SENSING AMPLIFIER FOR RANDOM ACCESS MEMORY

Abstract: A sensing amplifier for a random access memory (10) (RAM) having a first differential amplifying circuit (40) formed of a first pair of transistors (41, 42) having their sources con­ nected together, their gates supplied with differential input sig­ nals, and their drains connected with symmetrical type active loads (44, 45); and a second differential amplifying circuit (50) of a current mirror type formed of a pair of transistors (51, 52) which is connected in series with the first differential amplify­ ing circuit.

Description: This invention relates generally to sensing amplifiers and, more particularly, is directed to a sensing amplifier for a random access memory (RAM) that is suitably used to amplify a signal read out from a memory. In the prior art, a sensing amplifier is used to amplify a signal having quite a low level read out from in internal storage or external storage (memory) of an electronic computer such that the level of such a signal read out reaches the voltage level to be processable by a logic circuit. An example of a prior art sensing amplifier will be described with reference to Figure 1. Figure 1 is a schematic diagram showing a construction of such a prior art sensing amplifier. Referring to Figure 1, a memory cell 10 is provided, which is a component of a metal oxide semiconductor (MOS) memory matrix, not shown. The memory cell 10 includes a flip-flop circuit formed of load resistors 11 and 12, and metal oxide semiconductor field effect transistors (MOSFET's) 13 and 14. This memory cell 10 is of a static type memory which stores information each time the MOSFET's 13 and 14 are turned on or off. When the memory cell 10 is put in the reading mode, if an X address (word line) 21 is selected by an X decoder, not shown, MOSFET's 15 and 16 connected to this word line 21 are turned on, and the information stored in the memory cell 10 is transferred to Y addresses (bit lines) 22a and 22b. In this case, since the memory cells on the same word line 21 are all activated, a selection signal for predetermined Y addresses (bit lines) 22a and 22b are supplied through a selection line 23 to MOSFET's 24a and 24b from a Y decoder, not shown. Then, the MOSFET's 24a and 24b are turned on, supplying the information of the predetermined memory cell 10 through the bit lines 22a and 22b to the sensing amplifier. A first stage amplifying circuit 30 of the sensing amplifier is formed as a current mirror type differential amplifying circuit. To be more concrete, the first stage amplifying circuit 30 comprises N-channel differential input MOSFET's 31 and 32 whose gates are respectively connected to the bit lines 22a and 22b. The sources of both the MOSFET's 31 and 32 are connected together to a drain of a third N-channel MOSFET 33 that serves as a constant current source. The source of the MOSFET 33 is grounded and the gate thereof is connected to a voltage source terminal TP and thereby the MOSFET 33 is turned on. The drains of both the N-channel MOSFET's 31 and 32 are connected respectively to drains of a pair of P-channel MOSFET's 34 and 35, each of which is used as an active load. The gate and the drain of the P-channel MOSFET 34 are directly coupled together to serve as a diode, while the gate of the other P-channel MOSFET 35 is connected to the gate of the MOSFET 34. Then, the sources of both the P-channel MOSFET's 34 and 35 are connected to the power source terminal TP. Thus, the current mirror circuit is constructed. In the above-mentioned differential amplifying circuit 30, potentials or voltages having voltage levels corresponding to the on or off state of both the input differential MOSFET's 13 and 14 in the memory cell 10 and generated on both the bit lines 22a and 22b are supplied to the gates of both the input differential MOSFET's 31 and 32 as input signals thereof. A difference signal between these input signals is amplified and from a connection point A between the MOSFET's 32 and 35, an unbalanced output signal of the differential amplifying circuit 30 is supplied to a driving stage, inverting amplifying circuit 36. The output signal from this inverting, amplifying circuit 36 is supplied to an output stage, buffer amplifying circuit 37. The output signal from the output amplifying circuit 37 has already reached a predetermined voltage level, and it is delivered through an output terminal 38 to a logic circuit, not shown. A sensing amplifier used for a memory capable of operating at high speed and producing multiple-outputs, the output amplifying circuit 37 has a large driving capacity. Thus, when a load having a large capacity is connected to the output terminal 38, a transient current having a large amplitude flows through the output amplifying circuit 37 to the voltage source and the ground, generating high frequency noises. The noises are supplied through the voltage source and the ground to the differential amplifying circuit 30 in the same phase. Of course, as is well known in the art, the differential amplifying circuit amplifies a difference signal between two input signals and it does not amplify the input voltages having the same phase. However, in the conventional current mirror type differential amplifying circuit 30 shown in Figure 1, the MOSFET 34 forming a part of the current mirror circuit is connected so as to operate as a diode and is low in impedance, while the other MOSFET 35 is high in impedance so that the loads to both the input differential FET's 31 and 32 are made asymmetric. As a result, when the high frequency response characteristic of the constant current source FET 33 is not sufficient, the above-mentioned noise components having the same phase can not be cancelled out within the differential amplifying circuit 30 and appear at the junction A between the MOSFET's 32 and 35, preventing a signal from being read out correctly from the memory. DE-A-33 46 529 discloses a sensing amplifier for use with a random access memory, said amplifier comprising: a symmetrical first stage differential amplifying circuit comprising a pair of amplifying transistors receiving read-out signals at their gates and a pair of MOSFET's as loads, and a driving stage differential amplifying circuit. The loads in the first stage differential amplifying circuit are established by transistors receiving a supply voltage. Therefor, it is not possible to modify the load by a bias voltage as it is in principle possible with the arrangement of Fig. 1 as explained above. However, the amplifier of DE-A-33 46 529 has the advantage that the first stage differential amplifying circuit is symmetrical due to the transistors as loads, thereby preventing the disadvantages as discussed in connection with the sensing amplifier according to Fig. 1. It is the object of the present invention to provide a sensing amplifier for use with a random access memory, said amplifier having a symmetrical first stage to be able to remove noise components, and being arranged to have applied a bias voltage for adjustment purposes. The sensing amplifier of the present invention comprises the following features: a symmetrical first stage differential amplifying circuit comprising a pair of amplifying transistors receiving read-out signals at their gates, and comprising first and second MOSFET's as loads, a bias circuit for biasing said load-MOSFETs and comprising a third MOSFET having its gate and drain coupled to operate as a diode, and a driving stage differential amplifying circuit, whereby said three MOSFETs are all of the same type, are all connected to a power source terminal with their drains, all receive the bias voltage at their gate, and all occupy the same dimension or area. The invention is set forth in independent claim 1. Due to the symmetrical construction of the first stage differential amplifying circuit, noise components can be removed. Due to the special arrangement of the bias circuit and the first stage differential amplifying circuit, the latter circuit is still symmetrical when bias voltages are applied to it. This is described in detail in the description of Fig. 2. These and other objects, features and advantages of the present invention will become apparent from the following detailed description of the preferred embodiment taken in conjunction with the accompanying drawing. Figure 1 is a schematic diagram showing a construction of a prior art sensing amplifier; and Figure 2 is a schematic diagram showing an embodiment of a sensing amplifier for a random access memory according to the present invention. In Figure 2, like parts corresponding to those of Figure 1 are marked with the same references and will not be described in detail. A first stage amplifying circuit 40 is of a differential amplifying circuit type which includes symmetrical active loads. Specifically, the bit lines 22a and 22b are respectively connected to gates of N-channel differential input MOSFET's 41 and 42 in the first differential amplifying circuit 40. The sources of both the MOSFET's 41 and 42 are connected together to a drain of a third N-channel MOSFET 43 that serves as a constant current source. The source of the MOSFET 43 is grounded. The drains of both the N-channel MOSFET's 41 and 42 are connected respectively to drains of a pair of P-channel MOSFET's 44 and 45 which become symmetrical active loads. The sources of the MOSFET's 44 and 45 are both connected to a power source terminal TP. A bias circuit 46 supplies a bias voltage to the first stage amplifying circuit 40. The bias circuit 46 comprises one P-channel MOSFET 47 and two N-channel MOSFET's 48 and 49 which are connected in series between the power source terminal TP and the ground. In other words, in the bias circuit 46, the source and the drain of the P-channel MOSFET 47 are respectively connected to the power source terminal TP and the drain of the N-channel MOSFET 48, the source of the MOSFET 48 and the drain of the MOSFET 49 are connected together, and the source of the MOSFET 49 is grounded. The gate and the drain of the P-channel MOSFET 47 are directly coupled to each other so as to operate as a diode and from a junction B at the connection between the MOSFET's 47 and 48, a bias voltage is commonly supplied to the gates of the pair of P-channel MOSFET's 44 and 45 in the differential amplifying circuit 40. Also, from this junction B, the common bias voltage is supplied to the gate of the MOSFET 49 of the bias circuit 46 and to the gate of the MOSFET 43 serving as the constant current source of the differential amplifying circuit 40. The gate of the MOSFET 48 in the bias circuit 46 is connected to the gate of the MOSFET 42 in the differential amplifying circuit 40. The differential amplifying circuit 40 generates balanced outputs at a junction C between the input differential MOSFET 41 and the active load, i.e., MOSFET 44, and a junction D between the input differential MOSFET 42 and the active load, i.e., MOSFET 45. A driving stage amplifying circuit 50 is formed as a current mirror-type differential amplifying circuit similar to the foregoing first stage amplifying circuit 30 shown in Figure 1. Therefore, in the driving stage differential amplifying circuit 50 shown in Figure 2, like parts corresponding to those of the first stage amplifying circuit 30 are marked with reference numerals similar to those in Figure 1 in which reference numerals on the first order are not changed but reference numerals on the second order are replaced with "5". Thus, the circuit arrangement thereof will not be described in detail. In this differential amplifying circuit 50, the gate of the MOSFET 53 serving as the constant current source is supplied with the bias voltage from the MOSFET 54 whose gate and drain are coupled so as to operate as a diode in the current mirror circuit. The drain of the MOSFET 54 is connected to the drain of the MOSFET 51 and a junction E therebetween is connected to the gate of the MOSFET 53. In the driving stage differential amplifying circuit 50, the differential input MOSFET's 51 and 52 are supplied at their gates with the balanced outputs from the junctions C and D, respectively, of the first stage differential amplifying circuit 40. From the junction E between the drains of the MOSFET's 51 and 54, there is supplied a bias voltage to the gate of the MOSFET 55. Further, from a junction F between the MOSFET's 52 and 55, an unbalanced output of the differential amplifying circuit 50 is supplied to the output amplifying circuit 37. An output having a predetermined level from this output amplifying circuit 37 is delivered to the output terminal 38. The operation of the sensing amplifier according to the above embodiment will now be described. The active loads, i.e., MOSFET's 44 and 45 in the differential amplifying circuit 40 and the MOSFET 47 whose gate and drain are coupled so as to operate as a diode in the bias circuit 46 are respectively formed to occupy the same dimension or area. For this reason, drain currents I44 and I45 of both the MOSFET's 44 and 45 become equal to a drain current I47 of the MOSFET 47. Further, since a drain current equal to those of the respective input differential MOSFET's 41 and 42 flows through the MOSFET 48 in the bias circuit 46, DC voltage drops across both the input differential MOSFET's 41 and 42 and across the bias MOSFET 48 become equal to one another. Accordingly, also DC voltage drops across the active loads, i.e., the MOSFET's 44 and 45 in the first stage differential amplifying circuit 40 become equal to each other so that both the active loads, i.e., the MOSFET's 44 and 45 become sufficiently symmetrical to each other. Both the junctions C and D of the first stage differential amplifying circuit 40 are respectively connected to the gates of the differential input MOSFET's 51 and 52 in the driving stage differential amplifying circuit 50, so that the first stage differential amplifying circuit 40 is made sufficiently symmetrical (balanced) in the circuit arrangement from its input circuit to its output circuit. Therefore, according to this embodiment, even when the noise components which are the same in phase enter through the power source terminal and the ground, as mentioned before, the noise components having the same phase can be removed well within the first stage differential amplifying circuit 40 so that they can be prevented from being transmitted to the driving stage differential amplifying circuit 50. Moreover, since the driving stage amplifying circuit 50 is formed of a differential amplifying circuit and the DC voltages having the same phase applied to the input differential MOSFET's 51 and 52 thereof can be varied over a wide range, when the input differential MOSFET's 51 and 52 are respectively connected to the pair of junctions (output terminals) C and D of the first stage differential amplifying circuit 40, it is not necessary to take a DC level difference therebetween into consideration. According to the present invention as described above in detail, since the first stage differential amplifying circuit is formed as a symmetrical (balanced) type and the driving stage circuit is formed as a differential amplifying circuit, it is possible to obtain a sensing amplifier which can sufficiently remove noise components which are the same in phase and which is free from the consideration of the DC level difference between the stages.

Claims: A sensing amplifier for use with a random access memory, said amplifier comprising: a symmetrical first stage differential amplifying circuit (40), comprising a pair of amplifying transistors (41, 42) receiving read-out signals at their gates, and first and second MOSFETs (44, 45) as loads, and a driving stage differential amplifying circuit (50). characterized in that a bias circuit (46) is provided for biasing said load-MOSFETs, said bias circuit comprising a third MOSFET (47) having its gate and drain coupled to operate as a diode, whereby said three MOSFETs (44, 45, 47) are all of the same type, are all connected to a power source terminal (TP) with their drains, all receive the bias voltage at their gates, and all occupy the same dimension or area. A sensing amplifier according to claim 1, characterized in that said first stage differential amplifying circuit (40) comprises a constant current source having a transistor (43) whose drain is connected to the source of said amplifying transistors (41, 42), whose source is grounded and whose gate is supplied with the bias voltage. A sensing amplifier according to one of the claims 1 or 2, characterized in that said bias circuit (46) comprises a fourth MOSFET (48) having its gate connected to one of the read-out memory signals, its drain connected to the drain of the third MOSFET (47), and its source connected to the drain of a fifth MOSFET (49), said fifth MOSFET having its gate connected to the drains of said third and fourth MOSFETs and its source connected to ground.

Inventors: WATANABE, KAZUO
Applicants: SONY CORPORATION
Application year: 1986
Main classification: G11C7
IPC codes: G11C7, H03K5, G11C11