Semiconductor memory device
BACKGROUND OF THE INVENTION 
     The present invention relates to a semiconductor memory device operated synchronously with clock signals, and more particularly to an output circuit used therein to expand the duration period of the output signal. 
     In a conventional semiconductor memory device controlled by clock signals such as a metal-oxide semiconductor (MOS) dynamic memory device, since the output signal is output and shut off synchronously with the turning on and off of a clock signal, such as a column address strobe signal, respectively, the duration period of the output signal is short when the frequency of the clock signals is high. Therefore, it is impossible to make the frequency of the clock signals too high, limiting the operation speed of the memory device. 
     SUMMARY OF THE INVENTION 
     In order to eliminate the above-mentioned drawbacks of a conventional memory device, the present invention adopts the idea of adding a simple switch circuit to an output buffer circuit to give the output buffer circuit a latching function. 
     It is an object of the present invention to increase the length of output signals as much as possible without decreasing the frequency of clock pulses, thereby increasing the operating speed of a memory device. 
     According to the present invention, there is provided a semiconductor memory device including: a prestage output amplifier circuit receiving a readout signal from a memory cell; an output buffer circuit receiving the output signal of the prestage output amplifier circuit; and a switch circuit inserted between the prestage output amplifier circuit and the output buffer circuit, the switch circuit being turned on just before a drive signal is supplied from the prestage output amplifier circuit to the output buffer circuit and turned off after the output condition of the output buffer circuit is settled. The potential corresponding to the output data is maintained in the circuit between the switch circuit and the output buffer circuit. Therefore, the output condition of the output buffer circuit is retained even during the reset period of the prestage drive circuit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic structural diagram of a conventional dynamic random access memory (RAM) device; 
     FIGS. 2 through 5 are waveform diagrams of the operation of the RAM device of FIG. 1; 
     FIG. 6 is a partial circuit diagram of a semiconductor memory device as an embodiment of the present invention; 
     FIG. 7 is an electric circuit diagram of an output amplifier used in the memory device of FIG. 6; 
     FIG. 8 is a waveform diagram of the operation of the memory device of FIG. 6; and 
     FIG. 9 is a partial circuit diagram of a modified example of the memory device of FIG. 6. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Before describing the preferred embodiments, an explanation will be given of a conventional semiconductor memory device in reference to FIGS. 1 through 5. 
     FIG. 1 is a schematic block diagram of a conventional dynamic RAM device. In the drawing, reference numeral 1 designates a memory cell array (MCA) having a matrix arrangement wherein memory cells are connected to each of the intersections of a plurality of word lines WL and bit lines (not shown). Reference numeral 2 designates a sense amplifier, 3 a row decoder, 4 a column decoder, 5 an output amplifier, and 6 an output buffer circuit including MOS output transistors, Q 1  and Q 2 . 
     Clock pulses used in the above-mentioned RAM device typically include, as shown in FIG. 2, an RAS (row address strobe) signal and CAS (column address strobe) signal which define both the active period of the device and the timing of reading of external address signals. These two strobe signals are used to control the so called address multiplex system wherein two groups of address signals are provided via common address lines in a manner of time division. A dynamic RAM device other than the above-mentioned type may be controlled by a single strobe signal. In the memory device of FIG. 1, information from all the memory cells MC connected to the word line WL selected by the row decoder 3 is amplified and retained by the sense amplifier 2. One unit of the information is selected by the column decoder 4 and is sent to the output amplifier 5. OUT&#39; are the complementary output signals of the sense amplifier 2. One of the pair of the output signals OUT&#39; is maintained in an H (high) potential level, and the other becomes an L (low) potential level. Of course, these H or L levels of the output signals OUT&#39; are determined depending on the data &#34;1&#34; or &#34;0&#34; stored in the selected memory cell. The output amplifier 5 amplifies the output signals OUT&#39; and drives the output transistors Q 1  and Q 2  by using the amplified outputs OUT&#34; thereof. These MOS transistors Q 1  and Q 2  are serially connected between a power source V CC  and the ground and generate an H level output signal OUT when the transistor Q 1  is turned on and the transistor Q 2  is turned off and an L level output signal OUT when the transistor Q 1  is turned off and the transistor Q 2  is turned on. This output signal OUT is the output signal finally obtained. When the clock CAS rises, the output signal OUT is shut off because the amplifier 5 is reset in this time. 
     As shown in FIG. 2, when the signal RAS is, at first, generated, i.e., rendered the L potential level to initiate the active period, the row address data is read into the row decoder 3 in response to the falling edge of the signal RAS to the L potential level, whereby a word line WL is selected. Then, the signal CAS is rendered the L potential level and column address data is read into the column decoder 4 in a similar manner, so that bit line selection is performed. The output signals OUT&#39; of the memory cell arranged at the intersection between the thus selected word line and bit line appear after a short delay, and, thereafter, the output signal OUT is generated. When the signal CAS is released, this output signal OUT is also released after a short delay. These operations are repeated in accordance with the next generation of the signals RAS and CAS. 
     FIG. 3 is an enlarged view showing the output signals OUT&#39;, OUT&#34;, and OUT. When the final output signal OUT is shut off, both the transistors Q 1  and Q 2  are turned off and become the high impedance status, i.e., open status. 
     As apparent from the above description and the drawings, the duration period of the output signal OUT is relatively short when compared with the pulse widths of the clock signals RAS and CAS. The duration period of the output signal OUT becomes shorter when the clock frequency becomes higher. If the duration period becomes too small, the external circuit connected to the memory device cannot respond to the output signal OUT. Therefore, in a conventional memory device, it is impossible to greatly increase the clock frequency. 
     Various operating modes of a memory device are known for a high-speed access operation. One of them is the page mode and the other is the nibble mode. 
     FIG. 4 illustrates the readout operation in the page mode. In this mode, row address data RA is read in at the falling edge of the row address strobe signal RAS. Thereafter, a plurality of column address data CA used together with the row address data RA are continuously read in at the falling edges of the column address strobe signal CAS. Therefore, a plurality of memory cells commonly connected to a selected word line are sequentially accessed, and the readout data is successively output as the output signal OUT. In this case, each output signal OUT is generated a short time after the falling of the column address strobe signal CAS and is eliminated a short time after the rising of the signal CAS. 
     FIG. 5 illustrates the readout operation in the nibble mode. In this mode, a row address RA is read in at the falling edge of the signal RAS and a column address CA is read in at a falling edge of the signal CAS. Thereafter, it is possible to continuously obtain the output signal OUT of, for example, four bits having continuous column addresses, only by turning on and off the signal CAS. This operation is performed by operating a counter which counts the signals CAS and by generating column addresses which are +1, +2, . . . of the first read-in column address CA. 
     In these modes, the output signal OUT is eliminated a short time after the rising of the signal CAS. This is because internal circuits are reset at that timing to prepare for the next readout operation. Therefore, the duration period of the output signal OUT becomes relatively short. As mentioned above, if the duration period is too short, it is impossible to use the output signal OUT in external circuits. Thus, in the conventional memory device, it is practically necessary to determine the lower limit of the period of the clock signals, whereby the high-speed operation of the memory device is deteriorated. 
     According to the present invention, there is provided a semiconductor memory device which overcomes these problems. 
     FIG. 6 illustrates an output portion of a memory device as an embodiment of the present invention. In the embodiment, the circuit of FIG. 6 is used in place of the output amplifier 5 and the output buffer circuit 6 of the circuit of FIG. 1. In FIG. 6, reference numeral 7 designates a switch circuit composed of MOS transistors Q 3  and Q 4  for a switching operation, which are inserted into the path from an output amplifier 5a to an output buffer circuit 6. These transistors Q 3  and Q 4  are controlled by an output signal A of a switch drive circuit 8, which includes totempole connected MOS transistors Q 5  and Q 6  and a bootstrap capacitor C 3 , and which is operated by clock pulses φ 1 , φ R , and φ 2 . The output amplifier 5a is activated and reset by a clock pulse B applied thereto. 
     As illustrated in FIG. 7, the output amplifier 5a includes MOS driver transistors Q 7  and Q 8  which are mutually cross-coupled at the gates and drains, load transistors Q 9  and Q 10  connected to the drains of the transistors Q 7  and Q 8 , respectively, and input transistors Q 11  and Q 12  inserted between the gate of the transistor Q 9  and a data bus DB and between the gate of the transistor Q 10  and a data bus DB, respectively. The data buses DB and DB are used to supply an output signal OUT&#39; from the sense amplifier to the output amplifier 5a. 
     The operation of the circuits of FIGS. 6 and 7 is explained with reference to FIG. 8, illustrating waveforms of signals appearing in these circuits. 
     The clock signal A generated by the switch drive circuit 8 rises in response to the turning on of the transistor Q 5  caused by the rising of a clock signal φ 1 . Thereafter, the clock signal A falls in response to the turning on of the transistor Q 6  caused by the turning on of a clock signal φ R  after the turning off of the clock signal φ 1 . A clock signal φ 2  is a pulse signal slightly delayed from the clock signal φ 1  and is used to completely turn on the transistors Q 3  and Q 4  by utilizing the bootstrap effect caused by raising the potential of a point A higher than or equal to V CC . The switch drive circuit 8 is provided commonly for all output buffers, and, therefore, only two additional elements are needed for all output buffers. 
     The clock signal B rises shortly after the rising of the clock signal A. When the clock signal B rises and the output amplifier 5a is activated, the output signal OUT&#39; is applied to and amplified by the flip-flop circuit having the transistors Q 7  and Q 8 . Thereby, an output signal OUT&#34; is generated in response to the output signal OUT&#39;. Just before the output signal OUT&#34; is generated, the clock signal A is changed from the L to H level and the transisistors Q 3  and Q 4  are turned on. The potentials of the gates of both transistors Q 1  and Q 2  become low immediately after the turn-on of transistors Q 3  and Q 4  because the clock signal B is at a low level at this time. Then the clock signal B rises to generate the output signal OUT&#34;. Consequently, the output transistors Q 1  and Q 2  are operated by the output signal OUT&#34; applied to the gates thereof. After the output signal OUT is generated, the clock signal A is turned off before the falling of the clock signal B, and the output amplifiers 5a and the output buffer circuit 6 are separated, so that the H or L level of the output signal OUT&#34; is maintained by parasitic capacitors C 1  and C 2 . Thereby, the status of the output transistors Q 1  and Q 2  and, therefore, the output signal OUT is retained in the same condition until the transistors Q 3  and Q 4  are turned on at the next cycle. The output amplifier 5a is reset at the falling edge of the clock signal B. 
     Thus, in the memory device according to the present invention, the output signal OUT is retained to the time the next output signal is generated, as shown by the solid lines of FIG. 8. On the other hand, the output signal OUT of the conventional memory device is eliminated just after the reset of the output amplifier 5, as shown by the dotted lines of FIG. 8. 
     In the circuit of FIG. 6, when the clock signal A falls and the transistors Q 3  and Q 4  are turned off, the gate circuits of the transistors Q 1  and Q 2  assume the open circuit status, and the potentials of the gate circuits are retained only by the parasitic capacitors C 1  and C 2 . 
     In order to surely and reliably retain the potentials of the gate circuit in the H or L potential level, it is effective to provide a level ensuring circuit or latch circuit, as illustrated in FIG. 9. In FIG. 9, the circuit surrounded by a frame 9 of dot-dash lines is the level ensuring circuit. This is composed of a flip-flop circuit having load transistors Q A  and Q B  and driver transistors Q C  and Q D . When the output signal OUT&#34; is in a condition wherein the potential of the transistor Q 3  side circuit is H, and the potential of the transistor Q 4  side circuit is L, the transistor Q D  is turned on and the transistor Q C  is turned off. Therefore, even when the transistors Q 3  and Q 4  are turned off, the potential H or L applied to each of the gates of the output transistors Q 1  and Q 2  is retained with certainty in the same condition as that obtained when the transistors Q 3  and Q 4  are turned on. Similarly, when the output signal OUT&#34; is in a condition wherein the potential of the transistor Q 3  side circuit is L and the potential of the transistor Q 4  side circuit is H, the transistor Q C  is turned on and the transistor Q D  is turned off, and thereby the H and L level conditions are maintained even when the transistors Q 3  and Q 4  are turned off. Thus, the transistors Q A  and Q B  operate as an H level ensuring circuit, and the transistors Q C  and Q D  operate as an L level ensuring circuit. 
     As mentioned above, according to the present invention, it is possible to provide a latching function to an output buffer circuit by using a simple circuit structure. Therefore, the frequency of clock signals can be made considerably high, and the readout operation of a semiconductor memory device, such as a MOS dynamic RAM device, using the clock signals can be performed at a high speed.