Abstract:
Reference level generating circuit in a memory device includes a first amplifier and a second amplifier each for comparing and amplifying a reference bitline level and a fedback preliminary reference level. A reference level adjuster receives signals from the first and second amplifiers, adjusts the signals to desired reference levels, and feeds the signals back to the first and second amplifiers. A reference level stabilizer stabilizes a reference level from the reference level adjuster, and a pull-down circuit drops an output from the reference level stabilizer by a required level in bitline precharging. An operational controller controls operation of the first and second amplifiers, the reference level adjuster, the reference level stabilizer, and the pull-down circuit. The reference level generating circuit and devices using such circuit improves data sensing rate and device reliability.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a memory device, and more particularly, to a reference level generator in a memory device. 
     2. Background of the Related Art 
     A ferroelectric memory, i.e., an FRAM (Ferroelectric Random Access Memory), has in general a data processing speed similar to a DRAM dynamic Random Access Memory) which is used widely as a semiconductor memory. Because the FRAM can conserve data even if the power is turned off, much attention has been given to FRAM as a next generation memory. The FRAM, a memory having a structure similar to the DRAM, is provided with a capacitor of a ferroelectric material for utilizing a high residual polarization of the ferroelectric material. The residual polarization permits the conservation of a data even after removal of an electric field. 
     FIG. 1 illustrates a characteristic curve of a hysteresis loop of a general ferroelectric material. A polarization induced by an electric field is not erased, but a certain amount (‘d’ and ‘a’ states) remains, even if the electric field is removed owing to existence of the residual polarization (or spontaneous polarization). The ‘d’ and ‘a’ states correspond to ‘1’ and ‘0’, respectively, in application to memories. 
     FIG. 2 illustrates a system of unit cell of the related art non-volatile ferroelectric memory. The system of a unit cell of the related art non-volatile ferroelectric memory is provided with a bitline BL formed in one direction, a wordline W/L formed in a direction perpendicular to the bitline, a plateline P/L formed spaced from the wordline in a direction identical to the wordline, a transistor T 1  having a gate connected to the wordline W/L and a source connected to the bitline BL, and a ferroelectric capacitor FC 1  having a first terminal connected to a drain of the transistor T 1  and a second terminal connected to the plateline P/L. 
     FIG. 3A illustrates a timing diagram of a write mode operation of the related art ferroelectric memory. In a writing mode, when an external chip enable signal CSBpad changes from ‘high’ to ‘low’ and a write enable signal WEBpad changes from ‘high’ to ‘low’ simultaneously, the write mode is enabled or initiated. When an address decoding is started in the write mode, a pulse applied to a relevant wordline is transited from ‘low’ to ‘high’ to select a cell. Thus, during a period the wordline is held ‘high’, a relevant plateline has a ‘high’ signal applied thereto for one period and a ‘low’ signal applied thereto for the other period in succession. 
     In order to write a logical value ‘1’ or ‘0’ on the selected cell, a ‘high’ or ‘low’ signal synchronized to the write enable signal WEBpad is applied to a relevant bitline. If a ‘high’ signal is applied to the bitline and a signal applied to the plateline is ‘low’ in a period in which a signal applied to the wordline is ‘high’, a logical value ‘1’ is written on the ferroelectric capacitor. If a ‘low’ signal is applied to the bitline and a signal applied to the plateline is ‘high’, a logical value ‘0’ is written on the ferroelectric capacitor. 
     The operation for reading the data stored in the cell by the aforementioned write mode operation will be explained. 
     FIG. 3B illustrates a timing diagram of a read mode operation of the related art ferroelectric memory. If the chip enable signal CSBpad changes from ‘high’ to ‘low’ from outside of the chip, all bitlines are equalized to a ‘low’ voltage before a relevant wordline is selected. After the bitlines are disabled, an address is decoded, and the decoded address causes a ‘low’ signal on a relevant wordline to transit to a ‘high’ signal, to select a relevant cell. A ‘high.’ signal is applied to the plateline of the selected cell, to break a data corresponding to a logical value ‘1’ stored in the ferroelectric memory. If a logical value ‘0’ is in storage in the ferroelectric memory, a data corresponding to the logical value ‘0’ is not broken. As the data not broken and the data broken provide values different from each other according to the aforementioned hysteresis loop, the sense amplifier can sense a logical value ‘1’ or ‘0’. 
     The case of the data broken is a case when the value is changed from ‘d’ to ‘f’ in the hysteresis loop of FIG. 1, and the case of the data not broken is a case when the value is changed from ‘a’ to ‘f’ in the hysteresis loop of FIG.  1 . Therefore, if the sense amplifier is enabled after a certain time period has passed, in the case of the data broken, a logical value ‘1’ is provided as amplified, and in the case of the data not broken, a logical value ‘0’ is provided. After the sense amplifier provides the data and since an original data should be restored, the plateline is disabled from ‘high’ to ‘low’ in a state a ‘high’ signal is applied to a relevant wordline. 
     FIG. 4 illustrates a block diagram of a related art nonvolatile ferroelectric memory. The related art nonvolatile ferroelectric memory is provided with a main cell array  41  having a lower portion allocated for a reference cell array  42 , a wordline driver  43  on one side of the main cell array  41  for providing a driving signal to the main cell array  41  and the reference cell array  42 , and a sense amplifier unit  44  formed under the main cell array  41 . The wordline driver  43  provides a driving signal to the main wordline for the main cell array  41  and the reference wordline for the reference cell array  42 . The sense amplifier unit  44  has a plurality of sense amplifiers each for amplifying bitlines and bitbarlines. 
     The operation of the aforementioned nonvolatile ferroelectric memory will be explained with reference to FIG. 5, which illustrates a partial detail of FIG. 4, wherein the main cell array has a folded bitline structure. The reference cell array  42  also has a folded bitline structure, and two pairs of a reference cell wordline and a reference cell plateline are provided, which are defined as RWL_ 1 , RPL_ 1  and RWL_ 2 , RPL_ 2  . . . RWL_N−1, PRL_N−1, and RWL_N, RPL_N, respectively. 
     Provided that the main cell wordline MWL_N−1 and the main cell plateline MPL_N−1 is enabled, the reference cell wordline RWL_N−1 and the reference cell plateline RPL_N−1 are enabled. Therefore, a data from the main cell is loaded on the bitline BL, and a data from the reference cell is loaded on the bitbarline /BL. When the main cell wordline MWL_N and the main cell plateline MPL N are enabled, the reference cell wordline RWL_N and the reference cell plateline RPL_N are also enabled. Therefore, a data from the main cell is loaded on the bitbarline /BL, and a data from the reference cell is loaded on the bitline BL. In this instance, a bitline level REF caused by the reference cell is between bitline levels B_H(High) and B_L (Low) caused by the main cell. 
     In order to position the reference voltage REF between the bitline levels B_H and B_L, two reference cell operation methods can be used. The first method stores logic “1” in the capacitor of the reference cell, which can be achieved by providing a capacitor of a reference cell of which size is smaller than a capacitor size of the main cell. The second method stores a logic “0” in the capacitor of the reference cell, which can achieved by providing a capacitor of a reference cell of which size is larger than a capacitor size of the main cell. Thus, the related art nonvolatile ferroelectric memory can produce a reference voltage required by the sense amplifier unit  44  by using the foregoing two methods. 
     However, the aforementioned related art nonvolatile ferroelectric memory has various problems. For example, when a capacitor size of the reference cell is made smaller than a capacitor size of the main cell as the first method for providing a level of the reference voltage to be between the bitline levels B_H and B_L, when the reference cell capacitor is excessively switched, i.e., destructed, in comparison to the main cell, the reference cell experiences fatigue before the main cell, which can lead to an unstable reference voltage. Further, when a capacitor size of the reference cell is made larger than a capacitor size of the main cell as the second method for providing a level of the reference voltage to be between the bitline levels B_H and B_L, although fatigue may not be a problem, the capacitor size increases, leading to an increase in size of the FRAM. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to solve at least the above problems and/or disadvantages and to provide at least the advantages described hereinafter. 
     An object of the present invention is to provide a stable reference voltage. 
     Another object of the present invention is tor provide an improved data sensing reliability. 
     An object of the present invention and other advantages can be achieved in a whole or in parts by a reference level generating circuit for a memory device comprising: a first amplifier and a second amplifier, each for comparing and amplifying a reference bitline level and a fedback preliminary reference level; a reference level adjuster for receiving signals from the first and second amplifiers, adjusting the signals to desired reference levels, and feeding the signals back to the first and second amplifiers; and a reference level stabilizer for stabilizing a reference level from the reference level adjuster. 
     An object of the present invention and other advantages can be achieved in a whole or in parts by a reference level generating circuit in a nonvolatile ferroelectric memory includes a first amplifier and a second amplifier each for comparing and amplifying a reference bitline level and a fedback preliminary reference level, a reference level adjuster for receiving signals from the first and second amplifiers, adjusting the signals to desired reference levels, and feeding the signals back to the first and second amplifiers, a reference level stabilizer for stabilizing a reference level from the reference level adjuster, a pull-down circuit for dropping an output from the reference level stabilizer by a required level in bitline precharging, and an operation controller for controlling operation of the first and second amplifiers, the reference level adjuster, the reference level stabilizer, and the pull-down circuit. 
     An object of the present invention and other advantages can be achieved in a whole or in parts by a reference level generating circuit in a nonvolatile ferroelectric memory comprising: an operation controller including a first PMOS transistor for switching a power source voltage in response to a first control signal; a first amplifier including a second PMOS transistor having a source connected to an output terminal on the first PMOS transistor, and a gate and a drain connected in common, a third PMOS transistor connected in parallel with the second PMOS transistor with respect to an output terminal on the first PMOS transistor, a first NMOS transistor having a gate connected to a reference bitline and a source connected to the second PMOS transistor, a second NMOS transistor formed between a drain of the first NMOS transistor and a grounding terminal and controlled by a drain voltage of the second PMOS transistor, and a third NMOS transistor connected between the third PMOS transistor and the second NMOS transistor, for comparing and amplifying a signal from the reference bitline and a fedback signal; a second amplifier including a fourth PMOS transistor having a source connected to the output terminal on the operation controller, a fifth PMOS transistor connected in parallel with the fourth PMOS transistor with respect to the output terminal on the operation controller, a fourth NMOS transistor having a gate connected to the reference bitline and a source connected to a drain of the third PMOS transistor, a fifth NMOS transistor formed between a drain of the fourth NMOS transistor and a grounding terminal, and a sixth NMOS transistor formed between the fifth PMOS transistor and a drain of the fifth NMOS transistor, for comparing and amplifying a signal from the reference bitline and a fedback signal; a reference level adjuster including a seventh NMOS transistor formed between the output terminal on the operation controller and a gate of the third NMOS transistor and controlled by a drain voltage of the third PMOS transistor, an eighth NMOS transistor formed between a drain of the third PMOS transistor and a drain of the seventh NMOS transistor and controlled by a source voltage of the fourth NMOS transistor, a sixth PMOS transistor having a source connected to the output terminal on the operation controller and controlled by the first control signal, and a seventh PMOS transistor formed between the sixth PMOS transistor and a gate of the third NMOS transistor and controlled by a source voltage of the fourth NMOS transistor, for receiving a signal from the first and second amplifiers and adjusting to a desired level; a reference level stabilizer including a ninth NMOS transistor having a source connected to an output terminal on the reference level adjuster and controlled by a source voltage of the fourth NMOS transistor, a tenth NMOS transistor connected to the ninth NMOS transistor in series and controlled by a source voltage of the sixth NMOS transistor, and an eleventh NMOS transistor having a source connected to an output terminal on the reference level adjuster and a drain connected to a drain of the tenth NMOS transistor and controlled by an external second control signal, for stabilizing a reference level from the reference level adjuster; and a pull-down circuit including a twelfth NMOS transistor connected in parallel with a drain of the tenth NMOS transistor and controlled by the first control signal, and a thirteenth NMOS transistor formed between a drain of the twelfth NMOS transistor and a grounding terminal and having a gate and a drain connected in common, for dropping a reference level from the reference level stabilizer down to a threshold voltage level of an NMOS transistor in precharging the bitline. 
     An object of the present invention and other advantages can be achieved in a whole or in parts by a memory device comprising: a memory cell array having a main cell array of plurality of main memory cells and a reference cell array of a plurality of reference memory cells; a wordline driver that provides driving signals to select a corresponding number of main memory cells and reference memory cells; and a sense amplifier unit used for reading data from and writing data to the memory cell array, wherein the reference cell array is formed between the wordline driver and the main cell array. 
     An object of the present invention and other advantages can be achieved in a whole or in parts by a method of generating a reference voltage comprising: comparing and amplifying a reference bitline level and a feedback preliminary reference level in response to a switched voltage; adjusting the amplified bitline level and the feedback preliminary reference level; and stabilizing the feedback preliminary reference level. 
     Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described in detail with reference to the following drawings in which like reference numerals refer to like elements wherein: 
     FIG. 1 illustrates a characteristic curve of a hysteresis loop of a general ferroelectric material; 
     FIG. 2 illustrates a system of unit cell of a related art non-volatile ferroelectric memory; 
     FIG. 3A illustrates a timing diagram of a write mode operation of the related art ferroelectric memory; 
     FIG. 3B illustrates a timing diagram of a read mode operation of the related art ferroelectric memory; 
     FIG. 4 illustrates a block diagram of a related art nonvolatile ferroelectric memory; 
     FIG. 5 illustrates a partial detail of FIG. 4; 
     FIG. 6 illustrates a system of unit cell of a non-volatile ferroelectric memory in accordance with a preferred embodiment of the present invention; 
     FIG. 7 illustrates a simplified circuitry system of a non-volatile ferroelectric memory of the present invention; 
     FIG. 8 illustrates a circuitry system of a non-volatile ferroelectric memory of the present invention provided for the purpose of explanation; 
     FIG. 9 illustrates a system of a non-volatile ferroelectric memory of the present invention; 
     FIG. 10 illustrates a partial detail of FIG. 9; 
     FIG. 11 illustrates a system of a reference level generating circuit in a non-volatile ferroelectric memory of the present invention; and 
     FIG. 12 illustrates noise from a power source voltage vs. output from a reference level generator. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 6 illustrates a system of unit cell of a non-volatile ferroelectric memory in accordance with a preferred embodiment of the present invention. The unit cell of a non-volatile ferroelectric memory in accordance with a preferred embodiment of the present invention includes a first split wordline SWL 1  and a second split wordline SWL 2  spaced a distance from each other formed in a first direction, preferably a row direction, a first bitline BL 1  and a second bitline BL 2  formed in a direction crossing the first and second split wordlines SWL 1  and SWL 2 . 
     A first transistor T 1  has a gate connected to the first split wordline SWL 1  and a drain connected to the first bitline BL 1 , and a first ferroelectric capacitor FC 1  is connected between a source of the first transistor T 1  and a second split wordline SWL 2 . A second transistor T 2  has a gate connected to the second split wordline SWL 2  and a drain connected to the second bitline B 2 , and a second ferroelectric capacitor FC 2  connected between a source of the second transistor T 2  and the first split wordline SWL 1 . A plurality of the unit cells are formed to provide a cell array, wherein a unit cell has two transistors  2 T and two ferroelectric capacitor  2 C connected to one pair of split wordlines and two bitlines in view of structure. 
     FIG. 7 illustrates a simplified circuitry system of a non-volatile ferroelectric memory in accordance with a preferred embodiment of the present invention. The non-volatile ferroelectric memory includes a plurality of split wordline pairs, the pair being a first and a second split wordlines SWL 1  and SWL 2 , formed in a row direction, a plurality of bitlines BL 1 , BLn+1 formed in a direction crossing the split wordline pairs, and a sensing amplifiers SA, each disposed between the bitlines for sensing a data provided through both sides of bitlines and providing to a dataline DL or a databarline /DL. There may be a sensing amplifier enable unit for providing an enable signal SEN to enable the sense amplifiers, and a selection switching unit CS for selectively switching bitlines and datalines. 
     The operation of the nonvolatile ferroelectric memory of the present invention will be explained with reference to the timing diagram shown in FIG.  8 . The time period T 0  is a period before the first split wordline SWL 1  and the second split wordline SWL 2  are enabled to “H(High)”, when all bitlines are precharged to a threshold voltage level of an NMOS transistor. The time period T 1  is a period when both the first and second split wordlines SWL 1  and SWL 2  are transited to “H”, and the data in the ferroelectric capacitor of the main cell is provided to the main bitline, to change a level of the bitline. In this instance, the ferroelectric capacitor having data stored in a logic “High” is involved in breakdown of the ferroelectric polarity due to application of electric fields of opposite polarities to the bitline and the split wordline, resulting in a great current to flow thereto to induce a high voltage to the bitline. Opposite to this, the ferroelectric capacitor having data stored in a logic “Low” is not involved in breakdown of the ferroelectric polarity as electric fields of the same polarity are applied to the bitline and the split wordline, resulting in a small current to flow thereto to induce a slightly lower voltage to the bitline. 
     If cell data are loaded on the bitline sufficiently, a sensing amplifier enable signal SEN is transited to “High” for enabling the sensing amplifier to amplify a level of the bitline. In the meantime, since the brokendown logic “H” data of the cell can not be restored under a state the first split wordline SWL 1  and the second split wordline SWL 2  are in “High”, the data is restored in following time periods T 2  and T 3 . In the time period T 2 , the first split wordline SWL 1  is transited to low, the second split wordline SWL 2  is held in a high state, and the second transistor T 2  is in a turned on state. In this instance, if a relevant bitline is in a high state, a high data is provided to one side electrode of the second ferroelectric capacitor FC 2 , to restore a logic “1” state between a low state of the first split wordline SWL 1  and a high level of the bitline. In the time period T 3 , the first split wordline SWL 1  is transited to high again and the second split wordline SWL 2  is transisted to low, to transit the first transistor T 1  into a turned on state. In this instance, if a relevant bitline is in a high state, a high data will be provided to one side electrode of the first ferroelectric capacitor FC 1 , to restore the logic 1 state between the high levels of the second split wordline SWL 2 . 
     A preferred embodiment of a reference level generating circuit for supplying a reference voltage to the sense amplifier in the aforementioned nonvolatile ferroelectric memory in accordance with will be explained. FIG. 9 illustrates a system of a non-volatile ferroelectric memory in accordance with a preferred embodiment of the present invention for explaining the reference level generating circuit in accordance with a preferred embodiment of the present invention. 
     Referring to FIG. 9, the non-volatile ferroelectric memory of the present invention includes a main cell array  91  having a reference cell array  92  allocated at either side thereof, a split wordline driver  93  for applying a driving signal to the main cell array  91 , and a sensing amplifier unit  94  under (or over) the main cell array  91  having a plurality of sense amplifiers and reference voltage generators each for providing a reference voltage to the sense amplifier. An equalizing and precharging circuit is used for equalizing and precharging adjacent bitlines. 
     FIG. 10 illustrates a partial detail of FIG. 9, showing a reference bitline RBL and a plurality of main bitlines MBL 1 , MBL 2  . . . MBLm, running in a column direction, a plurality of split wordline pairs, one pair being the first split wordline SWL 1  and the second split wordline SWL 2 , running in a direction crossing the reference bitline RBL and the main bitlines MBL 1 , MBL 2  . . . MBLm cells each having a transistor and a ferroelectric capacitor formed between adjacent first and second split wordlines and two adjacent bitlines. An equalizing and precharging circuit  99  equalizes and precharges the bitlines, sensing amplifiers S/A, each connected to the main bitline senses data on the main bitline, and a reference level generating circuit  100  stabilizes a level of the reference voltage on the reference bitline and provides a reference voltage for the sense amplifiers. 
     In the aforementioned nonvolatile ferroelectric memory, when one pair of the split wordlines are enabled, the main cell and the reference cell are enabled. The data in the main cell is transferred to the main bitline, and therefrom to the sense amplifier, and the data in the reference cell is transferred to the reference bitline RBL. However, the data in the reference cell is not transferred to the sense amplifier directly through the reference bitline RBL. Instead, the data in the reference cell on the reference bitline RBL is provided to and amplified in the reference level generating circuit  100 , and provided to the sense amplifier. 
     In such an instance, the data on the reference bitline RBL has a state identical to a logic “0” of the main bitline. In other words, the sizes of the main cell and the reference cell are almost the same, and the reference cell is to store a logic “0”. Accordingly, the reference level generating circuit  100  senses a voltage on the reference bitline RBL and forwards the voltage boosted by ΔV, so that the boosted level is between a high and a low levels of the main bitline caused by the main cell. In, this instance, as the ferroelectric capacitor in the reference cell has no destruction operation exerted thereto, the ferroelectric capacitor is not involved in fatigue, and the level of the reference voltage can be stabilized because the same split wordline signals are received. 
     FIG. 11 illustrates a system of a reference level generating circuit in a non-volatile ferroelectric memory in accordance with a preferred embodiment of the present invention. The reference level generating circuit includes a first amplifier  100   a , a second amplifier  100   b , a reference level adjuster  100   c , a reference level stabilizer  100   d , a pull-down circuit  100   e , and an operational controller  100   f  for controlling the respective blocks. The operational controller  100   f  includes a PMOS transistor PM 1  for selectively switching power source voltage in response to a first control signal LS_EN which is preferably provided externally. 
     The first amplifier  100   a  is a current mirror type differential amplifier. A first transistor PM 2  has a source connected to an output terminal on the operational controller  100   f  and a gate and a drain connected in common, and a second transistor PM 3  is connected in parallel with the first transistor PM 2  with respect to the output terminal on the operation controller  100   f . A third transistor NM 1  has a gate connected to the reference bitline RBL and a source connected to a drain of the first transistor PM 2 , and a fourth transistor NM 2  is provided between a drain of the third transistor NM 1  and a grounding terminal and controlled by a drain voltage of the first transistor. A fifth transistor NM 3  is disposed between the second transistor PM 3  and the fourth transistor NM 2  and controlled by a signal fed back from the reference level controller  100   c . The first and second transistors PM 2  and PM 3  are PMOS transistors, and the third, fourth and fifth transistors NM 1 , NM 2 , and NM 3  transistors are NMOS transistors. 
     The reference level adjuster  100   c , disposed between an output terminal on the operation controller  100   f  and a gate of the fifth transistor NM 3  in the first amplifier  100   a , includes a first transistor NM 7  controlled by a signal from the first amplifier  10   b , and a second transistor NM 8  controlled by a signal from the second amplifier  100   b  for adjusting an output of the first transistor NM 7 . A third transistor PM 6  for switching an output of the operation controller  100   f  in response to an external control signal, and a fourth transistor PM 7  controlled by a signal form the second amplifier  100   b  and formed between an output terminal of the third transistor PM 6  and a gate of the fifth transistor NM 3  in the first amplifier  100   a . The first and second transistors NM 7  and NM 8  are NMOS transistors, and the third, and fourth transistors PM 6  and PM 7  are PMOS transistors. 
     The reference level stabilizer  100   d  controlled by a signal from the second amplifier  100   b  includes a first and a second transistors NM 9  and NM 10  connected to an output terminal on the reference level adjuster in series. A third transistor NM 11  is controlled by an external second control signal LSC and has a source connected to an output terminal of the reference level adjuster  100   c , and a drain connected to an output terminal on the second transistor NM 10 . The first, second, and third transistors NM 9 , NM 10 , and NM 11  are NMOS transistors. 
     The pull-down circuit  100   e  includes a first transistor NM 12  controlled by an external first control signal LS_EN and connected in parallel with an output terminal on the reference level stabilizer  100   d . A second transistor NM 13  is formed between a source of the first transistor and a grounding terminal and having a gate and a drain connected in common. The first and second transistors NM 12  and NM 13  are NMOS transistors. 
     The second amplifier  100   b  includes a first transistor NM 4  having a gate connected to the reference bitline, a drain connected to a gate of the first transistor NM 9  in the reference level stabilizer  100   d . A second transistor PM 4  is disposed between an output terminal on the operation controller  100   f  and a drain of the first transistor NM 4 , and a third transistor PM 5  is connected in parallel with the second transistor PM 4  with respect to the output terminal on the operation controller  100   f . A fourth transistor NM 5  is formed between a source of the first transistor NM 4  and a grounding terminal Vss and having a gate connected to a gate of the third transistor PM 5 . A fifth transistor NM 6  is formed between a drain of the third transistor PM 5  and a drain of the fourth transistor NM 5  and having a gate connected to an output terminal on the reference level adjuster  100   c . The second, third transistors PM 4  and PM 5  are PMOS transistor, and the first, fourth, and fifth transistors NM 4 , NM 5 , and NM 6  are NMOS transistors. 
     The operation of the reference level generating circuit in a nonvolatile ferroelectric memory in accordance with a preferred embodiment of the present invention having the aforementioned system will be explained. The reference level generating circuit of the present invention receives a signal from the reference bitline as a reference input REF_IN. In the control signal for the reference level generating circuit, there are a first control signal and a second control signal, wherein the first control signal is defined as LS_EN and the second control signal is defined as LSC. An output REF_OUT from the reference level generating circuit is used as a reference input to the plurality of sense amplifiers in the sense amplifier unit. 
     The LS_EN signal, the first control signal, either enables or disables the reference level generating circuit. If the LS_EN signal is high, the PMOS transistor in the operation controller  100   f  is disabled, to cut off a current flow from the power source Vcc to the grounding terminal. The LS_EN signal is high, the third transistor PM 6  in the reference level adjuster  100   c  is turned off, and the first transistor NM 12  in the pull-down circuit  100   e  is turned on. Therefore, an output REF OUT of the reference level generating circuit is discharged to the grounding terminal Vss through the second transistor NM 13 . 
     If the LS_EN, the first control signal, is low, the PMOS transistor in the operation controller  100   f  is enabled, to supply a power from the power source Vcc to the reference level generating circuit. The third transistor PM 6  in the reference level adjuster  100   c  is turned on and the first transistor NM 12  in the pull-down circuit  100   e  is turned off, cutting off discharge of the output REF_OUT from the reference level generating circuit to the grounding terminal through the second transistor NM 13  in the pull-down circuit  100   e . Once the reference voltage from the reference bitline RBL is provided to the gate of the third transistor NM 1  in the first amplifier  100   a  and a gate of the first transistor NM 4  in the second amplifier  100   b , respective amplifiers for amplification. Another input to the first amplifier  100   a  and the second amplifier  100   d , a fed back signal form the reference level adjuster  100   c , is provided to the gate of the fifth transistor NM 3  in the first amplifier  100   a  and the gate of the fifth transistor NM 6  in the second amplifier  100   b . An output from the first amplifier  100   a  and an output from the second amplifier  100   b  are provided to an input of the reference level adjuster  100  so that the outputs are adjusted to a desired reference level by the first and second transistors NM 7  and NM 8  and the third and fourth transistors PM 6  and PM 7  in the reference level adjuster  100   c.    
     In this instance, the first amplifier  100   a  and the second amplifier  100   b  compare and amplify a signal fedback from the reference level adjuster  100   c  and a signal on the reference bitline, which is continued repetitively until a desired output of the reference level adjuster  100   c  is obtained. Provided that the desired reference level is obtained through the repetitive comparison and amplification operation, an output from the reference level adjuster  100   c  is provided to the reference level stabilizer  100   d.    
     In the meantime, the first and second transistors NM 9  and NM 10  in the reference level stabilizer  100   d  provide an excellent effect in preventing unnecessary fluctuation of the reference level. In other words, the gates of the first and second transistors NM 9  and NM 10  in the reference level stabilizer  100   d  have a signal from the second amplifier  100   b  applied thereto. Therefore, since inputs to the first and second transistors NM 9  and NM 10  have phases opposite to each other, a transient response at the input terminal on the first transistor NM 9  is offset with a transient response at an output terminal on the second transistor NM 10  when the transient response at the input terminal on the first transistor NM 9  is provided to the output terminal on the second transistor NM 10 , preventing a sudden change of the output REF_OUT of the reference level generating circuit. However, if the voltage provided to the gates of the first and second transistors NM 9  and NM 10  in the reference level stabilizer  100   d  are too low, the output from the reference level adjuster  100   c  may not be shown as the output REF OUT of the reference level generating circuit adequately, the third transistor NM 11  is brought into a turned on state at a time when the transient response ends for providing a stable signal without loss as an output. 
     In order to provide a difference of ΔV between the input REF_IN to the reference level generating circuit and the output REF_OUT from the reference level generating circuit, the following method is used. Basically, sizes of the third transistor NM 1  in the first amplifier  100   a  and the first transistor NM 4  in the second amplifier  100   b  are made the same, and also sizes of the fifth transistor NM 3  in the first amplifier  100   a  and the fifth transistor NM 6  in the second amplifier  100   b  are made the same. Further, the driving powers of the fifth transistors NM 3  and NM 6  in the first and second amplifiers  100   a  and  100   b  are made smaller than driving powers of the third and first transistors NM 3  and NM 4  in the first and second amplifiers  100   a  and  100   b . By adjusting sizes of the first, third, fourth transistors NM 7 , PM 6  and PM 7  in the reference level adjuster  100   c , an output REF_OUT level of the reference level generating circuit can be elevated higher than an input level by ΔV. 
     FIG. 12 illustrates a noise from a power source voltage vs. an output from a reference level generator, wherefrom it can be known that the reference level generating circuit of the present invention can provide a stable reference level despite of the noise from the power source voltage. 
     The reference level generator in accordance with the preferred embodiment used in a nonvolatile ferroelectric memory of the present invention has various advantages. For example, a variation of a reference level can be adjusted with ease by adjusting a size of the NMOS transistors. Further, a fast response is available since the level of the reference voltage finally provided to the sense amplifier is stable without fluctuation. Moreover, the level of the final reference voltage can be maintained stable regardless of power source noise, and, if the reference voltage on the reference bitline is constant, the reference voltage is protected almost perfectly even if a noise is carried on the power source voltage. 
     The foregoing embodiments and advantages are merely exemplary and are not to be construed as limiting the present invention. The present teaching can be readily applied to other types of apparatuses. The description of the present invention is intended to be illustrative, and not to limit the scope of the claims. Many alternatives, modifications, and variations will be apparent to those skilled in the art. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents but also equivalent structures.