Abstract:
A sense amplifier connected to first and second bit lines, comprising means for precharging said bit lines to a high voltage, means for connecting one or the other of the bit lines to a memory cell, said connection causing according to the state of the memory cell a maintaining of the bit line at the high voltage or a voltage reduction, first and second transistors respectively controlled by the first and second bit lines, and, in series with the first and second transistors, a controllable means for the current through the transistor controlled by the bit line connected to the memory cell to be greater than the current through the other transistor when the voltages of the two bit lines are at the high voltage.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to the reading from memory circuits, and in particular to sense amplifiers.  
           [0003]    2. Discussion of the Related Art  
           [0004]    In a memory circuit array such as DRAM, each memory cell comprises a capacitor that can be electrically isolated by a switch. The capacitor of each cell is at a high supply voltage Vdd or at a low voltage, for example zero, according to whether the memory cell stores a “1” or a “0”. To read the information stored in a memory cell, the cell capacitor is connected to a bit line connected to a memory cell column, and a sense amplifier is used to detect the bit line voltage.  
           [0005]    [0005]FIG. 1 schematically shows a sense amplifier connected to a bit line BL likely to be connected by a switch SW to a memory cell M symbolized by a capacitor C. A single memory cell M is shown although, in practice, several memory cells are likely to be connected to bit line BL. The sense amplifier comprises an N-channel transistor T 0  having its gate connected to bit line BL and having its drain connected to a reference bit line BLref having the same electric characteristics as bit line BL. An N-channel transistor T 1  has its gate connected to line BLref and its drain connected to line BL. A P-channel transistor T 2  has its gate connected to the drain of transistor T 1 , its drain connected to the drain of transistor T 0 , and its source connected to a supply voltage Vdd. A P-channel transistor T 3  has its gate connected to the drain of transistor T 0 , its drain connected to the drain of transistor T 1 , and its source connected to voltage Vdd. The sources of transistors T 0  and T 1  are connected to the drain of an N-type transistor T 4 . The source of transistor T 4  is grounded and its gate receives a signal Sense for activating the sense amplifier. Precharge blocks Pr, activable by signals not shown, are connected to lines BL and BLref.  
           [0006]    Lines BL and BLref are conventionally precharged to a reference voltage by blocks Pr before reading of the information stored in the memory cell. The particularly simple case where the reference voltage is the high circuit supply voltage (Vdd) is here considered. Once the precharge is over, lines BL and BLref are isolated. To read the content of a cell, switch SW is turned on. If cell M stores voltage Vdd (state “1”), the voltage of line BL is not modified. However, if cell M stores the zero voltage (state “0”), line BL discharges into capacitor C to reach an equilibrium voltage Vdd-δV ranging between Vdd and 0V. A predetermined time period after the closing of switch SW, signal Sense is activated to turn on transistor T 4 .  
           [0007]    If bit line BL is at voltage Vdd-δV when transistor T 4  is on, transistor T 0  is controlled by a voltage Vdd- 6 V and transistor T 1  is controlled by voltage Vdd. Bit line BL then discharges to ground through transistor T 1  faster than bit line BLref discharges to ground through transistor T 0 . The voltage of line BL decreases faster than the voltage of line BLref, which turns on transistor T 2  before transistor T 3 . This forces lines BLref to voltage Vdd, forces transistors T 1  and T 3  respectively to the on and off states, and forces line BL to ground. The state of line BL can then be read by a digital means not shown, and the reading of state “0” from cell M is ended. The bit lines may again be precharged to read the information stored in another memory cell, not shown.  
           [0008]    If bit line BL is at voltage Vdd when transistor T 4  is on (the memory point stores a 1), transistors T 0  and T 1  are controlled by the same voltage Vdd. The dimensions of transistors T 0  and T 1  must be different for transistor T 0  to conduct a greater current than the current flowing through transistor T 1 . The voltage of line BLref thus decreases faster than the voltage of line BL, which turns on transistor T 3  before transistor T 2 . The turning-on of transistor T 3  forces line BL to voltage Vdd, which provides a digital value “1” to a read means not shown, and forces transistors T 0  and T 2  respectively to the on and off states. The operation of reading state “1” from cell M is then over. It should be noted that if transistors T 0  and T 1  were identical, one or the other of lines BL or BLref would switch to ground at the end of the reading of a “1”, in undetermined fashion. The reading of a “1” could then not be surely differentiated from the reading of a “0”.  
           [0009]    The use of the reference bit line, to which usable memory cells cannot be connected, reduces the integration density of a memory circuit comprising sense amplifiers such as in FIG. 1.  
           [0010]    Structures with symmetrical sense amplifiers connected to two functional bit lines enabling indifferent reading from a memory cell connected to one or the other of the bit lines are known. Such amplifiers enable reading from twice as many memory cells as the amplifier of FIG. 1, but they only operate if the bit lines are precharged to an intermediary voltage (for example, Vdd/2) between supply voltage Vdd and the ground. The generation of voltage Vdd/2 poses many problems, especially consumption and stability problems.  
         SUMMARY OF THE INVENTION  
         [0011]    An object of the present invention is to provide a sense amplifier connected to two functional bit lines without requiring use of an intermediary precharge voltage.  
           [0012]    To achieve this object, as well as others, the present invention provides a sense amplifier connected to first and second bit lines, comprising means for precharging said bit lines to a high voltage, means for connecting one or the other of the bit lines to a memory cell, said connection causing according to the state of the memory cell a maintaining of the bit line at the high voltage or a voltage reduction, and first and second transistors respectively controlled by the first and second bit lines, -and further comprising in series with the first and second transistors a controllable means for the current flowing through the transistor controlled by the bit line connected to the memory cell to be greater than the current flowing through the other transistor when the voltages of the two bit lines are at the high voltage.  
           [0013]    According to an embodiment of the present invention, the first and second transistors are identical MOS transistors of a first conductivity type and the controllable means comprises third and fourth transistors having their drain terminals connected to the source terminal of the first transistor, and fifth and sixth transistors having their drain terminals connected to the source terminal of the second transistor, the third and fifth transistors being of same dimensions and receiving on their gate terminal a signal for activating the sense amplifier, the fourth and sixth transistors being of same dimensions and respectively receiving on their gate terminal control signals for the reading from the first and second bit lines.  
           [0014]    The present invention also aims at a memory circuit comprising a plurality of memory cells connectable to a plurality of such sense amplifiers.  
           [0015]    According to an embodiment of the present invention, the first and second transistors are identical MOS transistors of a first conductivity type and the controllable means comprises seventh and eighth transistors having their drain terminals connected to the source terminal of the first transistor, and ninth and tenth transistors having their drain terminals connected to the source terminal of the second transistor, the eighth and tenth transistors being of same dimensions, the seventh and ninth transistors being of same dimensions and selected to be, when on, more conductive than the eighth and tenth transistors, the seventh and tenth transistors receiving on their gate terminal a control signal for reading from the first bit line and the eighth and ninth transistors receiving on their gate terminal a control signal for reading from the second bit line.  
           [0016]    According to an embodiment of the present invention, the sense amplifier further comprises an eleventh transistor arranged between the source terminal of the first transistor and the drain terminals of the seventh and eighth transistors, and a twelfth transistor arranged between the source terminal of the second transistor and the drain terminals of the ninth and tenth transistors, the eleventh and twelfth transistors receiving on their gate terminal a signal for activating the sense amplifier.  
           [0017]    The present invention also aims at a memory circuit comprising a plurality of memory cells connectable to a plurality of such sense amplifiers, in which the first and second transistors of each sense amplifier are connected to single seventh, eighth, ninth, and tenth transistors.  
           [0018]    According to an embodiment of the present invention, the drain terminals of the first and second transistors are respectively connected to the second and first lines and the gate terminals of the first and second transistors are respectively connected to the first and second bit lines, and the amplifier comprises thirteenth and fourteenth MOS transistors of a second conductivity type having their source terminals connected to a supply voltage, having their drain terminals respectively connected to the drain terminals of the first and second transistors and having their gate terminals respectively connected to the drains of the second and first transistors.  
           [0019]    The foregoing object, features, and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    [0020]FIG. 1, previously described, shows a conventional sense amplifier;  
         [0021]    [0021]FIG. 2 shows a first embodiment of a sense amplifier according to the present invention; and  
         [0022]    [0022]FIG. 3 shows a second embodiment of a sense amplifier according to the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0023]    Only those elements necessary to the understanding of the present invention have been shown.  
         [0024]    [0024]FIG. 2 schematically shows a sense amplifier according to an embodiment of the present invention, connected to two bit lines of same dimensions, BL 0  and BL 1 , each of which is likely to be connected by a switch SW 0 , SW 1  to the capacitor C of a memory cell, M 0 , M 1 , from among the memory cells, not shown, connected to bit lines BL 0 , BL 1 . A precharge block Pr is connected to each bit line. An N-channel transistor T 10  has its gate connected to line BL 0  and its drain connected to line BL 1 . An N-channel transistor T 11  has its gate connected to line BL 1  and its drain to line BL 0 . A P-channel transistor T 12  has its gate connected to the drain of transistor T 11 , its drain connected to the drain of transistor T 10 , and its source connected to a supply voltage Vdd. A P-channel transistor T 13  has its gate connected to the drain of transistor T 10 , its drain connected to the drain of transistor T 11 , and its source connected to voltage Vdd. Two N-channel transistors T 140 , T 150  have their drain connected to the source of transistor T 10  and their source connected to ground, and two N-channel transistors T 141 , T 151  have their drain connected to the source of transistor T 11  and their source connected to ground. Transistors T 140 , T 141 , of same dimensions, receive on their gate a signal Sense such as in FIG. 1. According to the present invention, transistors T 150 , T 151 , of same dimensions, respectively receive on their gates signals S 0 , S 1  respectively activated when line BL 0  or BL 1  is desired to be read from.  
         [0025]    The reading of information stored in a memory cell is preceded with a precharge of lines BL 0  and BL 1  to voltage Vdd by blocks Pr. Switch SW 0  or SW 1  is on according to whether the information stored in memory cell M 0  or that in cell M 1  is desired to be read. It is considered hereafter as an example that memory cell M 0  is read from. The voltage of line BL 0  remains at Vdd or falls to a voltage Vdd-δV according to whether cell M 0  is in a state “1” or “0” when switch SW 0  is on. Signals Sense and S 0  are then activated to turn on transistors T 140 , T 141  and T 150 . Upon reading from line BL 0 , no memory point is activated on line BL 1  which thus remains at the precharge voltage (Vdd in this example).  
         [0026]    In the case where bit line BL 0  is at voltage Vdd-δV when transistors T 140 , T 141 , and T 150  are on, transistor T 10  is controlled by a voltage Vdd-δV smaller than voltage Vdd controlling transistor T 11 . The voltage of line BL 0  decreases faster than the voltage of line BL 1 , which turns on transistor T 12  before transistor T 13  and forces line BL 1  to voltage Vdd and line BL 0  to ground.  
         [0027]    In the case where bit line BL 0  is at voltage Vdd when transistors T 140 , T 141 , and T 150  are on, transistors T 10  and T 11  are controlled by the same voltage Vdd. According to the present invention, however, the current flowing through transistor T 10  in series with transistors T 140  and T 150  is stronger than the current flowing through transistor T 11  in series with the sole transistor T 141 . The voltage of line BL 1  thus decreases faster than the voltage of line BL 0 , which turns on transistor T 13  before transistor T 12  and forces line BL 0  to voltage Vdd.  
         [0028]    The sense amplifier according to the present invention operates symmetrically for the reading from memory cell M 1 . The activation of the sense amplifier will then be performed by signals Sense and S 1 , and not Sense and S 0 , to turn on transistors T 140 , T 141 , and T 151 . The respective dimensions of transistors T 140 , T 141 , and T 150 , T 151  are selected according to the characteristics of the sense amplifier. In practice, transistors T 140 , T 141 , T 150 , and T 151  may be of same dimensions.  
         [0029]    A sense amplifier according to the present invention thus enables reading from a memory cell indifferently connected to one or the other of two bit lines precharged to the supply voltage, and enables reading from twice as many memory cells as the amplifier of FIG. 1 without having to use an intermediary precharge voltage.  
         [0030]    [0030]FIG. 3 schematically shows a sense amplifier (SA) according to another embodiment of the present invention. Same references represent same elements in FIGS. 2 and 3. The sense amplifier is connected to two bit lines BL 0 , BL 1 , each of which is likely to be connected by a switch SW 0 , SW 1  to the capacitor C of a memory cell M 0 , M 1 . Precharge blocks Pr are connected to lines BL 0  and BL 1 . The structure of the sense amplifier is the same as in FIG. 2 as concerns transistors T 10 , T 11 , T 12 , and T 13 . The gates and drains of transistors T 140  and T 141  are connected as in FIG. 2.  
         [0031]    Two N-channel transistors T 250 , T 260  have their drain connected to the source of transistor T 140  and their source connected to ground, and two N-channel transistors T 251 , T 261  have their drain connected to the source of transistor T 141  and their source connected to ground. Transistors T 260 , T 261  are equal. Transistors T 250  and T 251  are equal and selected to be, when on, more conductive than on transistors T 260 , T 261 . As an example, transistors T 250  and T 251  may have a gate of same length and of twice as small a width as transistors T 260  and T 261 . Transistors T 251 , T 260  are controlled by signal S 1  and transistors T 250 , T 261  are controlled by signal S 0 .  
         [0032]    Advantageously, transistors T 250 , T 251 , T 260 , T 261  may be connected to the sources of transistors T 140 , T 141  of several sense amplifiers, as shown in dotted lines. According to such an embodiment, each sense amplifier can receive a specific signal Sense activable to select the pair of bit lines connected to the amplifier, and signals S 0  and S 1  are activated to control the reading either from the first, or from the second bit lines belonging to the selected bit line pairs. Such an embodiment especially enables using a reduced number of transistors, and thereby taking up a reduced chip surface area.  
         [0033]    The sense amplifier operation is substantially the same as in FIG. 2. Considering a reading from memory cell M 0 , the voltage of line BL 0  is, after turning-on of switch SW 0 , at Vdd or at Vdd-δV according to whether cell M 0  was in a state “1” or “0”. Signals Sense-and S 0  are then activated to turn on transistors T 140 , T 141 , T 250 , and T 261  (preferably, S 0  will be activated before the signal Sense which will start the reading, then will remain unchanged until the next precharge).  
         [0034]    In the case where bit line BL 0  is at voltage Vdd-δV when transistors T 140 , T 141 , T 250 , and T 261  are on, the sense amplifier forces line BL 1  to voltage Vdd and line BL 0  to ground in the same way as the sense amplifier of FIG. 2.  
         [0035]    In the case where bit line BL is at voltage Vdd when transistors T 140 , T 141 , T 250 , and T 261  are on, the current flowing through transistors T 11 , T 141 , and T 261  is smaller than the current flowing through transistors T 10 , T 140 , and T 250 , due to the selection of transistors T 261  and T 250 , which turns on transistor T 13  before transistor T 12  and forces line BL 0  to voltage Vdd.  
         [0036]    The sense amplifier operation is symmetrical for the reading from memory cell M 1 .  
         [0037]    Of course, the present invention is likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. In particular, transistors T 140  and T 141  of the sense amplifier of FIG. 3 may be suppressed. Such an alternative, taking up a small surface area, is appropriate for a memory circuit in which a memory cell in all the first or all the second bit lines connected to the sense amplifiers connected to the same transistors T 250 , T 251 , T 260 , T 261  is simultaneously desired to be read from.  
         [0038]    Also according to an alternative, each of transistors T 250 , T 251  may be replaced with two transistors having the dimensions of transistors T 260 , T 261 . Thus, the imbalance will be created by the turning on on one side of two identical transistors and on the other side of a single transistor identical to the other two.  
         [0039]    The present invention has been described in relation with a specific sense amplifier structure, but it will easily apply to other structures. Especially, the drains of transistors T 10  and T 11  may be connected not to the bit lines but to digital read means.  
         [0040]    Further, a specific DRAM cell structure has been described, but the present invention is likely to be adapted to the reading from any memory cell, of DRAM type or of another type. In particular, the present invention may be used to read ROM- or SRAM-type memory cells.  
         [0041]    The present invention has been described in relation with a positive supply voltage Vdd and MOS transistors having specific conductivity types, but those skilled in the art will readily adapt the present invention to a negative supply voltage, using transistors of appropriate conductivity types.  
         [0042]    Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto.