IMPROVED ACTIVE PULL-UP CIRCUIT

Abstract: An improved active pull-up circuit which can be fabri­ cated with reduced number of elements and operate with a small power consumption. A first switch is provided between a refresh voltage terminal and a time circuit node to be pulled-up. A second switch controlled by a potential of a complementary circuit node is provided for operatively discharging the charge of a control electrode of the first switch. A pull-up clock is applied via a capacitor to the control electrode of the first switch.

Description: The present invention relates to an active pull-up circuit and more particularly to an active pull-up circuit suitable for a dynamic memory circuit composed of field effect transistors. Active pull-up circuits have been advantageously utilized in dynamic circuits, especially in dynamic memories in order to establish potentials equal to or above power supply voltages at predetermined circuit nodes. For example, in a dynamic memory, an active pull-up circuit is provided for each bit line. In this memory, a read-out signal produced on a bit line is once distinguished as one of the binary logic levels by a sense amplifier and thereafter in the case where the read-out signal is high, the potential of that bit line is pulled-up to above a power supply voltage (Vcc) by the active pull-up circuit so that the pulled-up potential on that bit line is re-written into the addressed memory cell, as well as being taken out to an output circuit. In dynamic memories, active pull-up circuits must be provided for the respective bit lines and hence the number of the active pull-up circuits has become very large because of the recent increase in memory capacity of dynamic memories. Therefore the whole area occupied by the active pull-up circuits and the whole power consumption caused by the active pull-up circuits in a dynamic memory have become important in evaluating a memory. For example, the typical prior art active pull-up circuit is composed of four field effect transistors and a capacitor and is adapted to operate by two or more control signals. Therefore, it has been difficult to fabricate such prior art active pull-up circuits in the memory chip with ease. EP-A-0.080.936 discloses a dynamic semiconductor memory device including two bit lines connected to a sense amplifier and an active pull-up circuit for each bit line. Each active pull-up circuit contains three transistors and a capacitor ie. a first field effect transistor connected between a bit line and a power voltage terminal and second and third field effect transistors connected in series between the bit line and the voltage terminal. The junction between the second and third transistors is connected to the gate of the first transistor. The capacitor has one end connected to the gate of the first transistor. A clock signal is applied to the other end of the capacitor. It is one object of the present invention to provide an active pull-up circuit which can be fabricated with reduced number of elements. It is another object of the present invention to provide an active pull-up circuit operable by a small power consumption and suitable for dynamic memories. According to the present invention there is provided an active pull-up circuit comprising a voltage terminal, a first circuit node, a first field effect transistor of an enhancement type having a drain-source path coupled between said first circuit node and said voltage terminal, a second field effect transistor of a depletion type having a drain-source path coupled between said first circuit node and a gate of said first transistor, a capacitor having a first terminal coupled to the gate of said first transistor, a first means for applying a control signal to a second terminal of said capacitor, a second means for operatively applying a first signal to said first circuit node, a third means for operatively applying a second signal to a second circuit node connected to a gate of said second transistor, each of said first and second signals having a first level which makes said second transistor conductive prior to a first time point, said first signal taking one of a second level, which is substantially the same as said first level and does not change the conductive state of said second transistor, and a third level, after said first time point, said second signal taking the other of said second level and said third level, which makes said second transistor non-conductive, after said first time point, whereby when said first signal is at the second level and said second signal is at said third level the second field effect transistor is non-conductive, the application of said control signal to the second terminal of said capacitor then causing the first transistor to conduct and causing the voltage at said voltage terminal to be applied to said first circuit node. When the first circuit node is at a high potential while the second circuit node is at a low potential, the second transistor assumes a nonconducting state and the clock signal is directly applied to the first transistor. Thus, the power voltage is effectively supplied to the fast circuit node. On the contrary, when the first and second circuit nodes are at low and high potentials, respectively, the second transistor is rendered conducting to charge the gate of the first circuit node at low level. Accordingly, the first transistor is kept non-conducting irrespective of the clock signal. The active pull-up circuit thus constructed is composed of two transistors and a capacitor and requires no special clock signal which is raised above the power voltage, the integrated circuit having the large integration structure and operable with a small power consumption can be obtained. The invention will now be described, by way of example, with reference to the accompanying drawings, in which:- Fig. 1 is a schematic circuit diagram showing the major part of a memory employing the prior art active pull-up circuit; Fig. 2 is a timing chart showing an operation of the circuit of Fig. 1; Fig. 3 is a schematic circuit diagram showing the major part of a memory employing the active pull-up circuit according to the invention; and Fig. 4 is a timing chart showing an operation of the circuit of Fig. 3. Fig. 1 shows a part of a memory circuit arrangement with the conventional active pull-up circuits. A pair of bit lines BL and BL are connected to a pair of input/output nodes Nb and Na of a sense amplifier composed of field effect transistors Q₁₀ to Q₁₃, respectively. Memory cells MC₁, MC₂₋₋₋ each composed of a cell transistor QM and a capacitor CM are arranged in a known manner with respect to word lines WL₁, WL₂ and the bit lines. Active pull-up circuits 10 and 10¹ are provided for the bit lines BL and BL , respectively. The pull-up circuit 10 operatively raises the potential of the bit line BL up to a power voltage Vcc when the potential of the bit line BL is higher than that of the bit line BL . Similarly, the pull-up circuit 10¹ operatively raises the potential of the bit line BL to the power voltage Vcc when the potential of the bit line BL is higher than that of the bit line BL. The pull-up circuit 10 is composed of transistors Q₁ to Q₄ and a capacitor C₁. Control clocks ø c and ø d are employed to operate the pull-up circuit 10. The pull-up circuit 10¹ connected to the bit line BL has the similar structure. The operation of the circuit shown in Fig. 1 is described with reference to Fig. 2. First, the signal ø c is kept above the potential Vcc to render the transistors Q₃ and Q₃1 so that the nodes N₁ and N₁1 are charged to Vcc. Namely, the capacitors C₁ and C₁1 are charged to Vcc. Also, the transistor Q₁₃ is rendered conductive to set the bit lines BL and BL at a potential of Vcc/2 in response to a high level of the signal ø a. Then, the signals ø a and ø c are changed to low level at a time point t₁ that is the end of reset period. Subsequently, the word line eg. WL₂ is driven towards a selection level from a time point t₂. In response thereto, a stored signal of the cell MC₂ storing "1" is read-out on the bit line BL to slightly raise the potential of the bit line BL from Vcc/2 at a time point t₃ while the potential of the bit line BL remains unchanged. Then, the signal ø b changes from low level to high level thereby to enable the sense amplifier at a time point t₄ so that the charge at the bit line BL is discharged to the ground potential by the sense amplifier. Then, at a time point t₅, the signal ø d is raised above the Vcc level so that the gate potential of the node N₁ is raised far above the Vcc level. Thus, the transistors Q₁ and Q₂ are conducting in non-saturated region, i.e. triode region to supply the bit line BL with the Vcc level. While, in this instance, the transistor Q₄1 in the pull-up circuit 10¹ assumes conducting state because the potential of the gate of the transistor Q₄1 is high so that the charge at the node N₁1 is discharged towards the ground potential through the transistors Q₄1 and Q₁₁. Thus, the potential the bit line BL remains low. Then, at a time point t₆, the word line WL₂ and the signals ø b and ø d return to low level to finish access operation. Then, at a time point t₇, the signal ø a changes low level to high level thereby to set the bit lines BL and BL the same level of Vcc/2 and the signals return to high level above the Vcc level. Thus, the reset period is introduced. As has been described, four transistors Q₁ to Q₄ are required to form the pull-up circuit 10 and hence it has been difficult to fabricate the pull-up circuit in a small area. Moreover, in order to operate the circuit 10, two clock signals ø c and ø d are necessary, and the circuit structure for generating such signals has been complicated and consuming a relatively large power. Referring to Fig. 3, a preferred embodiment of the present invention is described for the case where the active pull-up circuit of the invention is applied to a dynamic memory. A pair of bit lines BL and BL are connected to respective first circuit nodes (N3, N3') which are in turn connected to respective input/output nodes Nb and Na of a sense amplifier. The amplifier is composed of enhancement type FETs Q₂₁ to Q₂₇. The nodes Nb and Na are connected to the amplifier via transfer gate transistors Q₂₆ and Q₂₅ of enhancement type FETs, respectively. An active pull-up circuit 20 for the bit line BL is composed of an enhancement type FET Q₆ having a drain-source connected between the bit line BL and the power voltage Vcc, a depletion type FET Q₅ having a source connected to the bit line BL and a gate connected to the input/output node Na, and a capacitor C₂ coupled between the drain of the transistor Q₅ and a clock terminal ø 15. An active pull-up circuit 20¹ for the bit line BL has a similar structure. A threshold voltage VTD of the depletion transistors Q₅ and Q₅1 is defined by "0 > VTD > -1/2 Vcc". The operation of the circuit of Fig. 3 is described with reference to Fig. 4. Prior to a time point t₁, a clock signal ø 11 is at a high level so as to keep potentials of the bit lines BL and BL at the same 1/2 Vcc level. Also, the nodes Na and Nb are set at the 1/2 Vcc through the transistors Q₂₅ and Q₂₆. At a time point t₁, the signal ø 11 changes from a high level to a low level to terminate a reset operation. Then, at a time point t₂, the word line WL₂ is selected. In this case, it is assumed that the memory cell MC₂ addressed stores a "1" level, and therefore the bit line BL is raised slightly in potential while the potential of the bit line BL remains at 1/2 Vcc. The potentials of the bit lines BL and BL are applied to the first circuit nodes N3, N3' and transmitted to the nodes Nb and Na via the transistors Q₂₆ and Q₂₅, respectively. Then, a clock signal ø 12 changes from a high to a low level at a time point t₃ to render the transistors Q₂₅ and Q₂₆ non-conducting. Thus, the nodes Na and Nb are isolated from the nodes N3 and N3' and the bit lines BL and BL, respectively. Under this condition, a clock signal ø 13 changes from a low level to a high level at a time point t₄ to render the transistor Q₂₁ conducting. Thus, the sense amplifier commences amplification of the signal difference between the nodes Na and Nb. Then, at a time point t₅, the signal ø 12 returns to a high level so that the bit lines BL and BL and the nodes N3 and N3' are electrically connected to the nodes Nb and Na. Simultaneously, a clock signal ø 14 becomes active to render the transistor Q₂₂ conductive. The transistor Q₂₂ has a larger current capability, i.e. conductance, than the transistor Q₂₁, and hence the transistors Q₂₂ and Q₂₁ discharge the charge at the bit line BL effectively and rapidly. Thus, the bit line BL is discharged to the ground potential. Therefore, the transistor Q₅ is rendered non-conductive while the transistor Q₅1 remains conductive. Therefore, the potential at the node N2 is slightly higher than 1/2 Vcc while the potential at the node N2¹ is at ground potential. Under this condition, a control signal ø 15 is raised to the Vcc level at a time point t₆. Therefore, the potential at the node N2 is raised above the Vcc level through the capacitor C₂ so that the transistor Q₆ is rendered conducting in the non-saturated region. Thus, the Vcc level is applied to the bit line BL without level reduction. Further, since the transistor Q₅1 is conducting, the potential at the node N2¹ is kept at the ground level irrespective of the change of the control signal ø 15. Thus, a pull-up operation is achieved with respect to the bit line BL. Then, at a time point t₇, the word line WL₂ and the signals ø 13, ø 14 and ø 15 return to a low level and a reset period is introduced. Also, the signal ø 11 returns to a high level to render the transistor Q₂₇ conducting. Accordingly, the "1" level of the bit line BL and the "0" level of the bit line BL are short-circuit and both of the bit lines and the nodes Na and Nb are automatically changed to the 1/2 Vcc level. As described above, an active pull-up circuit according to the present invention can be fabricated by the reduced number of elements, i.e. two transistors and a capacitor and it promotes large integration structure. Furthermore, a pull-up circuit of the invention does not require any special clock signal which is raised above the power voltage Vcc. Accordingly, a circuit having simple circuit structure and operable with a small power consumption can be obtained.

Claims: An active pull-up circuit comprising a voltage terminal (Vcc), a first circuit node (N3, N3'), a first field effect transistor (Q6, Q6') of an enhancement type having a drain-source path coupled between said first circuit node (N3, N3') and said voltage terminal (Vcc), a second field effect transistor (Q5, Q5') of a depletion type having a drain-source path coupled between said first circuit node (N3, N3') and a gate of said first transistor (Q6, Q6'), a capacitor (C2, C2') having a first terminal coupled to the gate of said first transistor (Q6, Q6'), a first means for applying a control signal (ø15) to a second terminal of said capacitor (C2, C2'), a second means (Nb, Q26, Na, Q25) for operatively applying a first signal to said first circuit node (N3, N3'), a third means (Na, Nb) for operatively applying a second signal to a second circuit node connected to a gate of said second transistor (Q5, Q5'), each of said first and second signals having a first level which makes said second transistor (Q5, Q5') conductive prior to a first time point (t1), said first signal taking one of a second level, which is substantially the same as said first level and does not change the conductive state of said second transistor (Q5, Q5'), and a third level, after said first time point (t1), said second signal taking the other of said second level and said third level, which makes said second transistor (Q5,Q5') non-conductive, after said first time point (t1), whereby when said first signal is at the second level and said second signal is at said third level the second field effect transistor (Q5, Q5') is non-conductive, the application of said control signal (ø15) to the second terminal of said capacitor (C2, C2') then causing the first transistor (Q6, Q6') to conduct and causing the voltage at said voltage terminal (Vcc) to be applied to said first circuit node (N3, N3'). The active pull-up circuit according to claim 1, characterised in that said first means raises the potential at said second terminal to the potential at said voltage terminal (Vcc) after said first and second signals take one and the other of said second and third levels. The active pull-up circuit according to claim 1, characterised in that said first circuit node (N3, N3') and said gate of said second transistor (Q5, Q5') are connected to a pair of bit lines (BL, BL ) in a memory.

Inventors: FUKUZO, YUKIO C/O NEC CORPORATION, INOUE, YASUKAZU C/O NEC CORPORATION
Applicants: NEC CORPORATION
Application year: 1985
Main classification: G11C11
IPC codes: G11C11, H03K3, H03K19