SENSE AMPLIFIER FOR AMPLIFYING SIGNALS ON A BIASED LINE

Abstract: Sense amplifier for use in memory systems, particu­ larly static random access memories (RAM). The sense amplifier circuitry comprises two inverters, one (26) forming a regular amplifier, the other (25) providing a reference voltage (V Ref ) to the amplifier for determining the optimal operating point (A o ) on the inverter's transfer curve, and to a bit-line (40) associated with a memory cell being read for biasing the line at its optimal operating level. The inverters used in the sense amplifier as voltage reference and as amplifier as well as the inverter pairs forming the memory cells may have the same structure and properties and can be formed on the same semiconductor chip.

Description: The invention concerns a sense amplifier for amplifying signals on a biased line and for generating distinct level digital signals. Such amplifier circuits are widely used particularly in digital data processing systems and, more specifically, they are employed in memory systems where, during read-out, the bit storage cells deliver a signal representative of the stored binary value, "1" or "0", which signal needs to be properly recognized and amplified to obtain a digital signal of a distinct level for further processing. Numerous sense amplifiers, particularly amplifiers for use in semiconductor memory systems have been proposed, all designs aiming at satisfactorily meeting the essential requirements for an amplifier that can be used in today's high density, high speed memories. These amplifiers need to be reliable, fast, sensitive to small signals and they should be of simple structure requiring only a minimum of space on a semiconductor chip. In addition, the circuit must meet these requirements without demanding extremely tight device margins since the unavoidable fabrication process tolerances, which vary widely depending on the technology used, may otherwise not permit the use of high-volume manufacturing processes. Particularly critical in this regard are current GaAs technologies. A sense amplifier arrangement for use in a semiconductor memory that is typical for today's state of the art has been disclosed at the IEEE GaAs IC Symposium held at Boston, Massachusetts, October 23-25, 1984. Article "A GaAs 4K Bit static RAM with Normally-on and -off Combination Circuit", appearing on pages 117-120. of the Technical Digest of that conference describes a memory system including a storage cell matrix and a sense amplifier which is of the differential amplifier type. Its inputs are connected to the pair of bit-lines associated with the storage cells that are represented by the well-known and widely used "6 device cells". So-called 'pull-up' Field Effect Transistors (FET's) serve to bias the lines and to remove any charges on the line before starting the next read operation. Currently, differential sense amplifiers are frequently employed but this type of amplifiers generally needs a second power supply voltage, the circuitry is more complex than that of regular amplifier circuits and, furthermore, it is a true analog circuit which is more difficult to realize than digital circuits, particularly in GaAs technology. In the article "Estimation of GaAs Static RAM Performance", published in IEEE Transactions on Electron Devices, Vol. ED-29, No. 7, July 1982, pp. 1130-1135, another random access memory (RAM) system is described in which a regular amplifier is provided for each bit line. In contrast to the operation of the differential amplifiers, the regular amplifiers do not compare the two signals appearing on a bit-line pair but compare the received signals with a build-in reference. This becomes particularly critical when the signal to be sensed is small compared to the bias voltage that is applied to the bit line. When the reference and the bias voltage don't track, i.e., when deviations from the optimum nominal value are not self-compensated, the realization of amplifiers providing the required reliability becomes critical with today's manufacturing tolerances when several amplifiers are required on the same chip. The current state of the art is furthermore represented by the following documents: EP - A - 0 142 167 disclosing a level shifting circuit comprising two inverting amplifiers, one serving as amplifier, the other functioning as an auto-bias circuit with respect to the first amplifier; and US - A - 3 714 638 describing a feedback circuit comprising a pair of cross-coupled transistors connected to the two sense lines of an array of memory cells which clamp one sense line to a low level in response to a high signal level read-out on the other sense line. The main object of the present invention is to provide a sense amplifier wherein the most essential characteristics of the employed devices "track", i.e., any deviations in the characteristic of a device should be compensated for by a deviation of a characteristic of an associated device such that the desired result is achieved and virtually not effected by deviations. The deviations may be caused by manufacturing tolerances but also by operational conditions such as changes in temperature. It is a further object to provide a reliable, simple and fast sense amplifier that operates with small voltage swings to increase the memory operation speed. Still another object is to provide a sense amplifier of simple structure that is easy to manufacture with the same processes used to fabricate the associated memory array. The invention as claimed is intended to overcome the drawbacks of presently known sense amplifier arrangements and to meet the above objectives. One way of carrying out the invention is described in detail below with reference to drawings in which Fig. 1 shows a block diagram of a static RAM in which the present invention can be employed; the diagram is restricted to those functional elements that are essential for the operation of the sense amplifier. Fig. 2 shows the transfer characteristic of an inverter used as a regular amplifier; the operating points A and A o , of a prior art circuit and of a circuit in accordance with the present invention, respectively, are illustrated. Fig. 3 shows the circuit diagram of a sense amplifier arrangement designed in accordance with the invention; it comprises a regular amplifier and an associated reference voltage circuit. Referring now to Fig. 1, a block diagram of a prior art memory arrangement 1 is illustrated in which the inventive sense amplifier may be used. Shown are a single memory cell 2 with associated row bit-lines B and B , a word-line W, two bias circuits 3 and 4, one for each of the bit-lines, and a pair of sense amplifiers 5B and 5 B . In order to keep the diagram simple, it is limited to those functional elements that are essential for the read operations during which signals on bit-lines B and/or B that are representative of the bit value stored in the memory cell are to be sensed by the amplifier. Other functional units such as address decoders and write circuitry as well as further memory cells of the memory array are not shown. The memory cell 2 includes 6 FET's designated T1 through T6. The cell essentially comprises a pair of parallel depletion-load inverters I₁₃ and I₂₄ each consisting of two series connected FET's: T1 and T3 form inverter I₁₃, T2 and T4 form inverter I₂₄. T1 and T2 are depletion-type FET's, each with a gate-source connection and serving as load devices, whereas T3 and T4 are enhancement-type FET's. Cross-coupling transistors T3 and T4 by connecting their gates with the common points 6 and 7 of the transistors of the opposite inverter of the cell results in a flip-flop in which, in its stable states, current flows only in one of its two inverters. The two stable states may be referred to as stored bit values "1" and "0", respectively. Two additional pass transistors T5 and T6, normally non-conducting, are connected between the flip-flop and the bit-lines B and B , respectively, as indicated. Select pulses applied to word-line W render transistors T5 and T6 conductive. Bias circuits 3 and 4, connected to bit-lines B and B , respectively, serve to apply bias voltages V B and V B and to remove any charges before the next read operation is performed. Sense amplifiers 5B and 5 B may, in fact, take the form of a single two-input differential amplifier that generates a digital output representative of the bit value stored in cell 2 from the difference of the signals on the two bit-lines. On the other hand, a pair of regular amplifiers may be provided that, each operating independently, generate the digital signals corresponding to the stored value. The following explanations refer to an embodiment of the latter type. It is assumed that the two stable states of cell 2 that represent the stored bit values "1" and "0", respectively, are defined as follows: "1": current flows through inverter I₁₃ - the voltage at point 6 is well below V cc and also below the bit-line level V B ; inverter I₂₄ is "off" - point 7 is charged to V cc , i.e., higher than bit-line level V B . "0": current flows through inverter I₂₄, the voltage at point 7 is well-below V cc and also below the bit-line level V B ; inverter I₁₃ is "off" - point 6 is charged to V cc , i.e., higher than bit-line level V B . Reading from cell 2 in Fig. 1 is then initiated by applying a select pulse to word-line W, in which case the pass transistors T5 and T6 turn "on" thereby connecting the two inverters I₁₃ and I₂₄ of the flip-flop to the respective bit-lines B and B . Assuming that, prior to the read operation, inverter I₁₃ has been conducting (representing a stored "1"), current will now flow from voltage source V cc through bias circuit 3, bit-line B, transistor T5 and transistor T3 to ground. As a result, the voltage on bit-line B is lowered. This negative voltage swing also occurs at the input of sense amplifier 5B which will generate the desired digital signal for further processing. Since inverter I₂₄ is non-conducting and point 7 is up, there will be no current flow from bit-line B through T6 and T4 to ground. To the contrary, a current of opposite direction may flow through T6 since point 7 had been charged to V cc which is higher than the level on B . This charge may give rise to a positive voltage swing on line B which will, applied to the input of amplifier 5 B , cause the generation of a signal of a polarity opposite to that of the output of amplifier 5B. Whether such "complementary" voltage signal is generated depends on the transfer characteristic of the amplifiers and, more particularly, on the location of the operating point on the transfer curve. In case inverter I₂₄ is conducting (representing a stored "0") rather than inverter I₁₃, exactly the opposite to the just described process will take place. When reading a stored "0", amplifier 5 B will generate an output of the same polarity as amplifier 5B did when reading a "1", and amplifier 5B may provide a complementary output. After describing the general operation of the arrangement illustrated in Fig. 1, the problems currently encountered with such memory systems that employ two regular sense amplifiers connected to bit-lines B and B will now be outlined with the aid of Fig. 2. Fig. 2 is the transfer curve V OUT = f(V IN ) of each of the amplifiers 5B and 5 B which are assumed to be identical. The operating point A is determined by bit-line voltage level V B (or V B ) which equals the power supply voltage V cc minus the potential drop V bc at the respective bias circuit 3 or 4. As shown in Fig. 2, unless special measures are taken operating point A is located at a rather high voltage V IN separated from the steep slope, high gain part of the transfer curve. Its stable state output is V OUT ∼ 0. Problems arise whenever the (negative) voltage swing occuring on the associated bit-line, the direction of which is indicated in the drawing by arrow 20 is not sufficient to have the device "travel" through the steep (high gain) part of the curve. This is particularly critical since the position of A can vary substantially (± 100 mV) due to manufacturing tolerances and differing operating conditions whereas the voltage swing is limited (ordinarily in the order of 100 to 150 mV). With added circuitry, point A can be moved to a region where V OUT ≠ 0, but such measures (such as providing an additional voltage supply) will show the desired result only in limited areas on a semiconductor chip but not across the entire chip unless very extensive efforts are made. It is thus desirable to provide a sense amplifier where the operating point can be set on the steep part of the V OUT = f(V IN ) curve, and to define this operating point not only automatically but also largely uneffected by manufacturing tolerances. Such a desired operating point A o that can be obtained when applying the invention is also indicated in Fig. 2. It is situated at the crossing of the transfer curve with the V OUT = V IN line. A circuit providing for such operation is illustrated in Fig. 3 and will now be described in detail. Fig. 3 shows a circuit diagram of an embodiment of the present invention comprising in combination a reference voltage circuit 25 and an associated amplifier 26. In principle, the circuits correspond to blocks 3 (or 4) and 5B (or 5 B ) of Fig. 1, i.e., the reference voltage circuit provides a bit-line bias and the amplifier serves to amplify signals appearing on the associated bit-line. It should be apparent that one combined reference voltage circuit-amplifier as illustrated in Fig. 3 is required for each of the bit-lines B and B in Fig. 1. Both, the reference voltage circuit 25 and the amplifier 26 basically consist of a depletion-load inverter of the same configuration. In practice, they will be located on the same semiconductor chip in adjacent positions whereby its characteristics are virtually identical. Each of these inverters consists of a pair of series connected depletion- and enhancement-type FET's, 30-31 and 36-37, respectively, that are connected to a voltage source V cc and to ground as shown in Fig. 3. Gate and source of both depletion-type FET's are coupled via connections 32 and 38, respectively. The voltage reference or bias voltage is obtained by connecting, via lead 33, the output 34 of the reference inverter 25 to its input, i.e., to the gate of enhancement-type FET 31. This results in a circuit having a single stable state where V IN = V OUT = V Ref . Output 34, on V Ref potential, is connected via line 40 to the associated bit-line, e.g., line B, and, furthermore, via line 35 to the amplifier input, i.e., to the gate of FET 37. Prior to a read operation, the bit-line is thus charged to the reference or bias voltage level and the operating point of the amplifier is automatically set at A o (Fig. 2) as desired, i.e., at the intersection of the high gain slope of the transfer curve with the V OUT = V IN line. During read-out, the wordline W is addressed next as explained above in connection with the overall memory arrangement illustrated in Fig. 1. Depending on the binary information ("1" or "0") stored in the cell to be read, i.e., depending on whether T3 or T4 of that cell are conducting, the cell will either supply or draw current from the reference inverter. As a result, one bit-line will be raised in voltage while the other will be lowered. For example, when T3 is conducting (which may be defined as a stored "1"), the cell will draw current from bit-line B lowering its voltage to a level below V Ref whereas current will be supplied to bit-line B increasing its voltage to a level above V Ref . The negative voltage swing -V "1" on bit-line B will then be applied to the input line 35 of the amplifier which is set at operating point A o as illustrated in Fig. 2. This results in an amplified signal at output 39 of the amplifier. Further amplifier stages not shown will increase the signal to full logic levels. While, in the example, the amplifier connected to bit-line B amplifies the negative signal -V "1" , the second amplifier, connected to bit-line B , at the same time receives the positive signal +V "1" which, after further amplification results in the complementary logic level signal. In the case of a stored "0" where T4 of the storage cell is conducting, the amplifier connected to bit-line B will receive and amplify a positive signal +V "0" whereas the second amplifier amplifies negative signal -V "0" . Consequently, the true and complement output signals will be of opposite polarity than those obtained when reading a stored "1". As already explained above, the reference voltage circuit 25 represents the bias circuit 3 of Fig. 1. It is used instead of the commonly employed so-called pull-up load consisting of a load transistor or a resistor. With the pull-up load hitherto used, the bit-line is charged to a level V cc - V t (supply voltage minus voltage drop across pull-up load), i.e., a voltage that, firstly, sets the amplifier operating point at A (Fig. 2) and which, secondly, is substantially higher than the "toggle" voltage of the memory cell inverters. The toggle voltage being that voltage level that the input of the OFF-inverter must at least reach before switching can occur. In contrast thereto, when employing the inventive circuit of Fig. 3, the bit-line is charged to the level of the reference voltage V Ref which is lower than V cc - V t . It properly defines the operating point A o of the amplifier and, when depletion-load inverters of same characteristics are used in both the reference voltage circuit and the memory cells, it also sets the bit-line at the toggle point thereby optimizing the bit-line voltage for reliable high speed read-out operations. In summary: circuit 25, like a pull-up load, biases the bit-line assuring that no charge remains on the line, its advantage being that it automatically biases the line and the amplifier at optimal voltage levels. The inverters used in both, the reference voltage circuit and the amplifier, are preferably the same as those employed in the memory cells. In fact, they need not to be different from those devices currently used in static RAMs. This means, that the inventive circuitry can be fabricated during the same process steps used to produce the memory array and that, furthermore, well known and controllable processes can be employed. One of the main features of the inventive circuitry is the "tracking" of the semiconductor devices and circuits used. Whenever, due to manufacturing tolerances or operational conditions the characteristics of one of a number of associated and cooperating devices such as inverters deviate from the desired nominal values, the characteristics of the associated devices deviate "accordingly", i.e., in such a way as to guarantee the same result as if no deviation would have occurred. For example, if V Ref of the reference inverter "moves" due to manufacturing process tolerances, the transfer curve of the associated amplifier will move accordingly, i.e., the relative position of the operating point on the curve remains the same and the deviation does not have any effect on the performance. The use of "tracking" elements and circuits permits substantially larger process tolerances. Further distinctions and advantages of the herein described circuitry over known arrangements can be listed as follows: the setting of the amplifiers operating point and of the proper bit-line level is fully automatic and does not require any additional circuitry or supply lines which are difficult to adjust when proper operational levels are to be provided to devices distributed over a whole memory chip. The charging of the bit-lines to the memory cells' toggle point permit reliable high speed and low voltage swing read operations. Both, true and complement output signals are provided by the amplifier because of the position of the operating point on the transfer curve (where V OUT ≠ 0) this providing for simplification of the logic circuitry to which the amplified signals are applied. The use of inverters as amplifiers permits lower power supply voltages as compared to differential amplifiers and, furthermore, requires only a single power supply. Utilization of the herein above described sense amplifier circuitry is not restricted to static RAMs having the configuration shown in Fig. 1. Amplifiers designed in accordance with the teaching of the present invention can be employed in semiconductor array memories and other storage devices whenever a signal on a sense or bit-line has to be detected and amplified. In fact, the range of applications of the invention may even be extended to any situation where a deviation from a voltage level that is set by the sense amplifier circuitry itself is to be detected.

Claims: Sense amplifier system in combination with a static random access memory (1) having storage cells (2), each formed by a pair of cross-coupled inverter circuits (I₁₃, I₂₄) and connected, during read operations, to at least one biased bit-line(B, B ), the system consisting of two essentially identical FET inverter circuits, one of which (26) operating as a regular amplifier for signal amplification, the other (25) providing a reference voltage (V Ref ) to said biased bit-line (B, B ; 40) and to the input (35) of said regular amplifier, the input and output (34) voltage levels of said inverter circuit (25) providing said reference voltage being forced to be equal through an electrical connection (33) whereby the operating point (A₀) of said regular amplifier circuit (26) is set at the intersection of the amplifier's transfer course { V OUT =f(V IN ) } with the V OUT =V IN line, the system being characterized in that said reference Voltage (V Ref ), prior to memory read operations, biases said bit-line (B, B ) to a dc voltage level corresponding to the "toggle" voltage of the cell (2) being read, said essentially identical inverter circuits (25, 26) are of essentially the same structure, having the same properties as the inverters ( I₁₃, I₂₄) forming said storage cell (2), and in that said essentially identical inverter circuits (25, 26) are formed on the same semiconductor chip as said inverters (I₁₃, I₂₄ ) forming said storage cell.

Inventors: VAN ZEGHBROECK, BART JOZEF
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1985
Main classification: H03K19
IPC codes: H03K17, H03K19, G11C7, H03K5, G11C11