CMOS Current sense amplifiers.

Current sense amplifiers are generally well known in the prior art and are typically used in electronic circuits to read data out of semiconductor memories such as EPROMs and EEPROMs. However, these prior art sense amplifiers suffer from the shortcomings of having to trade a high-to-low transition with a low-to-high transition. Because of commercial demands for significant increases in access time for addressable memories fabricated on integrated circuit semiconductor chips, it has become very important to memory designers to have available a current sense amplifier circuit which operates at high speeds so as to improve efficiency of the overall memory circuit operation.

It would therefore be desirable to provide a current sense amplifier circuit whose design does not depend upon a trade-off between the high-to-low and low-to-high transitions.

According to one aspect of the present invention the low-to-high transition is made extremely fast by precharging all bit-lines to ground with the aid of clamping transistors, thereby requiring that all of the bit-lines charge up from the ground level. This allows the circuit designer to optimise the sense amplifier for the high-to-low transition without deteriorating the low-to-high transition. A sense amplifier circuit embodying the invention may therefore have a low-to-high transition time of approximately 6 nanosecond at a temperature of +30°C.

In a preferred embodiment of the invention there is provided a CMOS current sense amplifier circuit which includes a sense amplifier for providing a first output. A memory array formed of core transistors is arranged in n rows of word lines and m columns of bit-lines. A first pass transistor and a plurality of Y-pass transistors are coupled between the sense amplifier and the memory array. A dummy sense amplifier is used to provide a second output. A dummy bit-line formed of core transistors are arranged in parallel along the n rows of word lines. Second and third pass transistors are coupled between the dummy sense amplifier and the dummy bit-line. Clamping devices are coupled to the memory array and dummy bit-line for maintaining each column of bit-lines at a ground potential. The first pass transistor, second pass transistor and clamping devices are responsive to false and true clamping pulse signals for permitting the quick discharge of first output of the sense amplifier. A differential sense amplifier is responsive to the first and second outputs for generating a third output at an output terminal which has a fast response time when making a low-to-high transition.

In the accompanying drawings, by way of example only:- <ul id="ul0001" list-style="none"><li>Figure 1 is a schematic circuit diagram of a CMOS current sense amplifier circuit embodying the present invention; and</li><li>Figures 2(a) through 2(e) are a set of waveform diagrams useful in understanding of the operation of the sense amplifier circuit of Figure 1.</li></ul>

In Figure 1 a high speed CMOS current sense amplifier

circuit 10 has a very fast response time to a logic "1" state (unprogrammed EPROM cell). As a result, the sense amplifier circuit 10 makes a low-to-high (zero to one) transition almost instantaneously. The sense amplifier circuit includes a sense amplifier arrangement 12, a dummy sense amplifier arrangement 14 and an operational amplifier arrangement 16.

The sense amplifier arrangement 12 includes a pair of P-channel MOS transistors 18 and 20 having their gate electrodes connected together and to a ground potential during the read operation. The source of the transistor 18 is connected to a supply voltage or potential V_CC. The drain of the transistor 18 is connected to the source of the transistor 20. The sense amplifier arrangement 12 further includes a pair of N-channel MOS transistors 22 and 24. The transistor 22 has its drain connected to the drain of the transistor 20 and to the gate of the transistor 24. The transistor 22 has its source connected to the ground potential and its gate electrode connected to the source of the transistor 24. The drain of the transistor 24 is connected to the supply potential V_CC.The junction of the drain of the transistor 18 and the source of the transistor 20 is tied to the gate of an output transistor 26 formed of a N-channel MOS transistor. The source of the transistor 26 is connected to the source of the transistor 24. The drain of the transistor 26 is joined to the gate and drain electrodes of a load transistor 28 formed of a P-channel MOS transistor. The source of the transistor 28 is also connected to the supply potential V_CC. The junction of the common drains of the transistors 26 and 28 serves as the output (node A) of the sense amplifier arrangement 12.

The dummy sense amplifier arrangement 14 includes a pair of P-channel MOS transistors 30 and 32 having their gates connected together and to the ground potential during the read operation. The source of the transistor 30 is connected to the supply potential V_CC. The drain of the transistor 30 is connected to the source of the transistor 32. The dummy sense amplifier arrangement further includes a pair of N-channel MOS transistors 34 and 36. The transistor 34 has its drain connected to the drain of the transistor 32. The transistor 34 has its source connected to the ground potential and its gate electrode connected to the source of the transistor 36. The drain of the transistor 36 is connected to the supply potential V_cc .The junction of the drain of the transistor 30 and the source of the transistor 32 is tied to the gate of an output transistor 38 formed of a N-channel MOS transistor. The source of the transistor 38 is connected to the source of the transistor 36. The drain of the transistor 38 is joined to the gate and drain electrodes of a load transistor 40 formed of a P-channel MOS transistor. The source of the transistor 40 is also connected to the supply potential V_CC. The junction of the common drains of the transistors 38 and 40 serves as the output (node B) of the dummy sense amplifier arrangement 14.

The operational sense amplifier arrangement 16 includes current mirror N-channel MOS transistors 42 and 44 having their gate and source electrodes connected together. The common gates of the transistors 42 and 44 are connected to the drain of the transistor 42. The common sources of the transistors 42 and 44 are connected to the drain of a current source N-channel MOS transistor 46. The gate of the transistor 46 is connected to a reference voltage V_REF. The source of the transistor 46 is tied to the ground potential. A load transistor 48 formed of a P-channel MOS transistor has its source connected to the supply potential V_CC, its gate connected to the output (node A) of the sense amplifier arrangement 12, and its drain connected to the drain of the transistor 42. A load transistor 50 formed of a P-channel MOS transistor has its source also connected to the supply potential VCC,its gate connected to the output (node B) of the dummy sense amplifier arrangement 14, and its drain connected to the drain of the transistor 44. When the operational sense amplifier arrangement 16 is considered as functioning as a differential amplifier, the junction of the drain of the transistor 50 and the drain of the transistor 44 serves as the output (node C) of the differential amplifier.

The operational sense amplifier arrangement 16 further includes an output buffer stage consisting of transistor 52 formed of a N-channel MOS transistor and a transistor 54 formed of a N-channel MOS transistor. The output (node C) of the differential amplifier is connected to the gate of, the transistor 52. The transistor 52 has its drain connected to the supply potential V_CC and its source connected to the drain of the transistor 54 and to the output V_OUT (node D) of the sense amplifier circuit 10. The gate of the transistor 54 is connected to the reference voltage V_REF and the source of the transistor 54 is tied to the ground potential.

The sense amplifier circuit 10 further includes a memory array 56 formed of a plurality of core transistors which are arranged in n rows of word lines WL ₁, WL ₂ ... through WL n by m columns of bit-lines BL ₁, BL ₂ ... through BL_m . As can be seen, each of the core transistors Q₁₁ ... Q_1m in the first row (n = ₁) have their gate electrodes connected together and to an input data signal V_in which is applied to the word line WL1 . All of the sources of the transistors Q ₁₁ ... Q _lm are connected to the ground potential. The respective drains of the core transistors Q ₁₁ ... Q _1m are connected to corresponding bit-lines BL ₁ ... through BL _m. Similarly, each of the core transistors Q ₂₁ ... Q_2m to Q_n1 ... through Q_nm in all of the remaining rows are likewise connected.

The sense amplifier circuit 10 also includes a dummy bit-line 58 formed of a plurality of core transistors. Specifically, the core transistors are arranged in parallel along the n rows of word lines WL₁ , WL2 ... through WL_n . In particular, each of the transistors Q₁₁'... through Q_n1' has its respective gate connected to the input data signal V_in which is applied to the respective word lines WL₁, WL₂ ... through WL_n. All of the sources of the core transistors Q_11'... through Q_n1' are connected to the grpund potential. The respective drains of the transistors Q_11'... through Q_n1' are connected to bit-line BL_1'.

A first pass transistor 60 has its drain connected to the source of the transistor 26 of the sense amplifier arrangement 12, its source connected to all of the drains of Y-pass transistors Q_D1... through Q_Dm , and its gate connected to a control terminal 62. A false clamp signal CP is applied to the control terminal 62 from a clamp pulse generator (not shown). A second pass transistor 64 has its drain connected to the source of the transistor 38 of the dummy sense amplifier arrangement 14, its source connected to drain of a third pass transistor 65 and its gate electrode connected to a control terminal 66. The false clamp pulse signal CP is also applied to the control terminal 66. A plurality of bit-line clamp transistors B/L are provided so that all of their sources are connected to the ground potential. All of the gates of the clamp transistors B/L are connected together and to a control terminal 68. A true clamp pulse signal CP is applied to the terminal 68 from the clamp pulse generator. Each of the drains of the transistors B/L are connected to respective bit-lines BL₁ ... through BL_m and BL_1' of the respective memory array 56 and dummy bit-line 58.

As previously stated, all of the Y-pass transistors Q_D1...through Q_Dm have their drains connected together and to the source of the first pass transistor 60. The respective sources of the transistors Q_D1 ... through Q_Dm are connected to corresponding bit-lines BL₁... through BL_m. Each of the gates of the transistor Q_D1... through Q_Dm are connected to respective Y-decode signals Y_D1 ... through Y_Dm from a Y-decoder (not shown). The Y-decode signals are used to select an unprogrammed memory cell.The third pass transistor 65 has its drain connected to the source of the second pass transistor 64, its source connected the dummy bit-line BL_1' , and its gate connected to the supply potential V_CC.

Parasitic capacitance represented by a capacitor C_S has one of its ends connected to the output (node A) of the sense amplifier arrangement 12 and its other end connected to the ground potential. Parasitic capacitance represented by a capacitor C_BL1 has one of its ends connected to the common drains of the core transistors which are connected to the bit-line BL₁. The other end of the capacitor C_BLlis connected to the ground potential. The capacitor CBL1 is used to facilitate the fast response of the sense amplifier circuit by clamping all of the bit-lines to the ground potential. Thus, this ensures that all of the bit-lines are required to be charged up from the ground potential. Also, this facilitates the quick discharge of the capacitance C_S at the output of the sense amplifier arrangement during a low-to-high transition of the sense amplifier circuit 10.

Reference is now made to the waveform diagrams shown in Figure 2(a) through 2(e) which are useful in understanding of the operation of the sense amplifier circuit 10 of Figure 1. For ease of explanation, it will be assumed that it is desired to select the bit-line BL₁ wherein the gate of the Y-pass transistor Q_D1 receives the decode signal Y_D1 being in the logic "1" state. As a result, the Y-pass transistor Q_Dlwill be turned on. Further, since the gate of the third transistor 65 is tied to the supply potential V_CC, it will always be turned on. Initially, at time t = 0, all of the bit-line clamp transistors B/L in the array 56 and dummy bit-line 58 have a true bit-line clamp pulse signal CP at a logic "1" level (Fig. 2b) applied to their gates via the control terminal 68 so as to cause them to be turned on. This results in the clamping of all of the bit-lines BL₁ ... through BL_m and BL_1' to the ground potential. In other words, the capacitor C_BL1 connected to the bit-line BL₁ will be completely discharged and will be required to charge up from the ground potential. Simultaneously, a false bit-line clamp pulse signal CP at a logic "0" level (Fig. 2c) will be applied to the gates of the pass transistors 60 and 64 via respective concrol terminals 62 and 66 so as to cause them co be turned off. Thus, the capacitor C_Sconnected to the output of the sense amplifier arrangement 12 will be fully charged up. The outputs of the sense amplifier arrangement 12 at node A and the dummy sense amplifier arrangement 14 at node B are shown in curves 70 and 72 in Figure 2(c) respectively. The outputs of the differential amplifier at node C and the sense amplifier circuit 10 at node D are illustrated in curves 74 and 76 in Figure 2(e) respectively.

Assuming that at time t = 1 the input data signal V_in makes a transition from low-to-high (zero to one), the true clamp pulse signal CP will make a high-to-low transition, and the false clamp signal CP will make a low-to-high transition at a time t = 2. As a result, the bit-line transistor B/L connected to the first bit-line BL_l will be turned off causing the capacitor C_BL1 to start charging from the ground potential. Further, the pass transistors 60 and 64 will be turned on, permitting the outputs of the sense amplifier arrangement and dummy sense amplifier sense arrangement to discharge quickly. As a consequence, the output at node D of the sense amplifier circuit 10 responds substantially instantaneously causing the very fast low-to-high transition to appear as indicated by edge 78 in Figure 2(e).

The sense amplifier circuit 10 has the following advantages over the conventional prior art sense amplifiers: <ul id="ul0002" list-style="none"><li>(a) it uses a plurality of bit-line clamp transistors for grounding all of the bit-lines;</li><li>(b) it provides a very fast response time when making a low-to-high transition;</li><li>(c) it has a fast response due to the quick discharging of the capacitance Cs connected to the output of a sense amplifier arrangement; and</li><li>(d) the fast response during the low-to-high transition does not adversely affect the high-to-low transition.</li></ul>

, From the foregoing detailed description, it can thus be seen that the circuit 10 provides an improved CMOS current sense amplifier circuit which has a fast response time when making a low-to-high transition. The current sense amplifier utilizes a plurality of bit-lines clamp transistors for grounding all of the bit-lines so as to require the bit-lines to charge up from the ground level.

While there has been illustrated and described what is at present to be a preferred embodiment of the present invention, it will be understood by those skilled in the art that various changes and modifications may be made, and equivalents may be substituted for elements thereof without departing from the true scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the central scope thereof. Therefore, it is intended that this invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the- invention will include all embodiments falling within the scope of the appended claims.