WO2011056852A2 - High performace lvds driver for scalable supply - Google Patents

High performace lvds driver for scalable supply Download PDF

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Publication number
WO2011056852A2
WO2011056852A2 PCT/US2010/055276 US2010055276W WO2011056852A2 WO 2011056852 A2 WO2011056852 A2 WO 2011056852A2 US 2010055276 W US2010055276 W US 2010055276W WO 2011056852 A2 WO2011056852 A2 WO 2011056852A2
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WIPO (PCT)
Prior art keywords
coupled
transistor
base
differential
current
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PCT/US2010/055276
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French (fr)
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WO2011056852A3 (en
Inventor
Bhavesh G. Bhakta
Mark W. Morgan
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Texas Instruments Japan Ltd
Texas Instruments Inc
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Texas Instruments Japan Ltd
Texas Instruments Inc
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Priority to CN201080050127.4A priority Critical patent/CN102598510B/en
Publication of WO2011056852A2 publication Critical patent/WO2011056852A2/en
Publication of WO2011056852A3 publication Critical patent/WO2011056852A3/en
Anticipated expiration legal-status Critical
Ceased legal-status Critical Current

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Classifications

    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03F—AMPLIFIERS
    • H03F3/00—Amplifiers with only discharge tubes or only semiconductor devices as amplifying elements
    • H03F3/45—Differential amplifiers
    • H03F3/45071—Differential amplifiers with semiconductor devices only
    • H03F3/45076—Differential amplifiers with semiconductor devices only characterised by the way of implementation of the active amplifying circuit in the differential amplifier
    • H03F3/45179—Differential amplifiers with semiconductor devices only characterised by the way of implementation of the active amplifying circuit in the differential amplifier using MOSFET transistors as the active amplifying circuit
    • H03F3/45183—Long tailed pairs
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L25/00—Baseband systems
    • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
    • H04L25/0264—Arrangements for coupling to transmission lines
    • H04L25/028—Arrangements specific to the transmitter end
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03F—AMPLIFIERS
    • H03F2200/00—Indexing scheme relating to amplifiers
    • H03F2200/408—Indexing scheme relating to amplifiers the output amplifying stage of an amplifier comprising three power stages
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03F—AMPLIFIERS
    • H03F2200/00—Indexing scheme relating to amplifiers
    • H03F2200/513—Indexing scheme relating to amplifiers the amplifier being made for low supply voltages
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03F—AMPLIFIERS
    • H03F2203/00—Indexing scheme relating to amplifiers with only discharge tubes or only semiconductor devices as amplifying elements covered by H03F3/00
    • H03F2203/45—Indexing scheme relating to differential amplifiers
    • H03F2203/45644—Indexing scheme relating to differential amplifiers the LC comprising a cross coupling circuit, e.g. comprising two cross-coupled transistors
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03F—AMPLIFIERS
    • H03F2203/00—Indexing scheme relating to amplifiers with only discharge tubes or only semiconductor devices as amplifying elements covered by H03F3/00
    • H03F2203/45—Indexing scheme relating to differential amplifiers
    • H03F2203/45648—Indexing scheme relating to differential amplifiers the LC comprising two current sources, which are not cascode current sources
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03F—AMPLIFIERS
    • H03F2203/00—Indexing scheme relating to amplifiers with only discharge tubes or only semiconductor devices as amplifying elements covered by H03F3/00
    • H03F2203/45—Indexing scheme relating to differential amplifiers
    • H03F2203/45702—Indexing scheme relating to differential amplifiers the LC comprising two resistors

Definitions

  • the invention relates generally to low voltage differential signal (LVDS) drivers and, more particularly, to LVDS drivers that operate with supply ranges between about 3.3V and about 1.8V.
  • LVDS low voltage differential signal
  • LVDS drivers are commonly used with many different types of circuits, such as clocking circuitry. Looking specifically to clock products, especially those with multiple output channels, signal integrity from the LVDS drivers themselves as well as isolation of each LVDS driver from the other drivers are important. Lowering the supply voltage (1.8V, for example) would be desirable to help isolate the drivers, but drivers that operate in these voltage ranges encounter signal degradation issues (especially at high frequencies).
  • the driver 102 generally comprises current source 106 and complementary metal oxide semiconductor (CMOS) transistors Ml through M4.
  • CMOS complementary metal oxide semiconductor
  • driver 102 receives complementary differential signals D and D (which are rail-to-rail signals) at the gates of transistors Ml through M4 and supplies a signal over transmission line 106 (which is terminated by resistor Rl to generally prevent line reflections) to receiver 104.
  • CMOS complementary metal oxide semiconductor
  • D and D which are rail-to-rail signals
  • Rl resistor
  • a preferred embodiment of the invention accordingly, provides an apparatus.
  • the apparatus comprises a first supply rail having a voltage between about 1.8V and about 3.3V; a second supply rail; an input stage coupled between that first and second supply rails that receives a differential input signal, wherein the input stage includes at least one differential input pair of complementary metal oxide semiconductor (CMOS) transistors; and a first output stage that is coupled to the input stage and to at least one of the first and second supply rails, wherein the first output stage includes a plurality of diode-connected transistors; a second output stage that is coupled between the first and second supply rails and to the first output stage, wherein the second output stage includes a first plurality of transistors; a third output stage that is coupled between the first and second supply rails and to the first and second output stages, wherein the third output stage includes a second plurality of transistors; and a pair of differential output terminals that are coupled to each of the second and third stages, wherein the pair of differential output terminals carry a differential current, and CM
  • the input stage further comprises a first NMOS transistor that is coupled to the first supply rail at its drain and that receives at least a portion of the differential input signal at its gate; a second NMOS transistor that is coupled to the first supply rail at its drain and the source of the first NMOS transistor at its source and that receives at least a portion of the differential input signal at its gate; and a current source that is coupled between the sources of the first and second NMOS transistors and the second supply rail.
  • the input stage further comprises a third NMOS transistor that is coupled to at least one of the plurality of diode- connected transistors at its drain and to the drain of the first NMOS transistor at its gate; a fourth NMOS transistor that is coupled to at least one of the plurality of diode-connected transistors at its drain and to the drain of the second NMOS transistor at its gate; and a second current source that is coupled between the sources of the third and fourth NMOS transistors and the second supply rail.
  • the first output stage further comprises a first diode-connected PNP transistor that is coupled to the first supply rail at its emitter and the first input stage at its base and collector; a second diode-connected PNP transistor that is coupled to the second supply rail at its emitter and the first input stage at its base and collector; a first current source that is coupled to the emitter and base of the first PNP transistor and the second supply rail; and a second current source that is coupled to the emitter and base of the second PNP transistor and the second supply rail.
  • the second output stage further comprises a first PNP transistor that is coupled to the first supply rail at its emitter and the base of at least one of the plurality of diode-connected transistors at its base; and a second PNP transistor that is coupled to the first supply rail at its emitter, the base of the first PNP transistor at its base, and one of the differential output terminals at its collector; and a current mirror that is coupled to the collector of the first PNP transistor and one of the differential output terminals.
  • the third output stage further comprises a first PNP transistor that is coupled to the first supply rail at its emitter and the base of at least one of the plurality of diode-connected transistors at its base; and a second PNP transistor that is coupled to the first supply rail at its emitter, the base of the first PNP transistor at its base, and one of the differential output terminals at its collector; and a current mirror that is coupled to the collector of the first PNP transistor and one of the differential output terminals.
  • the differential output terminals further comprises a positive output terminal and a negative output terminal.
  • the first direction of the differential current is a first current traveling into the apparatus through the positive output terminal and a second current traveling out of the apparatus through the negative output terminal.
  • the second direction of the differential current is a first current traveling out of the apparatus through the positive output terminal and a second current traveling into the apparatus through the negative output terminal.
  • an apparatus comprising an input stage having: a first differential pair of CMOS transistors that receive a differential input signal; a second differential pair of CMOS transistors that are coupled to the first differential pair; a first output stage having a first diode-connected bipolar transistor and a second diode-connected bipolar transistor, wherein each of the first and second diode- connected bipolar transistors is coupled to one of CMOS transistors from the second differential pair; a second output stage having: a first set of bipolar transistors, wherein each bipolar transistor from the first set is coupled to the first diode-connected transistor at its base; and a first current mirror coupled that is coupled the collector of at least one of the bipolar transistors from the first set; and a third output stage having: a second set of bipolar transistors, wherein each bipolar transistor from the first set is coupled to the first diode-connected transistor at its base, wherein the collector of at least one bipolar transistor from
  • the first output stage further comprises a first diode-connected PNP transistor that is coupled to at least one of the CMOS transistors at its base and collector; a second diode-connected PNP transistor that is coupled to at least one of the CMOS transistors at its base and collector; a first current source that is coupled to the emitter and base of the first PNP transistor; and a second current source that is coupled to the emitter and base of the second PNP transistor.
  • the first set of bipolar transistors further comprises a first PNP transistor that is coupled to the base of at least one of the plurality of diode-connected bipolar transistors at its base and the first current mirror at its collector; and a second PNP transistor that is coupled to the base of the first PNP transistor at its base and the second current mirror at its collector.
  • the second set of bipolar transistors further comprises a first PNP transistor that is coupled to the base of at least one of the plurality of diode-connected bipolar transistors at its base and the second current mirror at its collector; and a second PNP transistor that is coupled to the base of the first PNP transistor at its base and the first current mirror at its collector.
  • an apparatus comprises signal generating circuitry; and a plurality of low voltage differential signal (LVDS) drivers that are each coupled to the signal generating circuitry, wherein each LVDS driver includes: a positive input terminal that is coupled to the signal generating circuitry; a negative input terminal that is coupled the signal generating circuitry; a first supply rail having a voltage between about 1.8V and about 3.3V; a second supply rail; a positive output terminal; a negative output terminal; a first NMOS transistor that is coupled to the positive input terminal at its gate and the first supply rail at its drain; a second NMOS transistor that is coupled to the negative input terminal at its gate and the first supply rail at its drain; a first current source that is coupled between the sources of the first and second NMOS transistors and the second supply rail; a third NMOS transistor that is coupled to the drain of the first NMOS transistor at its gate; a fourth NMOS transistor that is coupled to the drain of the
  • the signal generating circuitry further comprises: a phase locked loop (PLL); and intermediate circuitry coupled between the PLL and each of the LVDS drivers.
  • PLL phase locked loop
  • a method comprises receiving a differential signal by a differential input pair of CMOS transistor, wherein the differential input signal has a state; mirroring one of a first current and a second current, generated in a first output stage from a supply voltage between about 1.8V and about 3.3V, in a corresponding one of a second output stage and a third output stage based at least in part on the state of differential signal; and generating a differential current at a first output terminal and a second output terminal, wherein direction of the differential current is based at least in part on the state of the differential signal, and wherein the second and third output stages are coupled to each of first and second output terminals.
  • the state of the differential signal further comprises a first state and a second state
  • the step of mirroring further comprises generating the first current in a first diode-connected bipolar transistor for the first state; mirroring the first current in the second output stage for the first state; generating the second current in a second diode-connected bipolar transistor in the second state; and mirroring the second current in the third output stage for the second state.
  • the step of generating further comprises: generating the differential current in a first direction for the first state; and generating the differential current in a second direction for the second state.
  • FIG. 1 is a block diagram of a conventional circuit using an LVDS driver with CMOS transistors
  • FIG. 2 is an example of a block diagram of a system in accordance with a preferred embodiment of the invention.
  • FIG. 3 is an example of a circuit diagram of the LVDS drivers of FIG. 2.
  • the reference numeral 200 generally designates a system in accordance with a preferred embodiment of the invention.
  • the system 200 generally comprises a phased locked loop (PLL) 202, intermediate circuitry 204, and drivers 300-1 to 300- N.
  • PLL 202 can generally operate to provide one or more clock signals to intermediate circuit 204 (which can be comprised of a variety of different types of circuit).
  • the intermediate circuitry 204 can then distribute signals to divers 300-1 to 300-N for transmission across differential transmission lines 206-1 to 206-N (respectively).
  • Driver 300 (which is generally the same as each of drivers 300-1 to 300-N) can be seen.
  • Driver 300 is generally divided into an input stage 302 and three output stage 304, 306, and 308.
  • This driver 300 takes advantage of the characteristics of bipolar and CMOS transistors so that the supply voltage can be between about 1.8V and about 3.3V (which is provided on supply rail VDD).
  • the input stage is generally comprised of differential input pairs of CMOS transistors M5/M6 and M7/M8, resistors Rl and Rl, and current sources 310 and 312.
  • the first input stage 304 is generally comprised of resistors R4 and R5, diode-connected PNP transistors Ql and Q2, and current sources 314 and 316.
  • the second input stage 306 is generally comprised of PNP transistors Q3 and Q4, resistors R5 and R6 and current mirror Q7, Q8, R10, and Rl l
  • the third output stage 308 generally comprises resistors R8, R9, R14, and R15, transistors Q5 and Q6, and current mirror Q9, Q10, R12, and R13.
  • transistors M5 and M6 can be replaced with bipolar transistors.
  • differential input signals are received by input terminals INM and INP so that an output signal having a differential current can be provided by or carried by output terminals OUTP and OUTM.
  • the state of the differential signal (which does not need to be fully rail-to-rail for switching), as applied to terminals INM and INP, influences the direction of the differential current carried by terminals OUTP and OUTM.
  • the relative currents carried by the terminals OUTP and OUTM generally comprise the differential current with the direction of the differential current being related to the relative directions carried by terminals OUTP and OUTM.
  • the collector of PNP transistor Q5 is coupled to terminal OUTP, the first current is carried out of the driver 300 by terminal OUTP. Additionally, because the collector of PNP transistor Q6 is coupled to the diode-connected NPN transistor Q10, the current mirrored by PNP transistor Q6 is provided to diode-connected NPN transistor Q10 and mirrored by NPN transistor Q9 (which is coupled to terminal OUTM at its collector), allowing the second current to be carried into the driver 300 by terminal OUTM.
  • the collector of PNP transistor Q3 is coupled to the diode-connected NPN transistor Q7, the current mirrored by PNP transistor Q3 is provided to diode-connected NPN transistor Q7 and mirrored by NPN transistor Q8 (which is coupled to terminal OUTP at its collector), allowing the first current to be carried into the driver 300 by terminal OUTP.
  • driver 300 also includes several other features that enhance its operation.
  • current sources 314 and 316 are coupled to the gates and collectors of diode- connected PNP transistors Ql and Q2, respectively. These current sources 316 and 314 (which are coupled to supply rail VSS that is typically at ground) are provided to allow a standing current to remain in transistors Ql and Q2 (partially saturated), which, in turn, causes a quiescent current to remain in transistors Q3 to Q10.
  • current sources 314 and 316 small swing differential signals may be applied to transistors M7 and M8, and much more rapid switching can take place because of the partial saturation of transistors Ql through Q10.
  • resistors R14 and R15 are coupled between terminals OUTP and OUTM so as to provide a common mode voltage to common mode terminal VCM.
  • driver 300 As a result of the configuration of the driver 300, several advantages over conventional LVDS drivers can be realized. For example, in systems (such as system 200), there is better channel-to-channel isolation or reduced electromagnetic interference because of small- differential swings (which are generally not rail-to-rail) that generate minimal aggressor noise and because the fully-differential signaling is more immune to noise from adjacent channels. Additionally, there is lower additive jitter and less phase noise at a IMHz offset by avoiding the use of short-channel CMOS devices in a critical path. Moreover, driver 300 maintains a generally constant amplitude at high frequencies (i.e., up to 100MHz). Additionally, the supply voltage is scalable (generally down to about 1.8V), and the phase noise remains generally constant across the supply.

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  • Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Signal Processing (AREA)
  • Amplifiers (AREA)
  • Logic Circuits (AREA)

Abstract

An apparatus has a first supply rail (VDD) supplying a voltage between about 1.8V and about 3.3V; a second supply rail (Vss); and an input stage (302) coupled between that first and second supply rails that receives a differential input signal (INP, INM). The input stage includes at least one differential input pair of complementary metal oxide semiconductor (CMOS) transistors (M5, M6). A first output stage (304) is coupled to the input stage and to at least one of the first and second supply rails. The first output stage includes a plurality of diode-connected transistors (Ql, Q2). A second output stage (306) is coupled between the first and second supply rails and to the first output stage. The second output stage includes a first plurality of transistors (Q3, Q4). A third output stage (308) is coupled between the first and second supply rails and to the first and second output stages. The third output stage includes a second plurality of transistors (Q5, Q6) and a pair of differential output terminals (OUTP, OUTM)coupled to each of the second and third stages. The pair of differential output terminals carry a differential current that travels in a first direction and is supplied through the second output stage when the differential input signal is in a first state, and travels in a second direction and is supplied through the third output stage when the differential input signal is in a second state.

Description

HIGH PERFORMACE LVDS DRIVER FOR SCALABLE SUPPLY
[0001] The invention relates generally to low voltage differential signal (LVDS) drivers and, more particularly, to LVDS drivers that operate with supply ranges between about 3.3V and about 1.8V.
BACKGROUND
[0002] LVDS drivers are commonly used with many different types of circuits, such as clocking circuitry. Looking specifically to clock products, especially those with multiple output channels, signal integrity from the LVDS drivers themselves as well as isolation of each LVDS driver from the other drivers are important. Lowering the supply voltage (1.8V, for example) would be desirable to help isolate the drivers, but drivers that operate in these voltage ranges encounter signal degradation issues (especially at high frequencies).
[0003] Turning FIG. 1, an example conventional circuit 100 employing an LVDS 102 can be seen. The driver 102 generally comprises current source 106 and complementary metal oxide semiconductor (CMOS) transistors Ml through M4. In operation, driver 102 receives complementary differential signals D and D (which are rail-to-rail signals) at the gates of transistors Ml through M4 and supplies a signal over transmission line 106 (which is terminated by resistor Rl to generally prevent line reflections) to receiver 104. Because of the use of these CMOS transistors or switches, though, amplitude degradation and noise increase with frequency, and there are significant switching transients. Another conventional alternative is to use a driver that employs bipolar transistors; however, these types of drivers generally require high voltages (typically 3.3V or greater).
[0004] Some other examples of conventional circuits are: U.S. Patent No. 6,617,888; U.S.
Patent No. 6,791,377; and U.S. Patent Pre-Grant Publ. No. 2003/0227303.
SUMMARY
[0005] A preferred embodiment of the invention, accordingly, provides an apparatus. The apparatus comprises a first supply rail having a voltage between about 1.8V and about 3.3V; a second supply rail; an input stage coupled between that first and second supply rails that receives a differential input signal, wherein the input stage includes at least one differential input pair of complementary metal oxide semiconductor (CMOS) transistors; and a first output stage that is coupled to the input stage and to at least one of the first and second supply rails, wherein the first output stage includes a plurality of diode-connected transistors; a second output stage that is coupled between the first and second supply rails and to the first output stage, wherein the second output stage includes a first plurality of transistors; a third output stage that is coupled between the first and second supply rails and to the first and second output stages, wherein the third output stage includes a second plurality of transistors; and a pair of differential output terminals that are coupled to each of the second and third stages, wherein the pair of differential output terminals carry a differential current, and wherein the differential current travels in a first direction and is supplied through the second output stage when the differential input signal is in a first state, and wherein the differential current travels in a second direction and is supplied through the third output stage when the differential input signal is in a second state.
[0006] In accordance with a preferred embodiment of the invention, the input stage further comprises a first NMOS transistor that is coupled to the first supply rail at its drain and that receives at least a portion of the differential input signal at its gate; a second NMOS transistor that is coupled to the first supply rail at its drain and the source of the first NMOS transistor at its source and that receives at least a portion of the differential input signal at its gate; and a current source that is coupled between the sources of the first and second NMOS transistors and the second supply rail.
[0007] In accordance with a preferred embodiment of the invention, the input stage further comprises a third NMOS transistor that is coupled to at least one of the plurality of diode- connected transistors at its drain and to the drain of the first NMOS transistor at its gate; a fourth NMOS transistor that is coupled to at least one of the plurality of diode-connected transistors at its drain and to the drain of the second NMOS transistor at its gate; and a second current source that is coupled between the sources of the third and fourth NMOS transistors and the second supply rail.
[0008] In accordance with a preferred embodiment of the invention, the first output stage further comprises a first diode-connected PNP transistor that is coupled to the first supply rail at its emitter and the first input stage at its base and collector; a second diode-connected PNP transistor that is coupled to the second supply rail at its emitter and the first input stage at its base and collector; a first current source that is coupled to the emitter and base of the first PNP transistor and the second supply rail; and a second current source that is coupled to the emitter and base of the second PNP transistor and the second supply rail. [0009] In accordance with a preferred embodiment of the invention, the second output stage further comprises a first PNP transistor that is coupled to the first supply rail at its emitter and the base of at least one of the plurality of diode-connected transistors at its base; and a second PNP transistor that is coupled to the first supply rail at its emitter, the base of the first PNP transistor at its base, and one of the differential output terminals at its collector; and a current mirror that is coupled to the collector of the first PNP transistor and one of the differential output terminals.
[0010] In accordance with a preferred embodiment of the invention, the third output stage further comprises a first PNP transistor that is coupled to the first supply rail at its emitter and the base of at least one of the plurality of diode-connected transistors at its base; and a second PNP transistor that is coupled to the first supply rail at its emitter, the base of the first PNP transistor at its base, and one of the differential output terminals at its collector; and a current mirror that is coupled to the collector of the first PNP transistor and one of the differential output terminals.
[0011] In accordance with a preferred embodiment of the invention, the differential output terminals further comprises a positive output terminal and a negative output terminal.
[0012] In accordance with a preferred embodiment of the invention, the first direction of the differential current is a first current traveling into the apparatus through the positive output terminal and a second current traveling out of the apparatus through the negative output terminal.
[0013] In accordance with a preferred embodiment of the invention, the second direction of the differential current is a first current traveling out of the apparatus through the positive output terminal and a second current traveling into the apparatus through the negative output terminal.
[0014] In accordance with a preferred embodiment of the invention, an apparatus is provided. The apparatus comprises an input stage having: a first differential pair of CMOS transistors that receive a differential input signal; a second differential pair of CMOS transistors that are coupled to the first differential pair; a first output stage having a first diode-connected bipolar transistor and a second diode-connected bipolar transistor, wherein each of the first and second diode- connected bipolar transistors is coupled to one of CMOS transistors from the second differential pair; a second output stage having: a first set of bipolar transistors, wherein each bipolar transistor from the first set is coupled to the first diode-connected transistor at its base; and a first current mirror coupled that is coupled the collector of at least one of the bipolar transistors from the first set; and a third output stage having: a second set of bipolar transistors, wherein each bipolar transistor from the first set is coupled to the first diode-connected transistor at its base, wherein the collector of at least one bipolar transistor from the second set is coupled to the first current mirror; and a second current mirror coupled that is coupled the collector of at least one bipolar transistors from the first set and at least one bipolar transistor from the second set.
[0015] In accordance with a preferred embodiment of the invention, the first output stage further comprises a first diode-connected PNP transistor that is coupled to at least one of the CMOS transistors at its base and collector; a second diode-connected PNP transistor that is coupled to at least one of the CMOS transistors at its base and collector; a first current source that is coupled to the emitter and base of the first PNP transistor; and a second current source that is coupled to the emitter and base of the second PNP transistor.
[0016] In accordance with a preferred embodiment of the invention, the first set of bipolar transistors further comprises a first PNP transistor that is coupled to the base of at least one of the plurality of diode-connected bipolar transistors at its base and the first current mirror at its collector; and a second PNP transistor that is coupled to the base of the first PNP transistor at its base and the second current mirror at its collector.
[0017] In accordance with a preferred embodiment of the invention, the second set of bipolar transistors further comprises a first PNP transistor that is coupled to the base of at least one of the plurality of diode-connected bipolar transistors at its base and the second current mirror at its collector; and a second PNP transistor that is coupled to the base of the first PNP transistor at its base and the first current mirror at its collector.
[0018] In accordance with a preferred embodiment of the invention, an apparatus is provided. The apparatus comprises signal generating circuitry; and a plurality of low voltage differential signal (LVDS) drivers that are each coupled to the signal generating circuitry, wherein each LVDS driver includes: a positive input terminal that is coupled to the signal generating circuitry; a negative input terminal that is coupled the signal generating circuitry; a first supply rail having a voltage between about 1.8V and about 3.3V; a second supply rail; a positive output terminal; a negative output terminal; a first NMOS transistor that is coupled to the positive input terminal at its gate and the first supply rail at its drain; a second NMOS transistor that is coupled to the negative input terminal at its gate and the first supply rail at its drain; a first current source that is coupled between the sources of the first and second NMOS transistors and the second supply rail; a third NMOS transistor that is coupled to the drain of the first NMOS transistor at its gate; a fourth NMOS transistor that is coupled to the drain of the second NMOS transistor at its gate; a second current source that is coupled to the sources of the third and fourth NMOS transistors; a first PNP transistor that is coupled to the drain of the third NMOS transistor at its base and collector and the first supply rail at its emitter; a second PNP transistor that is coupled to the drain of the fourth NMOS transistor at its base and collector and the first supply rail at its emitter; a third current source that is coupled to the emitter and base of the first PNP transistor; a fourth current source that is coupled to the emitter and base of the second PNP transistor; a third PNP transistor that is coupled to the first supply rail at its emitter and the base of the first PNP transistor at its base; a fourth PNP transistor that is coupled to the first supply rail at its emitter, the base of the first PNP transistor at its base, and the negative output terminal at its collector; a first current mirror that is coupled to the second supply rail and the collector of the third PNP transistor and the positive output terminal; a fifth PNP transistor that is coupled to the first supply rail at its emitter and the base of the second PNP transistor at its base; a sixth PNP transistor that is coupled to the first supply rail at its emitter, the base of the second PNP transistor at its base, and the positive output terminal at its collector; and a second current mirror that is coupled to the second supply rail, the collector of the fifth PNP transistor, and the negative output terminal.
[0019] In accordance with a preferred embodiment of the invention, the signal generating circuitry further comprises: a phase locked loop (PLL); and intermediate circuitry coupled between the PLL and each of the LVDS drivers.
[0020] In accordance with a preferred embodiment of the invention, a method is provided. The method comprises receiving a differential signal by a differential input pair of CMOS transistor, wherein the differential input signal has a state; mirroring one of a first current and a second current, generated in a first output stage from a supply voltage between about 1.8V and about 3.3V, in a corresponding one of a second output stage and a third output stage based at least in part on the state of differential signal; and generating a differential current at a first output terminal and a second output terminal, wherein direction of the differential current is based at least in part on the state of the differential signal, and wherein the second and third output stages are coupled to each of first and second output terminals.
[0021] In accordance with a preferred embodiment of the invention, the state of the differential signal further comprises a first state and a second state, and wherein the step of mirroring further comprises generating the first current in a first diode-connected bipolar transistor for the first state; mirroring the first current in the second output stage for the first state; generating the second current in a second diode-connected bipolar transistor in the second state; and mirroring the second current in the third output stage for the second state.
[0022] In accordance with a preferred embodiment of the invention, the step of generating further comprises: generating the differential current in a first direction for the first state; and generating the differential current in a second direction for the second state.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] FIG. 1 is a block diagram of a conventional circuit using an LVDS driver with CMOS transistors;
[0024] FIG. 2 is an example of a block diagram of a system in accordance with a preferred embodiment of the invention; and
[0025] FIG. 3 is an example of a circuit diagram of the LVDS drivers of FIG. 2.
DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS
[0026] Referring to FIG. 2 of the drawings, the reference numeral 200 generally designates a system in accordance with a preferred embodiment of the invention. The system 200 generally comprises a phased locked loop (PLL) 202, intermediate circuitry 204, and drivers 300-1 to 300- N. PLL 202 can generally operate to provide one or more clock signals to intermediate circuit 204 (which can be comprised of a variety of different types of circuit). The intermediate circuitry 204 can then distribute signals to divers 300-1 to 300-N for transmission across differential transmission lines 206-1 to 206-N (respectively).
[0027] Turning to FIG. 3, an example of the drivers 300 (which is generally the same as each of drivers 300-1 to 300-N) can be seen. Driver 300 is generally divided into an input stage 302 and three output stage 304, 306, and 308. This driver 300 takes advantage of the characteristics of bipolar and CMOS transistors so that the supply voltage can be between about 1.8V and about 3.3V (which is provided on supply rail VDD). The input stage is generally comprised of differential input pairs of CMOS transistors M5/M6 and M7/M8, resistors Rl and Rl, and current sources 310 and 312. The first input stage 304 is generally comprised of resistors R4 and R5, diode-connected PNP transistors Ql and Q2, and current sources 314 and 316. The second input stage 306 is generally comprised of PNP transistors Q3 and Q4, resistors R5 and R6 and current mirror Q7, Q8, R10, and Rl l, and the third output stage 308 generally comprises resistors R8, R9, R14, and R15, transistors Q5 and Q6, and current mirror Q9, Q10, R12, and R13. Alternatively, transistors M5 and M6 can be replaced with bipolar transistors.
[0028] In operation, differential input signals are received by input terminals INM and INP so that an output signal having a differential current can be provided by or carried by output terminals OUTP and OUTM. The state of the differential signal (which does not need to be fully rail-to-rail for switching), as applied to terminals INM and INP, influences the direction of the differential current carried by terminals OUTP and OUTM. The relative currents carried by the terminals OUTP and OUTM generally comprise the differential current with the direction of the differential current being related to the relative directions carried by terminals OUTP and OUTM.
[0029] For a state of the differential input signal where a high signal is applied to input terminal INP and a low signal is applied to input terminal INM, a first current would travel out through terminal OUTP, and a second current would travel in through terminal OUTM. To accomplish this, the high and low signals are applied to the gates of NMOS transistors M5 and M6, respectively. As a result, high and low signals are respectively applied to the gates of transistors M7 and M8 (which are coupled to the drains of transistors M6 and M5, respectively). Current, then, flows through resistor R4 and diode-connected PNP transistor Ql, which is mirrored by PNP transistors Q5 and Q6. Because the collector of PNP transistor Q5 is coupled to terminal OUTP, the first current is carried out of the driver 300 by terminal OUTP. Additionally, because the collector of PNP transistor Q6 is coupled to the diode-connected NPN transistor Q10, the current mirrored by PNP transistor Q6 is provided to diode-connected NPN transistor Q10 and mirrored by NPN transistor Q9 (which is coupled to terminal OUTM at its collector), allowing the second current to be carried into the driver 300 by terminal OUTM.
[0030] Alternatively, for a state of the differential input signal where a low signal is applied to input terminal INP and a high signal is applied to input terminal INM, the first current would travel in through terminal OUTP, and the second current would travel out through terminal OUTM. To accomplish this, the high and low signals are applied to the gates of NMOS transistors M6 and M5, respectively. As a result, high and low signals are respectively applied to the gate of transistors M8 and M7. Current, then, flows through resistor R5 and diode- connected PNP transistor Q2, which is mirrored by PNP transistors Q3 and Q4. Because the collector of PNP transistor Q4 is coupled to terminal OUTM, the second current is carried out of the driver 300 by terminal OUTM. Additionally, because the collector of PNP transistor Q3 is coupled to the diode-connected NPN transistor Q7, the current mirrored by PNP transistor Q3 is provided to diode-connected NPN transistor Q7 and mirrored by NPN transistor Q8 (which is coupled to terminal OUTP at its collector), allowing the first current to be carried into the driver 300 by terminal OUTP.
[0031] As shown, driver 300 also includes several other features that enhance its operation. For example, current sources 314 and 316 are coupled to the gates and collectors of diode- connected PNP transistors Ql and Q2, respectively. These current sources 316 and 314 (which are coupled to supply rail VSS that is typically at ground) are provided to allow a standing current to remain in transistors Ql and Q2 (partially saturated), which, in turn, causes a quiescent current to remain in transistors Q3 to Q10. By providing current sources 314 and 316, small swing differential signals may be applied to transistors M7 and M8, and much more rapid switching can take place because of the partial saturation of transistors Ql through Q10. Additionally, resistors R14 and R15 are coupled between terminals OUTP and OUTM so as to provide a common mode voltage to common mode terminal VCM.
[0032] As a result of the configuration of the driver 300, several advantages over conventional LVDS drivers can be realized. For example, in systems (such as system 200), there is better channel-to-channel isolation or reduced electromagnetic interference because of small- differential swings (which are generally not rail-to-rail) that generate minimal aggressor noise and because the fully-differential signaling is more immune to noise from adjacent channels. Additionally, there is lower additive jitter and less phase noise at a IMHz offset by avoiding the use of short-channel CMOS devices in a critical path. Moreover, driver 300 maintains a generally constant amplitude at high frequencies (i.e., up to 100MHz). Additionally, the supply voltage is scalable (generally down to about 1.8V), and the phase noise remains generally constant across the supply.
[0033] Those skilled in the art will appreciate that many other embodiments and variations are also possible within the scope of the claimed invention. Embodiments having different combinations of one or more of the features or steps described in the context of example embodiments having all or just some of such features or steps are also intended to be covered hereby.

Claims

CLAIMS What is claimed is:
1. An apparatus comprising:
an input stage having a first differential pair of CMOS transistors that receive a differential input signal; and a second differential pair of CMOS transistors coupled to the first differential pair;
a first output stage having a first diode-connected bipolar transistor and a second diode- connected bipolar transistor; wherein each of the first and second diode-connected bipolar transistors is coupled to one of the CMOS transistors of the second differential pair;
a second output stage having a first set of bipolar transistors; wherein each bipolar transistor of the first set is coupled to the first diode -connected bipolar transistor at its base; and a first current mirror coupled to the collector of at least one of the bipolar transistors of the first set; and
a third output stage having a second set of bipolar transistors; wherein each bipolar transistor of the first set is coupled to the first diode-connected transistor at its base; wherein the collector of at least one bipolar transistor from the second set is coupled to the first current mirror; and wherein a second current mirror is coupled the collector of at least one bipolar transistor of the first set and at least one bipolar transistor of the second set.
2. The apparatus of Claim 1, wherein the first differential pair of CMOS transistors further comprises:
a first NMOS transistor that receives at least a portion of the differential input signal at its gate;
a second NMOS transistor that is coupled to the source of the first NMOS transistor at its source and that receives at least a portion of the differential input signal at its gate; and
a current source that is coupled to the sources of the first and second NMOS transistors.
3. The apparatus of Claim 2, wherein the second differential pair of CMOS transistors further comprises: a third NMOS transistor that is coupled to at least one of the plurality of diode-connected bipolar transistors at its drain and to the drain of the first NMOS transistor at its gate;
a fourth NMOS transistor that is coupled to at least one of the plurality of diode- connected bipolar transistors at its drain and to the drain of the second NMOS transistor at its gate; and
a second current source that is coupled to the sources of the third and fourth NMOS transistors.
4. The apparatus of Claim 3, wherein the first output stage further comprises:
a first diode-connected PNP transistor that is coupled to at least one of the CMOS transistors at its base and collector;
a second diode-connected PNP transistor that is coupled to at least one of the CMOS transistors at its base and collector;
a first current source that is coupled to the emitter and base of the first PNP transistor; and
a second current source that is coupled to the emitter and base of the second PNP transistor.
5. The apparatus of Claim 4, wherein the first set of bipolar transistors further comprises:
a first PNP transistor that is coupled to the base of at least one of the plurality of diode- connected bipolar transistors at its base and the first current mirror at its collector; and
a second PNP transistor that is coupled to the base of the first PNP transistor at its base and the second current mirror at its collector.
6. The apparatus of Claim 5, wherein the second set of bipolar transistors further comprises:
a first PNP transistor that is coupled to the base of at least one of the plurality of diode- connected bipolar transistors at its base and the second current mirror at its collector; and
a second PNP transistor that is coupled to the base of the first PNP transistor at its base and the first current mirror at its collector.
7. An apparatus comprising signal generating circuitry and a plurality of low voltage differential signal (LVDS) drivers that are each coupled to the signal generating circuitry, wherein each LVDS driver includes:
a positive input terminal that is coupled to the signal generating circuitry;
a negative input terminal that is coupled the signal generating circuitry;
a first supply rail for supplying a voltage;
a second supply rail;
a positive output terminal;
a negative output terminal;
a first NMOS transistor that is coupled to the positive input terminal at its gate and the first supply rail at its drain;
a second NMOS transistor that is coupled to the negative input terminal at its gate and the first supply rail at its drain;
a first current source that is coupled between the sources of the first and second NMOS transistors and the second supply rail;
a third NMOS transistor that is coupled to the drain of the first NMOS transistor at its gate;
a fourth NMOS transistor that is coupled to the drain of the second NMOS transistor at its gate;
a second current source that is coupled to the sources of the third and fourth NMOS transistors;
a first PNP transistor that is coupled to the drain of the third NMOS transistor at its base and collector and the first supply rail at its emitter;
a second PNP transistor that is coupled to the drain of the fourth NMOS transistor at its base and collector and the first supply rail at its emitter;
a third current source that is coupled to the emitter and base of the first PNP transistor; a fourth current source that is coupled to the emitter and base of the second PNP transistor;
a third PNP transistor that is coupled to the first supply rail at its emitter and the base of the first PNP transistor at its base; a fourth PNP transistor that is coupled to the first supply rail at its emitter, the base of the first PNP transistor at its base, and the negative output terminal at its collector;
a first current mirror that is coupled to the second supply rail and the collector of the third PNP transistor and the positive output terminal;
a fifth PNP transistor that is coupled to the first supply rail at its emitter and the base of the second PNP transistor at its base;
a sixth PNP transistor that is coupled to the first supply rail at its emitter, the base of the second PNP transistor at its base, and the positive output terminal at its collector; and
a second current mirror that is coupled to the second supply rail, the collector of the fifth PNP transistor, and the negative output terminal.
8. A method comprising:
receiving a differential signal by a differential input pair of CMOS transistor, wherein the differential input signal has a state;
mirroring one of a first current and a second current, generated in a first output stage from a supply voltage between about 1.8V and about 3.3V, in a corresponding one of a second output stage and a third output stage based at least in part on the state of differential signal; and generating a differential current at a first output terminal and a second output terminal, wherein direction of the differential current is based at least in part on the state of the differential signal, and wherein the second and third output stages are coupled to each of first and second output terminals.
9. The method of Claim 8, wherein the state of the differential signal further comprises a first state and a second state, and wherein the step of mirroring further comprises: generating the first current in a first diode-connected bipolar transistor for the first state; mirroring the first current in the second output stage for the first state;
generating the second current in a second diode-connected bipolar transistor in the second state; and
mirroring the second current in the third output stage for the second state.
10. The method of Claim 9, wherein the step of generating further comprises: generating the differential current in a first direction for the first state; and generating the differential current in a second direction for the second state.
PCT/US2010/055276 2009-11-05 2010-11-03 High performace lvds driver for scalable supply Ceased WO2011056852A2 (en)

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US20110102083A1 (en) 2011-05-05

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