Low voltage rail-to-rail CMOS input stage

[00002] This application is a Continuation of U.S. patent application Ser. No. 9/516,008 entitled LOW VOLTAGE RAIL-TO-RAIL CMOS INPUT STAGE, filed on Feb. 29, 2000 now U.S. Pat. No. 6,366,167. This application is related to co-pending U.S. patent application Ser. No. 9/515,961 entitled LOW VOLTAGE RAIL-TO-RAIL CMOS OUTPUT STAGE, filed on an even day herewith on behalf of Troy L. Stockstad, the disclosure of which is incorporated herein by reference.

[00003] The present invention relates generally to integrated circuits and, more particularly to amplifiers having a low voltage CMOS differential amplifier input stage.

[00004] Today, electronic systems encompassing operational amplifiers generally have lower operating voltage supplies than they have in the past, mainly do to low power battery sources. Moreover, power supply voltages requirements continue to decrease, while dynamic range requirements remain essentially constant. Fortunately, various manufacturing processes for integrated circuits make it possible to create rail-to-rail differential input stages.

[00005] Although the various types of input stages operate from a single supply voltage source, the low voltage limit for amplifier operation differs for each type of input stage and each integrated circuit manufacturing process. Present op amp input stage designs exhibit voltage operation limits that hinder their application in products powered by batteries having an end life of near one volt. For example, an op amp using complimentary bipolar transistor differential pairs amplifying signals near positive and negative supplies has low operating voltage limitations imposed by standard transistor base to emitter voltage drops.

[00006] One conventional solution for rail-to-rail input stages has been the use of depletion-mode MOSFETs to provide amplification of the differential input. FIG. 1 is a schematic diagram showing a prior art input stage 100 for a low voltage operational amplifier. The input stage 100 includes a differential input signal V_IN coupled to the gates of two N-channel depletion-mode MOSFETs 2 and 4. The drain of MOSFET 2 is coupled to one terminal of current source 6, and the drain of MOSFET 4 is coupled to one terminal of current source 8. The second terminals for both current sources 6 and 8 are coupled to operating potential V_CC. Both source terminals of MOSFETs 2 and 4 are coupled to one terminal of current sink 10, while the other terminal of current sink 10 is coupled to ground reference. The bulk, or well, terminals of both MOSFET 2 and MOSFET 4 are also coupled to the ground reference.

[00007] The differential pair of MOSFETs 2 and 4 receives the input signal V_IN and provides a differential output current from the drain terminals of MOSFETs 2 and 4 at inputs 14 and 16 to the rest of the system. While the prior art input stage 100 provides an input transconductance, input stage 100 has limited uses. For example, input stage 100 requires the input NMOS transistors 2 and 4 to have a particular combination of threshold voltage and bulk concentration to have proper common mode range and function correctly. This causes the process requirements for input stage 100 to be strictly defined so that the common mode input range is maximized.

[00008] In view of the forgoing, what is needed is a versatile operational amplifier input stage that can be used in a variety of applications powered from battery sources. In addition, the amplifier input stage should allow near rail-to-rail performance and increased design flexibility over that provided by prior art input stages.

[00009] The present invention addresses these needs by providing a low voltage rail-to-rail CMOS input stage. In one embodiment, a low voltage operational amplifier input stage is disclosed. The input stage includes a differential pair of P-channel metal oxide semiconductor field effect (PMOS) transistors, which produces a differential current. The input stage further includes two N-channel depletion-mode metal oxide semiconductor field effect (NMOS) transistors, coupled to the bulk terminals of the differential pair of PMOS transistors, for receiving an input signal. The depletion-mode NMOS transistors further act as source follower devices to drive the bulk terminals of the differential pair of PMOS transistors.

[00010] In another embodiment, a method for providing an output signal from an input stage of a low voltage operational amplifier is disclosed. The method includes providing an input signal to two NMOS transistors coupled to bulk terminals of a differential pair of PMOS transistors. The method further includes providing first and second alternating current signals using the differential pair of PMOS transistors.

[00011] An application specific integrated circuit (ASIC) having an input stage for a low voltage operational amplifier input stage is disclosed. The ASIC includes a differential pair of P-channel metal oxide semiconductor field effect (PMOS) transistors, which produces a differential output current. The ASIC further includes two N-channel depletion-mode metal oxide semiconductor field effect (NMOS) transistors, coupled to the bulk terminals of the pair of PMOS transistors, for receiving an input signal. The depletion-mode NMOS transistors further act as source follower devices to drive the bulk terminals of the differential pair of PMOS transistors.

[00012] In yet another embodiment, a low voltage operational amplifier input stage is disclosed. The input stage includes a differential pair of NMOS transistors, which produces a differential current. The input stage further includes two depletion-mode PMOS transistors coupled to the bulk terminals of the differential pair of NMOS transistors, for receiving an input signal. The depletion-mode PMOS transistors further act as source follower devices to drive the bulk terminals of the differential pair of NMOS transistors.

[00013] In a still further embodiment, a low voltage operational amplifier input stage is disclosed. The input stage includes a differential pair of NMOS transistors, which produces a differential current. The input stage further includes two JFET transistors coupled to the bulk terminals of the differential pair of NMOS transistors, for receiving an input signal. The JFET transistors further act as source follower devices to drive the bulk terminals of the differential pair of NMOS transistors.

[00014] Advantageously, the present invention provides a versatile operational amplifier input stage that can be used in a variety of applications, including applications powered by low power battery sources. Moreover, the input stage of the present invention provides essentially rail-to-rail performance and increased design flexibility over that provided by conventional input stages.

[00023] An invention is disclosed for a low voltage rail-to-rail CMOS input stage. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art, that the present invention may be practiced without some or all of these specific details. In other instances, well known process steps and devices have not been described in detail in order not to unnecessarily obscure the present invention.

[00024] FIG. 1 has been described in terms of the prior art. FIG. 2 is a block diagram showing an operational amplifier 150, in accordance with one embodiment of the present invention. The operational amplifier 150 includes an input stage 200 and an output stage 201.

[00025] In operation, the input stage 200 receives a differential input signal V_IN. The input stage 200 then converts the differential input signal to an output signal and supplies the output signal to the output stage 201. The output stage 201 receives the input stage output signal and converts it to an amplified output voltage V_O.

[00026] The output stage 201 provides essential rail-to-rail performance, and is capable of operating with a voltage supply as low as slightly more than a single V_GS voltage. As described in greater detail subsequently, the use of an output sink and source network by the output stage 201 allows this functionality.

[00027] FIG. 3 is a block diagram showing an input stage 200, in accordance with an embodiment of the present invention. The input stage 200 includes a voltage input network 202 and transconductance network 204 coupled to the voltage input network 202. The voltage input network 202 receives a differential input signal V_IN, while the transconductance network 204 provides differential current I₀ to the rest of the system, such as to an operational amplifier.

[00028] It is desirable to generate a current based on the differential input voltage over the entire common-mode range of the amplifier, including V_CC and V_EE. Thus, in operation, input stage 200 provides current for rail-to-rail operation at low voltage, as is often required by low power battery sources. Conventional transconductance networks generally have a limited input common mode voltage range over which they can generate current, which does not include V_CC and V_EE. However, the present invention allows essentially full rail-to-rail performance because the voltage input network 202 allows the transconductance network 204 to generate current over a voltage range of essentially from V_EE to V_CC.

[00029] FIG. 4 is a schematic diagram showing an input stage 300 in accordance with another embodiment of the present invention. The input stage 300 includes a voltage input network 202 and a transconductance network 204. The voltage input network 202 includes depletion-mode NMOS transistor 302 and depletion-mode NMOS transistor 304. The drains of depletion-mode NMOS transistors 302 and 304 are coupled to V_CC and the sources of depletion-mode NMOS transistors 302 and 304 are coupled to the transconductance network 204. Finally, the differential input signal V_IN is provided to the voltage input network 202 through the gates of depletion-mode NMOS transistors 302 and 304.

[00030] In use, the depletion-mode NMOS transistors 302 and 304 are used as source follower devices to drive the transconductance current network 204. Advantageously, the depletion-mode NMOS transistors 302 and 304 have a source potential greater than V_EE when their gate voltages are equal to V_EE. Moreover, when the gate voltage of the depletion-mode NMOS transistors 302 and 304 transitions to near V_CC, the back-gate effect on these transistors causes the threshold to become positive, thus making the source voltage less than the gate voltage. As described in greater detail subsequently, this property of the depletion-mode NMOS transistors 302 and 304 allows the input stage of the present invention to operate over a common mode voltage range, which includes V_CC and V_EE.

[00031] FIG. 5 is graph 400 showing a plot of the source voltage vs. the gate voltage of a depletion-mode NMOS transistor, in accordance with an embodiment of the present invention. The graph 400 includes a voltage-gate axis 402, a voltage-source axis 404, and a plot of the source voltage 406 in relation to the gate voltage. As shown by graph 400, when the gate voltage is at V_EE, the source voltage is positive, at about 200 mV, and when the gate voltage is at 1V, the source voltage is about 900 mV. Thus, the source range for the depletion-mode NMOS transistors of the present invention is about 200 mV to 900 mV, which allows these devices to drive the bulk terminals of PMOS transistors, as described in greater detail subsequently. Essentially, the depletion mode NMOS transistors compress the full scale input voltage to a range within the supply voltages V_CC and V_EE.

[00032] Depletion-mode NMOS transistors are generally built on a silicon substrate having four terminals represented as gate, drain, source, and bulk. A processing mask layer defines the region for implanting N-type doping material, such as arsenic, into the silicon to form source and drain regions. The MOS gate region is also defined by a processing mask layer such that the gate conductor and gate oxide physically separate the source and drain regions. N-channel source and drain regions are confined within a well region for receiving a p-type material implant, such as boron. The background concentration of the well region determines the back gate effect, which controls the change in the threshold voltage determined by the source to bulk voltage. Low resistance conducting materials, such as aluminum metal, provide electrical connections to the gate terminal, source terminal, drain terminal, and the bulk terminal.

[00033] FIG. 6A is a schematic diagram of a low voltage rail-to-rail CMOS input stage 500, in accordance with one embodiment of the present invention. The input stage 500 includes a voltage input network 202 and a transconductance network 204. The voltage input network 202 includes depletion-mode NMOS transistor 302 and depletion-mode NMOS transistor 304. The drains of depletion-mode NMOS transistors 302 and 304 are coupled to V_CC and the sources of depletion-mode NMOS transistors 302 and 304 are coupled to the transconductance network 204. Finally, the differential input signal V_IN is provided to the voltage input network 202 through the gates of depletion-mode NMOS transistors 302 and 304.

[00034] In use, the depletion-mode NMOS transistors 302 and 304 are used as source follower devices to drive the transconductance network 204. Advantageously, the depletion-mode NMOS transistors 302 and 304 have a source potential greater than V_EE when their gate voltages are equal to V_EE. Moreover, when the gate voltage the depletion-mode NMOS transistors 302 and 304 transitions to near V_CC, the back-gate effect on these transistors causes their threshold voltages to become positive, thus making their source voltages less than their gate voltages. This property of the depletion-mode NMOS transistors 302 and 304 allows the input stage of the present invention to operate at essentially rail-to-rail.

[00035] The transconductance network 204 includes a differential PMOS transistor pair 502 and 504. The gates of PMOS transistors 502 and 504 are coupled to the ground reference V_EE, and the sources of PMOS transistors 502 and 504, are coupled to current source 506. The drains of PMOS transistors 502 and 504 provide a differential output I₀ to the rest of the system, such as to an operational amplifier. Finally, the bulk terminal of PMOS transistor 502 is coupled to the source of depletion-mode NMOS transistor 302, and the bulk terminal of PMOS transistor 504 is coupled to the source of depletion-mode NMOS transistor 304.

[00036] Essentially, it is desirable to generate a current based on a differential input voltage over the entire common-mode range of the amplifier, including V_CC and V_EE. The present invention addresses this by driving the bulk terminals of the differential pair PMOS transistors 502 and 504, and coupling their gates to the ground reference. Thus, in operation, the depletion-mode NMOS transistors 302 and 304 are utilized as source follower devices to drive the bulk terminals of PMOS transistors 502 and 504. By modulating the bulk voltages of PMOS transistors 502 and 504 via the source follower NMOS transistors 302 and 304, the channels of transistors 502 and 504 are sufficiently modulated to generate an input transconductance for the amplifier.

[00037] The bulk terminals of transistors 502 and 504 can be treated as another gate input to the PMOS transistor, thus, allowing the present invention to operate over the entire input common-mode range of V_CC to V_EE. In the present invention, the range of voltages over which a current can be generated matches the input ranges of the depletion-mode transistors.

[00038] Since the bulk terminals of NMOS transistors 302 and 304 are coupled to V_EE, when the gate voltage of these transistors is at V_EE the source voltage will be positive (i.e., above the gate). Moreover, when the gate voltage transistors 302 and 304 transitions to near V_CC, the back-gate effect on these transistors causes their threshold voltages to become positive, thus making their source voltages less than their gate voltages, as discussed previously with reference to FIG. 5.

[00039] These depletion-mode devices are primarily used for a voltage level shifter in the present invention, rather than to control the transconductance. This allows for greater flexibility than is possible with prior art configurations. For example, the devices may have different sizes and thus have a different transconductance without effecting the bandwidth of the operational amplifier. This is because the current in the differential pair transistors 502 and 504, and the size of these devices, set the bandwidth of the operational amplifier, along with the compensation network in output stage 201.

[00040] FIG. 6B is a schematic diagram of a low voltage rail-to-rail CMOS input stage 600, in accordance with another embodiment of the present invention. The embodiment in FIG. 6B is the dual of the embodiment of in FIG. 6A. The input stage 600 includes a voltage input network 602 and a transconductance network 604. The voltage input network 602 in FIG. 6B includes PMOS transistors 308 and 310. Also, the transconductance network 604 includes a differential NMOS transistor pair 510 and 512. As can be appreciated by those with skill in the art, the embodiment of FIG. 6B performs similar to the embodiment of FIG. 6A. In yet another embodiment, the PMOS transistors in FIG. 6B can be replaced with JFET transistors.

[00041] The choice between the embodiment in FIG. 6A and the embodiment in FIG. 6B essentially depends on the process being implemented. Depending on the process, one of the embodiments may be easier to implement. However, both will provide the essentially same function.

[00042] FIG. 7 is a schematic diagram showing a one volt rail-to-rail CMOS input stage 700, in accordance with another embodiment of the present invention. The input stage 700 includes depletion-mode NMOS transistor 302, depletion-mode NMOS transistor 304, and a transconductance network 204. The drains of depletion-mode NMOS transistors 302 and 304 are coupled to V_CC and the sources of depletion-mode NMOS transistors 302 and 304 are coupled to the transconductance network 204. Finally, the differential input signal V_IN is provided through the gates of depletion-mode NMOS transistors 302 and 304.

[00043] In use, the depletion-mode NMOS transistors 302 and 304 are used as source follower devices to drive the transconductance network 204. Advantageously, the depletion-mode NMOS transistors 302 and 304 have a source potential greater than V_EE when their gate voltages are equal to V_EE. Moreover, when the gate voltages of the depletion-mode NMOS transistors 302 and 304 transitions to near V_CC, the back-gate effect on these transistors causes their threshold voltages to become positive, thus making their source voltages less than their gate voltages. This property of the depletion-mode NMOS transistors 302 and 304 allows the input stage of the present invention to operate at essentially rail-to-rail.

[00044] The transconductance network 204 includes a differential PMOS transistor pair 502 and 504. The gates of PMOS transistors 502 and 504 are coupled to the ground reference V_EE, and the sources of PMOS transistors 502 and 504, are coupled to current source 506. The drains of PMOS transistors 502 and 504 provide differential output current I₀ to the rest of the system, such as to an operational amplifier. Finally, the bulk terminal of PMOS transistor 502 is coupled to the source of depletion-mode NMOS transistor 302, and the bulk terminal of PMOS transistor 504 is coupled to the source of depletion-mode NMOS transistor 304.

[00045] Essentially, it is desirable to generate a current based on a differential input voltage over the entire common-mode range of the amplifier, including V_CC and V_EE. The present invention addresses this by driving the bulk terminals of the differential pair PMOS transistors 502 and 504, and coupling their gates to the ground reference. Thus, in operation, the depletion-mode NMOS transistors 302 and 304 are utilized as source follower devices to drive the bulk terminals of PMOS transistors 502 and 504. By modulating the bulk voltages of PMOS transistors 502 and 504 via the source follower NMOS transistors 302 and 304, the channels of transistors 502 and 504 are sufficiently modulated to generate an input transconductance for the amplifier.

[00046] The bulk terminals of transistors 502 and 504 can be treated as another gate input to the PMOS transistor, thus, allowing the present invention to operate over the entire input common-mode range of V_CC to V_EE. In the present invention, the range of voltages over which a current can be generated matches the input ranges of the depletion-mode transistors.

[00047] Since the bulk terminals of NMOS transistors 302 and 304 are coupled to V_EE, when the gate voltage of these transistors is at V_EE the source voltage will be positive (i.e., above the gate). Moreover, when the gate voltage transistors 302 and 304 transitions to near V_CC, the back-gate effect on these transistors causes their threshold voltages to become positive, thus making their source voltages less than their gate voltages, as discussed previously with reference to FIG. 5.

[00048] The input stage 700 further includes NMOS transistors 702 and 704, which operate as current sources, a current mirror 705 having PMOS transistors 706 and 708, and a folded cascode 709 having NMOS transistors 710 and 712. In operation, the current mirror 705 is used to create a differential-to-single-ended conversion to the output V_O, where transistors 714 and 716 act as current sources. The current from the transconductance network 204 subtracts from the drain currents of transistors 714 and 716. The differential current is then applied to the folded cascode 709, where the current from transistor 712 is replicated by the current mirror 705, and compared to the current in transistor 710 at the output V_O.

[00049] While the present invention has been described in terms of several preferred embodiments, there are many alterations, permutations, and equivalents which may fall within the scope of this invention. It should also be noted that there are many alternative ways of implementing the systems and apparatuses of the present invention. It is therefore intended that the following appended claims be interpreted as including all such alterations, permutations, and equivalents as fall within the true spirit and scope of the present invention.