL at the points 10, 20, 30 and 40, respectively. These circuits provide voltage pulses the levels of which are dependent on the voltage of the ground voltage line GL at the points where they are respectively connected. The voltage of the ground voltage line GL is not uniquely determined because of distributed resistance of the line and because of current flow through the line. The resistances R 1  to R 5  represent lumped resistances equivalent to the distributed resistance of the line GL. 
     Although to the line GL is also connected to several other circuits which generate control signals for read out from and write-in to the memory cells, these circuits are not shown for simplicity. If some of the circuits connected to the line GL operate, current flows through the line GL and introduces voltage drops along the line. Therefore, for example, the low level V L  of the voltage pulse VP varies depending on how many of these other circuits are operating, and generally is not equal to zero volts. 
     According to the above embodiment, however, the discharging transistor Q D1  does not conduct if the low level V L  of the pulse VP is greater than 0.5 volts, which is equal to the threshold voltage V T  of the transistor, but is less than 1 volt. Therefore, according to the above embodiment, discharging by the transistor Q D1  is stably controlled in spite of the voltage drift of the ground voltage line GL. 
     In the above embodiment, the pulse circuits PC 1  to PC 4  are responsive to corresponding clock signals provided by a circuit which is not shown in FIG. 1 for simplicity. The circuits PC 1  to PC 4  can be easily constructed by those skilled in the art by referring to the following literatures. 
     (1) IEEE Journal of Solid State Circuit, vol. SC-8, No. 5, pp. 292-331, 1973. 10; 
     (2) William N. Carr et al., &#34;MOS/LSI Design &amp; Application&#34;, published by McGraw Hill; and 
     (3) Robert H. Crawford, &#34;MOS-FET in Circuit Design&#34;, published by McGraw Hill. 
     Another embodiment of the present invention can be developed by modifying the latch circuit LCH of FIG. 1 in the manner shown by the latch circuit LCH&#39; of FIG. 5. Elements, in FIG. 5 with the same reference numerals or letters as those in FIG. 1 correspond to those elements in FIG. 1. The gate of field effect transistor Q S  &#39; is connected to the line PAL by way of a line 200, and is simultaneously precharged to the voltage V DD  when the line PAL is precharged by the transistor Q P1  (FIG. 1). During the precharge period, the transistor Q S  &#39; is conductive and the transistor Q D1  is shunted by a low on-state resistance R L  of the transistor Q S  &#39;. When the voltage pulse level raises and the voltage level of the line DCL at the point 60 becomes greater than the threshold voltage of the transistor Q D1 , the transistor Q D1  becomes conductive and the line PAL begins to discharge. At that time, the voltage level of the line PAL begins to fall to the ground level. According to the fall of the voltage of the line PAL, the resistance of the transistor Q S  &#39; becomes larger and the voltage at the point 60 becomes larger. Finally, the transistor Q D1  becomes fully conductive and the transistor Q S  &#39; becomes fully non-conductive. Thus, latch circuit LCH&#39; is effectively decoupled from the line DCL. 
     From the description above, it can be understood that the discharge control line DCL and the latch circuit LCH or LCH&#39; forms a signal transformation circuit which transforms the pulse signal provided by the pulse circuit PC 2  into another pulse signal which has levels lower and higher than the threshold voltage of the discharge transistor Q D1  when the pulse provided by the circuit PC 2  has lower and higher levels, respectively. 
     The present invention is not limited to the above embodiments, but includes various kinds of embodiments which those skilled in the art can make within the scope of the claims defined below. For example, the present invention can be applied to a memory system which uses P channel field effect transistors.