# Patent

## Title
Low current wide VREF range input buffer

## Abstract

A low-current input buffer is disclosed. The buffer uses self-biased N and P channel differential pairs with their outputs tied together. The self-biasing assists in reducing current consumption. The combination of N and P-channel differential pairs results in symmetry across a wide range of reference and supply voltages.

## Background
This application is a divisional of application Ser. No. 10/161,601, filed on Jun. 5, 2002, now U.S. Pat. No. 6,864,725, which is incorporated herein by reference.


FIELD OF THE INVENTION
The invention relates to a low-current differential buffer that works across a wide range of reference and supply voltages.
BACKGROUND OF THE INVENTION
There is a need for differential buffers that have uniform, symmetrical rise and fall trigger characteristics. Bazes buffers sense both low-to-high and high-to-low transitions equally, symmetrically, and with high speed. However, Bazes buffers tend to consume a lot of current, and also sometimes function inconsistently depending on the reference voltage and Vcc supplied thereto. Therefore, a low-current symmetrical buffer that can work across a wide range of reference and supply voltages is desired.
BRIEF SUMMARY OF THE INVENTION
In one aspect, the invention provides a lower power buffer in which all differential amplifiers contained therein are self-biased. The differential amplifiers can be either fully or half self-biased, where the half self-biased embodiment consumes slightly more power but requires less transistors to create. In another aspect, the buffer is double- rather than single-ended, which provides a more reliable output and faster switching speed. Additional aspects of the present invention locate enable gates at a variety of positions within the buffer.



BRIEF DESCRIPTION OF THE DRAWING
The foregoing and other features and advantages of the invention will become more apparent from the detailed description of the exemplary embodiments of the invention given below in connection with the accompanying drawings in which:
FIG. 1 is a schematic diagram depicting a portion of an input buffer with a resistor bias;
FIG. 2 is a schematic diagram depicting a portion of an input buffer with a self-biasing transistor;
FIG. 3 is a schematic diagram depicting an differential input buffer;
FIG. 4 is a schematic diagram depicting a Bazes buffer;
FIG. 5 is a schematic diagram depicting a differential buffer biased using resistors to choke off current;
FIG. 6 is a schematic diagram depicting a buffer according to a first embodiment of the present invention;
FIG. 7 is an additional schematic diagram of the buffer of FIG. 6, while FIGS. 7A, 7B, 7C, 7D, and 7E show variations of the buffer of FIG. 7;
FIG. 8 is a schematic diagram of an additional embodiment of the present invention, while FIG. 8A, 8B, 8C, 8D, and 8E show variations of that embodiment;
FIG. 9 is a schematic diagram of an additional embodiment of the present invention, while FIGS. 9A, 9B, and 9C show variations of that embodiment;
FIG. 10 is a schematic diagram of an additional embodiment of the present invention, while FIGS. 10A, 10B, 10C, 10D, and 10E show variations of that embodiment;
FIG. 11 is a schematic diagram of an additional embodiment of the present invention, while FIGS. 11A, 11B, and 11C show variations of that embodiment;
FIG. 12 is a schematic diagram of an additional embodiment of the present invention, while FIGS. 12A, 12B, 12C, and 12D show variations of that embodiment;
FIG. 13 is a schematic diagram of an additional embodiment of the present invention, while FIGS. 13A, 13B, and 13C show variations of that embodiment;
FIG. 14 is a schematic diagram of an additional embodiment of the present invention, while FIGS. 14A, 14B, 14C, 14D, and 14E show variations of that embodiment;
FIG. 15 is a schematic diagram of an additional embodiment of the present invention, while FIGS. 15A, 15B, 15C, 15D, and 15E show variations of that embodiment;
FIG. 16 show the present invention as part of a processor system.



DETAILED DESCRIPTION OF THE INVENTION
Differential input buffer circuits are useful in digital circuits for determining whether an unknown input voltage VIN is either above or below a fixed reference voltage VREF. Specifically, when VIN>VREF, a definite output is expected, and when VIN<VREF, another, opposite output is expected. However, to guarantee that the differential input buffer circuit works properly, VIN must differ from VREF by an offset of no less than a predetermined voltage, e.g. 300 mV.
It is a necessary feature of differential buffer circuits to be biased in order that the transistors contained or known therein will be in operational mode at all times. FIG. 1 shows a portion of buffer circuit 100 in which an n-channel differential amplifier 108 is biased using a resistor 104. This arrangement, however, has the disadvantage that it is difficult to consistently fabricate resistors having the exact same resistance value. Also, the resistance value of resistor 104 may not remain consistent all voltage and temperature ranges during operation, therefore impacting the circuit's bias stability.
FIG. 2 shows portion of another known buffer circuit 200 which is an improvement over FIG. 1, in that a portion of an input buffer circuit 200 contains an n-channel differential amplifier 204 which is self-biased. As shown in FIG. 2, this self-biasing is accomplished by connecting the source 208 of one of the ndiff transistors (n-channel transistors within a differential amplifier circuit) to the gate 212 of a biasing transistor 216. Thus, improved biasing and switching within a buffer circuit can be achieved.
FIG. 3 shows an n-channel differential pair 300, where the current mirror portion 308 ensures that the amount of current through both p-channel transistors is equal. The differential pair 300 is enabled by a biasing transistor 304. Substituting a transistor 304 for the resistor 104 of FIG. 1 stabilizes some of the fabrication problems associated with resistor biasing, but is still tied to an unstable reference resistance tree 316. The circuit 300 also does not operate symmetrically in that it senses low-to-high transitions of VIN differently than it senses high-to-low transitions. This problem is exacerbated when the differential pair 300 is operated across a wide range of reference voltages.
FIG. 4 is a schematic diagram depicting a Bazes buffer 400 employing a circuit using the self-biasing principles of FIG. 2. As shown in FIG. 4, the gates of p- and n-channel bias transistors 412, 416 are connected at a point between the p- and n-channel differential amplifiers 404, 408. P-channel transistor 412 is connected to Vcc, while n-channel transistor 416 is connected to ground. With this biasing technique, the Bazes buffer 400 responds to inputs in a more symmetrical fashion, but works best when VREF is exactly Vcc/2. As VREF varies from Vcc/2, the symmetry is reduced.
FIG. 4 also shows that the sources of the transistors within the p-channel differential amplifier 404 are connected together at VH, and that the drains of the transistors within the n-channel differential amplifier 408 are connected together at VL. In order for the circuit of FIG. 4 to be biased in a stable fashion, the currents through transistors 412 and 416 steer the current to enable stable switching of the output based on the Vbias level. When Vbias goes higher, the n-channel supplies more current to the ground side. As Vbias goes lower, the p-channels provide more current to the Vcc side. The two bias-voltage inputs are connected to the internal amplifier bias node VBIAS. This self-biasing creates a negative-feedback loop that stabilizes the bias voltages. Additionally, any variations in processing parameters or operating conditions that shift the bias voltages away from their nominal values result in a shift in VBIAS which corrects the bias voltages through negative feedback.
In the Bazes buffer of FIG. 4, transistors 412 and 416 operate in the linear region. Consequently, the voltages VH and VL may be set very close to the supply voltages. Since these two voltages determine the output swing of the amplifier, the output swing can be very close to the difference between the two supply rails. This large output swing simplifies interfacing the Bazes amplifier to other types of logic gates, since it provides a large margin for variations in the logic threshold of the gates.
Another consequence of the linear range operation of transistors 412 and 416 is that the Bazes amplifier 400 can provide output switching currents which are significantly greater than its quiescent current. In contrast, conventional CMOS differential amplifiers cannot provide switching currents which exceed the quiescent current set by the current-source drive, which operates in the saturation region. This capability of supplying momentarily large current pulses makes the Bazes amplifier especially suitable for high-speed comparator applications where one of the inputs VIN is a reference voltage, and where it is necessary to rapidly charge and discharge output capacitive loads without consuming inordinate amounts of power. Bazes buffers such as that shown in FIG. 4 detect transitions faster, symmetrically, and with less distortion than other buffers. These are very desirable characteristics for high speed data buffers such as input/output buffers for memory devices and processors. Unfortunately, Bazes buffers have the disadvantage that they consume a large amount of current. Also, as VREF moves away from Vcc/2, Bazes buffers lose stability due to current mismatches, because rise and fall times do not track as well as desired.
FIG. 5 shows a known buffer 500. Resistors 504 and 508 assist in achieving bias stability, but as noted with respect to FIG. 1 it is difficult to consistently fabricate resistors having the exact same resistance value. Also, the current supplied through those resistors value may not remain consistent during operation, therefore impacting the circuit's bias stability especially across a range of values for Vcc. Additionally, the buffer 500 still has the problem of consuming excessive amounts of current, as well as inconsistent performance across a range of reference voltages. For example, when VREF is low, the buffer 500 maintains fairly good symmetry but consumes a lot of current. When VREF=Vcc/2 which is the generally accepted optimum condition of operation, timing data regarding both output conditions is gathered at the output node 512. This data shows that rise time Trise (the result when VIN>VREF) is faster than fall time Tfall (the result when VIN<VREF), which causes the buffer 500 to behave asymmetrically. Finally, as VREF increases, the buffer 500 continues to have its bias nodes 516, 520 go lower, yet the output node 512 stays very close to ground. This results in the drive ratios of the combined differential amplifiers 524 and 528 becoming skewed so that the buffer ceases to switch well, because Tfall increases while Trise decreases. Accordingly, the response again becomes asymmetrical. Even worse, as VREF continues to increase, the P-channel amplification characteristics become so strong that the buffer ceases to work at all. Thus, the drive ratios necessary to make the buffer work well at the lower VREFs inhibit the buffer's performance at the higher VREFs.
These problems are mitigated by the input buffers of the present invention as shown in the following Figures. In FIG. 6, the input buffer 600 has two transistors 604, 608 for the p-differential amplifier 620, and two other transistors 612, 616 for the n-differential amplifier 624. The transistors 604, 608, 612, and 616 are included specifically for the purpose of self-biasing, which provides for more consistent performance across a wide range of reference voltages. These transistors result in both p- and n-differential amplifiers 620, 624 being fully self-biased. In contrast, the known circuit of FIG. 5 shows both p- and n-differential amplifiers 524, 528 not self-biased, but instead being choked by resistors 504 and 508. These current-choking resistors 504, 508 cause the differential amplifiers 524, 528 to consume less current, but do not produce a measurable biasing effect. The advantage of being self-biased is that, as VREF increases, bias nodes 628, 632 stay fairly constant, perhaps increasing slightly. During this time the n-channel node 636 node greatly increases, yet the drive ratios of the p-channel to n-channel amplifiers do not change. It is well known that a p-channel differential amplifier works better at lower reference voltages, while n-channel differential amplifiers work better at higher reference voltages. The present invention combines these two characteristics while minimizing the disadvantages associated therewith. This enables the buffers of the present invention, including all of the following embodiments, to work with either synchronous dynamic random access memory (SDRAM), double data rate random access memory (DDRRAM), and low power mobile random access memory (LPMRAM), all of which require differing reference voltages.
FIG. 7 shows the circuit of FIG. 6 reworked to fit horizontally on a page rather than vertically, and also with the addition of “enable gate” transistors 704 and 708. The enable gate transistor 704 couples biasing transistor 604 and 612 to VCC, while enable gate transistor 708 couples biasing transistor 608 and 616 to ground. Enable gates allow input buffers such as the buffer 600 and 700 to be disabled to save current in certain operating modes. Various embodiments of the present invention will be shown in which portions of the n- and p-channel differential amplifiers either share enable gates or have their own enable gates. Sharing enable gates allow the buffers to consume less current and assist the bias node in tracking variances in the reference voltage VREF and then making any necessary drive ratio adjustments resulting from those variances, thereby resulting in more consistent rise and fall times. The buffer of FIG. 7 works reliably with an input voltage VIN and a reference voltage VREF, but also can work with complementary VIN and VIN− inputs.
Additionally, it is important to note that all of the following embodiments have the output side of the buffer is tied to the side of the differential pair that is close to the input signal VIN, not the reference signal VREF. In this way, the output of the buffer is always inverting. However, the present invention could also be arranged to have the output side of the buffer tied to the side of the differential pair that is close to the reference signal VREF, not the input signal VIN. This arrangement would always be noninverting.
Additional variations of the FIG. 7 circuit contemplated by the invention include replacing one of the two biasing transistors 604 and 608 with a direct connection between buffer 620 and transistor 704 and buffer 620 and enable gate 708 respectively, which would then make the p-differential amplifier 620 only half self-biased, while the n-differential amplifier 624 remains fully self-biased. Similarly, one of the two biasing transistors 612 and 616 could also be replaced with a direct connection between amplifier 624 and either enable gate 704 or 708, which would then make the n-differential amplifier 624 only half self-biased, while the p-differential amplifier 620 would remain fully self-biased.
FIG. 7 shows both p- and n-differential amplifiers 620, 624 respectively sharing enable gates 704 and 708. However, each differential amplifier could have its own enable gate 704′ or 708′ (for p-differential amplifier 620) and 704′′, 708′′ (for n-differential amplifier 624), as shown in FIG. 7A. Additionally, each differential amplifier 620, 624 could share one enable gate 704 as in FIG. 7, but also have separate enable gates 708′, 708′′ instead of enable gate 708 as in FIG. 7A, or vice versa.
As another variation to the FIG. 7 circuit, one of the enable gates 704, 708 but not both could be replaced by a resistor 704b or 708c, as shown in FIGS. 7B and 7C. Furthermore, either enable gate 704 or 708 of FIG. 7 could be replaced by series transistors. In the case where the enable gate 704 is replaced by p-channel series transistors, the transistors would have their gates tied to VA as shown in FIG. 7D, where ground <=VA<VTP (threshold voltage of the p-channel transistors). In the case where the enable gate 708 is replaced by n-channel series transistors, the transistors would have their gates tied to VB as shown in FIG. 7E, where threshold voltage of the n-channel transistors VTN<VB<=Vcc.
Another embodiment of the present invention is shown in FIG. 8, in which a differentially folded input buffer 800 has mirrored p-differential amplifiers 804, 808, as well as mirrored n-differential amplifiers 812, 816. As shown in FIG. 8, the outputs of differential amplifiers 804 and 812 are combined, and the outputs of differential amplifiers 808 and 816 are combined. The amplifiers 804, 812 are coupled to provide the differential output VOUT, while the amplifiers 808, 816 are combined to provide the complementary signal VOUT−. The FIG. 8 circuit is advantageous for improving reliability and accuracy of the two output signals, and also for creating two complementary outputs as opposed to a single ended output.
The FIG. 8 embodiment has the differential amplifiers 804, 808 fully self-biased by transistors 820, 824, and the differential amplifiers 812, 816 filly self-biased by the transistors 828 and 832. FIG. 8 also has enable gates 836 and 840, which couple bias transistors 820 and 828 to Vcc and bias transistors 824, 832 to ground, respectively.
Variations of the FIG. 8 circuit which also form part of the invention include replacing either one of the two biasing transistors 820 and 824 with a direct connection to enable gate 836 or 840, which would then make the mirrored p-differential amplifiers 804, 808 only half self-biased, while the mirrored n-differential amplifiers 812, 816 would remain fully self-biased. Similarly, one of the two biasing transistors 828 and 832 could also be replaced with a direct connection to enable gate transistor 836 or enable transistor 840, which would then make the mirrored n-differential amplifiers 828, 832 only half self-biased, while the mirrored p-differential amplifiers 820, 824 would remain fully self-biased.
FIG. 8 shows all mirrored differential amplifiers 804, 808, 812, 816 sharing enable gates 836 and 840. However, each set of 804, 808 mirrored differential amplifiers (804, 808 and 812, 816) could have its own enable gates 836′, 840′ and 836′′, 840′′ as shown in FIG. 8A. Additionally, each mirrored differential amplifier 804, 808, 812, 816 and FIG. 8 could share enable gate 836 as shown in FIG. 8, but have separate enable gates 840′, 840′′ as shown in FIG. 8A instead of enable gate 840, or vice versa.
Furthermore, one but not both of the enable gates 836, 840 of FIG. 8 could be replaced by a resistor 836b, 840c or series transistors 836d, 840e as shown in FIG. 8B-8E. In the case where the enable gate 836 to Vcc of FIG. 8 is replaced by p-channel series transistors 836d (FIG. 8D), the transistors would have their gates tied to VA, where ground <=VA<VTP. In the case where the enable gate 840 to ground of FIG. 8 is replaced by n-channel series transistors 840e (FIG. 8E), the transistors would have their gates tied to VB, where VTN=VB<=Vcc.
Another embodiment of the present invention is shown in FIG. 9, in which an input buffer 900 has a p-differential amplifier 904 as well as an n-differential amplifier 908. Both amplifiers 904, and 908, however, are only half self-biased, as shown by the dashed circles 909 and 910 which are meant to draw attention to the lack of an electronic component located there. Being half self-biased is advantageous in that fewer transistors are used, but results in slightly higher current consumption. As stated, all of the embodiments of the present invention using half rather than fill self-biasing have the disadvantage of slightly higher current consumption. However, response symmetry is not measurably affected by half self-biasing. Both amplifiers 904 and 908 share a p-channel enable gate 914 to Vcc.
Additional variations of the FIG. 9 embodiment could include each differential amplifier 904 and 908 having its own enable gate 914′, 914′′, as shown in FIG. 9A. Furthermore, the enable gate 914 of FIG. 9 could be replaced by a resistor 914b or series transistors 914c with their gates tied to VA where ground <=VA<VTP, as shown in FIGS. 9B and 9C, respectively.
Another embodiment of the present invention is shown in FIG. 10, wherein a p-differential amplifier 1004 is half self-biased, while an n-differential amplifier 1008 is fully self-biased. Both amplifiers 1004 and 1008 share an n-channel enable gate 1012 to ground as well as a p-channel enable gate 1016 to Vcc. This embodiment, like all of the embodiments having half rather than fill self-biasing, has the disadvantage of slightly higher current consumption yet requires less transistors and maintains satisfactory response symmetry.
Additional variations of the FIG. 10 embodiment could include each differential amplifier 1004, 1008 having its own enable gates 1016′, 1016′′, 1012′, 1012′′, as shown in FIG. 10A. Additionally, each differential amplifier could share enable gate 1016 as in FIG. 10, but have separate enable gates 1012′, 1012′′ as shown in FIG. 10A, or vice versa. Furthermore, either (but not both) enable gates 1012, 1016 of FIG. 10 could be replaced by a resistor 1004b, 1008c or series transistors 1004d, 1008e as shown in FIGS. 10B-10E. In the case where the enable gate to Vcc 1016 of FIG. 10 is replaced by p-channel series transistors 1016d (FIG. 10D), the transistors would have their gates tied to VA, where ground <=VA<VTP. In the case where the enable gate 1012 to ground of FIG. 10 is replaced by n-channel series transistors 1012e (FIG. 10E), the transistors would have their gates tied to VB, where VTN<VB<=Vcc.
Another embodiment of the present invention is shown in FIG. 11, wherein a p-differential amplifier 1104 is fully self-biased, while an n-differential amplifier 1108 is also fully self-biased. Both amplifiers 1104 and 1108 share an n-channel enable gate 1112 to ground. However, the bias transistors 820, 828 are directly tied to Vcc.
Additional variations of the FIG. 11 embodiment could include replacing any one of the four biasing transistors 820, 828, 824, 832 with a direct connection to Vcc (for replaced transistors 820, 828) or enable gate transistor 1112 (for transistors 824, 832), which would then make one of the two differential amplifiers 1104 and 1108 half self-biased.
Additionally, FIG. 11 shows both p- and n-differential amplifiers 1104, 1108 sharing the enable gate 1112. However, each differential amplifier could have its own enable gate 1112′, 1112′′ as shown in FIG. 11A. Furthermore, enable gate 1112 of FIG. 11 could be replaced by a resistor 1112b or series transistors 1112c, as shown in FIGS. 11B and 11C respectively. In the case where the enable gate 1112 to ground is replaced by n-channel series transistors 1112c, the transistors would have their gates tied to VB, where VTN<VB<=Vcc.
Another embodiment of the present invention is shown in FIG. 12, wherein a p-differential amplifier 1204 is fully self-biased, and an n-differential amplifier 1208 is also fully self-biased. Both amplifiers 1204 and 1208 have their own enable gates to both Vcc (1205, 1209) and to ground (1206, 1210).
Additional variations of the FIG. 12 embodiment could include replacing any one of the biasing transistors 1220, 1221, 1222, 1223 with a direct connection, which would then make one of the differential amplifiers only half self-biased, while the other would remain fully self-biased. Furthermore, one or more of the enable gates 1205, 1206, 1209, 1210 could be replaced by a resistor or series transistors, as shown in FIGS. 12A, 12B for replacing one or both of transistor enable gate 1206, 1209 and FIGS. 12C, 12D for replacing one or both of enable gate transistors 1205, 1210. In the case where an enable gate to Vcc is replaced by p-channel series transistors, the transistors would have their gates tied to ground. In the case where an enable gate to ground is replaced by n-channel series transistors, the transistors would have their gates tied to VB, where VTN<VB<=Vcc.
Another embodiment of the present invention is shown in FIG. 13, wherein a p-differential amplifier 1304 is fully self-biased by transistors 1320, 1321, and an n-differential amplifier 1308 is also fully self-biased by transistor 1322, 1323. The amplifiers 1304 and 1308 share a p-channel enable gate 1312 to Vcc while bias transistors 1321, 1323 are tied to ground.
Additional variations of the FIG. 13 embodiment include replacing one of the biasing transistors 1320, 1321, 1322, and 1323 with a direct connection, which would then make one of the differential amplifiers 1304, 1308 only half self-biased, while the other would remain fully self-biased. Additionally, FIG. 13 shows both differential amplifiers 1304, 1308 sharing the enable gate 1312. However, each differential amplifier could have its own enable gate, 1312′, 1312′′ as shown in FIG. 13A. Furthermore, the enable gate 1312 could be replaced by a resistor 1312b (FIG. 13B) or series transistors 1312c (FIG. 13C) with their gates tied to VA, where ground <=VA<VTP.
Another embodiment of the present invention is shown in FIG. 14, wherein a p-differential amplifier 1404 is fully self-biased, while an n-differential amplifier 1408 is half self-biased. The amplifiers 1404 and 1408 share a p-channel enable gate 1412 to Vcc and an n-channel enable gate 1416 tied to ground.
Additional variations of the FIG. 14 embodiment include each differential amplifier having its own enable gate 1412′, 1412′′, 1416′, and 1416′′ as shown in FIG. 14A. Additionally, each differential amplifier 1464, 1408 could share enable gate 1412 of FIG. 14, but have separate enable gates 1416′, 1416′′ instead of enable gate 1416 as in FIG. 14A, or vice versa. Furthermore, either (but not both) enable gates 1412, 1416 of FIG. 14 could be replaced by a resistor or series transistors, as shown in FIGS. 14B and 14D for enable gate 1412, and FIGS. 14C and 14E for enable gate 1416. In the case where the enable gate 1412 to Vcc is replaced by p-channel series transistors, the transistors would have their gates tied to VA, where ground <=VA<VTP. In the case where the enable gate 1416 to ground is replaced by n-channel series transistors (FIG. 14E), the transistors would have their gates tied to VB, where VTN<VB<=Vcc.
Another embodiment of the present invention is shown in FIG. 15, wherein a p-differential amplifier 1504 is fully self-biased by transistors 1520, 1521, and an n-differential amplifier 1508 is also fully self-biased by transistor 1521, 1523. The amplifiers 1504 and 1508 share an n-channel enable 1512 gate to ground, while the n-differential amplifier 1508 has its own separate p-channel enable gate 1516 to Vcc.
Additional variations of the FIG. 15 embodiment include replacing one of the biasing transistors 1520, 1521, 1522, and 1523 of FIG. 15 with a direct connection, which would then make one of the differential amplifiers 1504, 1508 only half self-biased, while the other would remain fully self-biased. Additionally, FIG. 15 shows both differential amplifiers 1504, 1508 sharing enable gate 1512. However, each differential amplifier could have its own enable gate 1512′, 1512′′, as shown in FIG. 15A. Furthermore, the enable gate 1516 of FIG. 15 could be replaced by a resistor 1516b (FIG. 15B) or series transistors 1516d (FIG. 15D) with their gates tied to VA, where ground <=VA<VTP. Additionally, the enable gate 1512 of FIG. 15 could be replaced by a resistor 1512c (FIG. 15C) or series, transistors 1512e (FIG. 15E) with their gates tied to VB, where VTN<VB<=Vcc.
The present invention can be utilized within any integrated circuit which receives an input signal from an external source. FIG. 16 illustrates an exemplary processing system 1600 which may utilize an electronic device comprising a self-biasing buffer constructed in accordance with any of the embodiments of the present invention disclosed above in connections with FIG. 6 through 15E. The processing system 1600 includes one or more processors 1601 coupled to a local bus 1604. A memory controller 1602 and a primary bus bridge 1603 are also coupled the local bus 1604. The processing system 1600 may include multiple memory controllers 1602 and/or multiple primary bus bridges 1603. The memory controller 1602 and the primary bus bridge 1603 may be integrated as a single device 1606.
The memory controller 1602 is also coupled to one or more memory buses 1607. Each memory bus accepts memory components 1608 which include at least one memory device 1631 contains a buffer device of the present invention. The memory components 1608 may be a memory card or a memory module. Examples of memory modules include single inline memory modules (SIMMs) and dual inline memory modules (DIMMs). The memory components 1608 may include one or more additional devices 1609. For example, in a SIMM or DIMM, the additional device 1609 might be a configuration memory, such as a serial presence detect (SPD) memory. The memory controller 1602 may also be coupled to a cache memory 1605. The cache memory 1605 may be the only cache memory in the processing system. Alternatively, other devices, for example, processors 1601 may also include cache memories, which may form a cache hierarchy with cache memory 1605. If the processing system 1600 include peripherals or controllers which are bus masters or which support direct memory access (DMA), the memory controller 1602 may implement a cache coherency protocol. If the memory controller 1602 is coupled to a plurality of memory buses 16016, each memory bus 16016 may be operated in parallel, or different address ranges may be mapped to different memory buses 1607.
The primary bus bridge 1603 is coupled to at least one peripheral bus 1610. Various devices, such as peripherals or additional bus bridges may be coupled to the peripheral bus 1610. These devices may include a storage controller 1611, an miscellaneous I/O device 1614, a secondary bus bridge 1615, a multimedia processor 1618, and an legacy device interface 1620. The primary bus bridge 1603 may also coupled to one or more special purpose high speed ports 1622. In a personal computer, for example, the special purpose port might be the Accelerated Graphics Port (AGP), used to couple a high performance video card to the processing system 1600. In addition to memory device 1631 which may contain a buffer device of the present invention, any other data input device of FIG. 16 may also utilize a buffer device of the present invention including the CPU 1601.
The storage controller 1611 couples one or more storage devices 1613, via a storage bus 1612, to the peripheral bus 1610. For example, the storage controller 1611 may be a SCSI controller and storage devices 1613 may be SCSI discs. The I/O device 1614 may be any sort of peripheral. For example, the I/O device 1614 may be an local area network interface, such as an Ethernet card. The secondary bus bridge may be used to interface additional devices via another bus to the processing system. For example, the secondary bus bridge may be an universal serial port (USB) controller used to couple USB devices 1617 via to the processing system 1600. The multimedia processor 1618 may be a sound card, a video capture card, or any other type of media interface, which may also be coupled to one additional devices such as speakers 1619. The legacy device interface 1620 is used to couple legacy devices, for example, older styled keyboards and mice, to the processing system 1600. In addition to memory device 1631 which may contain a buffer device of the invention, any other data input device of FIG. 16 may also utilize a buffer device of the invention, including a CPU 1601.
The processing system 1600 illustrated in FIG. 16 is only an exemplary processing system with which the invention may be used. While FIG. 16 illustrates a processing architecture especially suitable for a general purpose computer, such as a personal computer or a workstation, it should be recognized that well known modifications can be made to configure the processing system 1600 to become more suitable for use in a variety of applications. For example, many electronic devices which require processing may be implemented using a simpler architecture which relies on a CPU 1601 coupled to memory components 1608 and/or memory buffer devices 304. These electronic devices may include, but are not limited to audio/video processors and recorders, gaming consoles, digital television sets, wired or wireless telephones, navigation devices (including system based on the global positioning system (GPS) and/or inertial navigation), and digital cameras and/or recorders. The modifications may include, for example, elimination of unnecessary components, addition of specialized devices or circuits, and/or integration of a plurality of devices.
While the invention has been described and illustrated with reference to specific exemplary embodiments, it should be understood that many modifications and substitutions can be made without departing from the spirit and scope of the invention. Accordingly, the invention is not to be considered as limited by the foregoing description but is only limited by the scope of the appended claims.

## Claims

1. A differential input buffer, comprising:
a buffer circuit having separate p- and n-channel differential amplifiers arranged to each receive a pair of input signals, said p- and n-channel differential amplifiers each being self-biased and having a current mirror portion;
a resistive circuit, at least one of said p- and n-channel differential amplifiers being coupled to said resistive circuit; and
an output terminal for combining the outputs of said p- and n-channel differential amplifiers to form an output of said differential input buffer, said output terminal being coupled to a pair of series transistors,
wherein said p- and n-channel differential amplifiers are coupled to a shared p-channel enable transistor.

2. The input buffer of claim 1, wherein said resistive circuit is coupled to ground.

3. The input buffer of claim 1, wherein said resistive circuit is coupled to a supply voltage.

4. The input buffer of claim 1, wherein:
said resistive circuit is coupled to a supply voltage.

5. The input buffer of claim 1, wherein:
said resistive circuit is coupled to a ground terminal; and
said enable transistor is coupled to a supply voltage terminal.

6. A processing system circuit, comprising:
a processor; and
a memory circuit for exchanging data with said processor;
wherein at least one of said processor and memory circuit comprise an input buffer circuit, said input buffer circuit further comprising:
a buffer circuit having separate p- and n-channel differential amplifiers, wherein said separate p- and n-channel differential amplifiers are each self-biased and having a current mirror portion; and
a resistive circuit, at least one of said p- and n-differential amplifiers being coupled to said resistive circuit,
wherein the outputs of said differential amplifiers are combined to form an output of said differential input buffer, said output being coupled to a pair of series transistors, and
wherein said p- and n-channel differential amplifiers are coupled to a shared p-channel enable transistor.

7. The processor circuit of claim 6, wherein said resistive circuit is coupled to a ground terminal.

8. The processor circuit of claim 6, wherein said resistive circuit is coupled to a supply voltage terminal.

9. The processor circuit of claim 6, wherein said resistive circuit is coupled to ground.

10. The processor circuit of claim 6, wherein said resistive circuit is coupled to a supply voltage terminal.

11. The processor circuit of claim 6, wherein:
said resistive circuit is coupled to a ground terminal, and
said shared p-channel enable transistor is coupled to a supply voltage terminal.

12. The processor circuit of claim 6, wherein said resistive circuit is coupled to a ground terminal.

13. The processor circuit of claim 6, wherein said resistive circuit is coupled to a supply voltage terminal and wherein said p- and n-channel differential amplifiers are each directly connected to a ground terminal.

14. The processor circuit of claim 6, wherein:
said resistive circuit is coupled to a supply voltage terminal.

15. A method of operating an input buffer, comprising:
self-biasing a pair of separate p- and n-channel differential amplifiers, each of said differential amplifiers having a current mirror portion;
enabling at least one of said p- and n-channel differential amplifiers, wherein said p- and n-differential amplifiers are coupled to a resistive circuit and to at least one pair of series transistors at an output node;
detecting an input signal;
comparing said input signal with a reference signal using said separate p- and n-channel differential amplifiers; and
emitting the results of said comparison as an output signal,
wherein said p- and n-channel differential amplifiers are coupled to a shared p-channel enable transistor.

16. The method of claim 15, wherein said resistive circuit is coupled to ground.

17. The method of claim 15, wherein said resistive circuit is coupled to a voltage supply.

18. The method of claim 15, wherein said resistive circuit is coupled to ground.

19. The method of claim 15, wherein said resistive circuit is coupled to a supply voltage.

20. The method of claim 15, wherein:
said resistive circuit is coupled to ground, and
said shared enable transistor is coupled to a supply voltage.

21. The method of claim 15, wherein said resistive circuit is coupled to a supply voltage.