WO2002008846A1

WO2002008846A1 - Standard cmos compatible band gap reference

Info

Publication number: WO2002008846A1
Application number: PCT/US2001/023291
Authority: WO
Inventors: Sam Ochi
Original assignee: Ixys Corporation
Priority date: 2000-07-21
Filing date: 2001-07-23
Publication date: 2002-01-31
Also published as: AU2001277144A1

Abstract

The present invention provides a CMOS low noise band gap reference circuit (10) that outputs a substantially constant reference voltage VREF. Band gap reference circuits of the present invention have an amplifier that includes a differential pair of bipolar junction transistors and a feedback circuit that adjusts it current to compensate for variations in the provide a output reference voltage VREF that is substantially constant over a range of temperature and a range of supply voltage.

Description

STANDARD CMOS COMPATIBLE BAND GAP REFERENCE

CROSS-REFERENCES TO RELATED APPLICATIONS [01] This application claims priority from U.S. Provisional Patent Application No. 60/220,068, filed July 21, 2000, which is incorporated by reference herein.

BACKGROUND OF THE INVENTION [02] The present invention relates to band gap reference circuits, and more particularly, to band gap reference circuits that maintain a constant output voltage over a range of temperature and bias current.

[03] A band gap reference circuit provides a constant output reference voltage V_REF- Problems may arise if the output reference voltage V_REF varies even by a small amount such as a few hundred millivolts over a range of temperature or bias current.

Therefore, it is desirable to provide a band gap reference circuit that provides an output reference voltage V_RE_F that is substantially constant over a range of temperature and bias current.

[04] Previously known standard CMOS band gap reference circuits typically include an amplifier that comprises a differential pair of p-channel MOS transistors.

V_REF is determined by the voltage at the gate of one of the p-channel MOS transistors. Excess charge carriers can become trapped in the silicon to silicon dioxide (SiO₂) interface in

MOS transistors. The excess charge may cause variations in the threshold voltages of the

MOS transistors in the differential pair of the amplifier. For example, the threshold voltages of the two MOS transistors in the differential pair may differ by more than 5 mV. This difference introduces an offset voltage into the amplifier which appears at V_REF of the band gap reference circuit. The offset voltage can prevent the band gap reference circuit from being adjusted with trimming resistors so that V_REF remains constant with temperature changes.

[05] In addition, the charge trapped in the silicon/SiO₂ interface of the differential pair MOS transistors in the band gap reference amplifier can vary over time causing V_REF to change over time even at a constant temperature. These variations in V_REF cause undesirable 1/f output noise. Also, the p-channel MOS transistors in the differential pair may introduce thermal noise at V _EF due to the nature of MOS transistors, which is also undesirable.

[06] A further disadvantage of previously known standard CMOS band gap reference circuits is that they are sensitive to relatively small changes in the supply voltage Vcc- Small changes in Vcc cause variations in the bias current through the band gap reference circuit, which can cause undesirable changes in V_REF-

[07] It would therefore be desirable to provide a less noisy band gap reference circuit in CMOS technology that provides a substantially constant output reference voltage V_RE_F over a range of supply voltage and a range of temperature.

BRIEF SUMMARY OF THE INVENTION [08] The present invention provides CMOS low noise band gap reference circuits that output a substantially constant reference voltage V _EF- Band gap reference circuits of the present invention have an amplifier that includes a differential pair of bipolar junction transistors. Each of the bipolar junction transistors are coupled to a first or a second plurality of bipolar junction transistors or a first and second plurality of diodes. The first and second plurality of transistors or diodes are coupled to a plurality of resistors. When the temperature of the circuit varies over a range, the change in the voltage drop across the resistors compensates for the change in the voltage drop across the transistors or the diodes so that V_REF remains substantially constant.

[09] A feedback circuit is coupled to the amplifier. The feedback circuit adjusts its current to compensate for variations in the supply current so that the V_REF remains substantially constant. The band gap reference circuits of the present invention provide a output reference voltage V_REF that is substantially constant with variations over a range of temperature and supply voltage. Band gap reference circuits of the present invention may be fabricated using standard CMOS process techniques.

BRIEF DESCRIPTION OF THE DRAWINGS [10] FIG. 1. is a schematic of an embodiment of a band gap reference circuit of the present invention;

[11] FIGS. 2A-2B illustrate top down and cross sectional layout views, respectively, of a CMOS compatible lateral PNP bipolar junction transistor in accordance with the principles of the present invention; [12] FIG. 2C illustrates a schematic of the lateral PNP BJT of FIGS. 2A- 2B;

[13] FIGS. 3A-3B illustrate top down and cross sectional layout views, respectively, of a CMOS compatible vertical PNP bipolar junction transistor in accordance with the principles of the present invention;

[14] FIG. 3C illustrates a schematic of the vertical PNP bipolar junction transistor of FIGS. 3A-3B; and

[15] FIG. 4 is a schematic of another embodiment of a band gap reference circuit of the present invention.

DETAILED DESCRIPTION OF THE INVENTION [16] Band gap reference circuit 10 shown in FIG. 1 is an embodiment of the present invention. Reference circuit 10 receives supply voltage Vcc from an external voltage source. Bias current source 11, which has a finite impedance, provides a reference current source that outputs a current equal to 151 to reference circuit 10. For example, 151 may represent 150 μA at 25 °C. Bias current source 11 is proportional to absolute temperature. Therefore, changes in the temperature of circuit 10 or changes in Vcc cause the current through current source 11 to vary.

[17] The current through bias current source 11 is divided through p- channel MOS transistors M1-M8 and Ml 1 according to the predetermined proportions which are determined by the relative channel width-to-length (W L) ratios of MOSFET transistors M1-M8 and Ml 1. For example, the W/L ratio of transistors M1:M2:M3:M4:M5:M6:M7:M8 may be 4:1:1:1:1:1:1:1 which provides a current ratio of 41 : 1 : I : I : I : I : I : I as shown in FIG. 1. MOS transistor Ml 1 has a W/L that is eight times the W/L of MOS transistors M9 and Ml 0. Other suitable transistor ratios may be used, if desired, according to the principles of the present invention. In a further embodiment of the present invention, MOSFET transistors M1-M8 may be replaced with PNP bipolar junction transistors that are sized to provide the desired bias current ratio in circuit 10.

[18] Band gap reference circuit 10 includes PNP bipolar junction transistors (BJTs) Ql and Q2, which from a differential pair for an amplifier. When the voltages at the bases of Ql and Q2 are equal, a current equal to one half of current I (172) flows through both Ql and Q2, and n-channel MOS transistors M9 and M10, which form a current mirror. [19] Circuit 10 also includes PNP BJTs Q4-Q9. The base of transistors Q2 is coupled to the output reference voltage V_REF, which is determined by the equation (1). ' REF ~ ' Rl ^""^" ' BE-Q9 ^"*^" ' BE-Q1 ^"*^" ' BE-Q5

[20] V_R2 is the voltage drop across R2, V_BE-Q₉ equals the base-emitter voltage drop across Q9, V_BE-Q equals the base-emitter voltage drop across Q7, and V_BE-_QS equals the base-emitter voltage drop across Q5. The voltage at the base of transistor Ql is determined by equation (2).

" Q\ ~ y R\ ^"*^" ' R2 ^"*^" * BE-QZ ^"*^* * BE-Q6 ^""^" ' BE-Q4 (.*")

[21] V_QΪ is the base voltage of transistor Ql, V_RI is the voltage drop across resistor Rl, V_BE-_Q8 equals the base-emitter voltage drop across Q8, V_BE-_Q6 equals the base- emitter voltage drop across Q6, and V_BE-_Q4 equals the base-emitter voltage drop across Q4. [22] BJTs Q1-Q9 may be CMOS compatible lateral PNP bipolar junction transistors. FIGS. 2A-2B illustrate top down and cross sectional views of an embodiment of a CMOS compatible lateral PNP bipolar junction transistor that may be used to form BJTs Q1-Q9. FIG. 2C illustrates a schematic of a lateral PNP BJT. The lateral PNP BJT shown in FIGS. 2 -2B includes a P+ emitter diffusion region, an N-well base region, and a P+ lateral collector diffusion region. The lateral PNP BJT of FIGS. 2A-2B can be made using standard CMOS process techniques that are used to form a p-channel MOSFET transistor. No new layers or process steps are required. The gate terminal in FIGS. 2A-2B is biased so that the parallel PMOS device if kept off. The vertical collector terminal is not used. Lateral PNP BJTs have a relatively high base-to-collector current gain β (e.g., 100).

[23] In a further embodiment, BJTs Q4-Q9 may be compatible vertical PNP bipolar junction transistors. FIGS. 3A-3B illustrate top down and cross sectional views of an embodiment of a CMOS compatible vertical PNP bipolar junction transistor that may be used to form BJTs Q4-Q9. FIG. 3C illustrates a schematic of a vertical PNP BJT. The PNP transistor in FIGS. 3A-3B includes an emitter P+ diffusion region, an N-well base region, and a P+ collector region coupled to the P- substrate. Thus, the collector of the vertical PNP BJT is coupled to the P-substrate. Transistors Q4-Q9 can be vertical PNP BJTs, because their collectors are coupled directly to ground. Transistors Q1-Q3 cannot be vertical PNP BJTs, because their collectors are not coupled directly to ground. The vertical PNP BJT of FIGS. 2A-2B can be made using standard CMOS process techniques that are used to form a p- channel MOSFET transistor. Vertical PNP BJTs have a relatively high base-to-collector current gain β (e.g., 500). [24] BJTs Q4, Q6, and Q8 have base-emitter junction areas that are 8 times the base emitter junction areas of BJTs Q5, Q7, and Q9. Therefore, base-emitter voltage V_BE of each of transistors Q4, Q6, and Q8 are 26 mV ^• In (8) = 54 mV greater than the base- emitter voltages V_BE of each of transistors Q5, Q7, and Q9. The total voltage drop of VB_E-_QS + VBE-QO + NBE-Q4 is 162 mV greater than the total voltage of V_BE-Q9 + NBE-Q7 + V_BE-_Q5.

Therefore, the resistance of resistor Rl should be selected so that the voltage drop across Rl equals 162 mV so that the voltage at the base of Ql equals the voltage at the base of Q2. For example, the voltage drop across Rl is 162 mV when Rl is 4.05 kΩ and the current of through Rl is 40 μA. When the voltages at the bases of Ql and Q2 are equal, circuit 10 is in an equilibrium state and outputs a constant output voltage V_REF-

[25] When the temperature of circuit 10 increases, the base-emitter junction voltage drops across the bipolar junction transistors Q5, Q7, and Q9 decreases, and the voltage drop across resistors Rl and R2 increases. When the temperature of circuit 10 decreases, the voltage drop across base-emitter junctions of BJTs Q5, Q7, and Q9 increases, and the voltage drop across Rl and R2 decreases. If a temperature change in circuit 10 causes a voltage change in V_REF away from a desired value (e.g., 3.6 volts), a trimming resistor can be added in series or in parallel with resistor R2 to bring V_REF back up to the desired value. The trimming resistor can be coupled to R2 using fusible links that are isolated with respect to ground to reduce parasitic capacitance. [26] Once circuit 10 has been adjusted to reach a balance point (so that

V_REF is at the desired value), then temperature changes over a range (e.g., -40° C - 125° C) in circuit 10 do not cause voltage changes in V_REF- At the balance point of circuit 10, a change in the voltage drop across the base-emitter junctions of BJTs Q5, Q7, and Q9 is offset by a change in the voltage drop across resistor Rl when the temperature of circuit 10 changes such that the voltage of V_REF remains substantially constant (e.g., within a few millivolts). Therefore, trimming resistance may be added to circuit 10 to achieve a zero temperature coefficient. If desired, resistor Rl can be selected at a single temperature to achieve the balance point at which V_REF remains constant despite changes in temperature. Highly accurate measurements of resistances may be needed to achieve this result in one step. [27] The base-emitter threshold voltages of BJTs Ql and Q2 are the substantially the same, and therefore a low offset voltage is introduced into V_REF- Variations in the base-emitter threshold voltages of BJTs are on the order of 100-1000 times less than variations in the threshold voltages of MOS transistors. Circuit 10 uses triple emitter followers Q4/Q6/Q8 and Q5/Q7/Q9 that provide a three times increase in the delta V_BE (e.g., 3 ^• 54 mN) which reduces the effect of the small input offset voltages and noise voltages that are introduced by Ql and Q2 into V_REF-

[28] Thus, triple emitter followers Q4/Q6/Q8 and Q5/Q7/Q9 as shown in FIG. 1 is preferred. However, in a further embodiment, a first double emitter follower is coupled to the base of Ql (e.g., by eliminating transistors Q4 and M2 in circuit 10), and a second double emitter follower is coupled to the base of Q2 (e.g., by eliminating transistors Q5 and M8). Also, in another embodiment, only a single BJT is coupled between the base of Ql and Rl, and a single BJT is coupled between the base of Q2 and R2 (e.g., by eliminating transistors Q4, Q5, Q6, Q7, M2, M3, M7 and M8 in circuit 10). [29] BJTs Ql and Q2 emit low thermal noise, and therefore, circuit 10 exhibits noise performance levels comparable with bipolar band-gap reference circuits. In addition, BJTs Ql and Q2 do not contain the trapped charge that often exists in prior art MOS differential pairs. Therefore, V E_F in circuit 10 is stable with time and past use history, and does not contain long term drift components that cause the noise problems associated with variations in trapped charge over time that are caused by MOS differential pairs.

[30] In prior art band gap reference circuits that used an amplifier with a p- channel MOS differential pair, an offset voltage may be included in the value of VREF due to variations in the threshold voltages of the differential pair MOS transistors. V_RE_F in these circuits is determined by the base-emitter voltage drop across a BJT and the voltage drop across a resistor. When the temperature of the prior art band reference circuit changes, the voltage of VR_EF changes from a desired value due to changes in the voltage drops across the resistor and the BJT. The prior art circuits do not reach a balance point when trimming resistors are added to bring VR_EF back up to the desired value, because the offset voltage introduced by differential pair MOS transistors is included in V_REF- [31] V_REF in the prior art reference circuit cannot remain substantially constant with changing temperature, because it does not reach a point at which the decrease in the voltage drop across the BJT cancels out the increase in voltage drop across the resistor when V_REF is set at the desired value. The offset in V_RE_F introduced by the differential pair MOS transistors may cause a designer to add trimming resistance that cause V_RE_F to reach the desired value, but that is to much or too little trimming resistance to reach the balance point at which the effect of temperature changes are canceled out and no longer effect V_REF-

[32] Circuit 10 of the present invention is also substantially resistant to small first order variations in supply voltage V c- When Vcc increases, the current output of bias current source 11 increases. A small increase in the current through resistors Rl and R2 causes an increase in voltages at the bases of Ql and Q2. However, the voltage at the base of Ql increases more than Q2, because the increase in the voltage drop at the base of Q4 is greater than the voltage drop at the base of Q5. Therefore, the current through Ql decreases below the current through Q2, causing both the gate voltage of Ml 1 and the current through Ml 1 to increase. Diode coupled BJT Q3 is coupled to transistor Ml 1. The channel width-to- length (W/L) ratio of transistors M9 and Ml 0 are designed to be equal. By scaling W/L of Ml 1 to be eight times the W/L of M9 or M10, V_Ds of M10 substantially matches V_Ds of M9, so that the collector current of Q3 is approximately eight times larger than the collector current of Q2 or Ql, minimizing any imbalance of Ql and Q2 in the feedback loop. [33] The current through Ml 1 is several times the magnitude of the current through M10 and M9. The current through Mil increases as much as the current through current source 11 increases. Therefore, all of the excess current through current source 11 flows through Ml 1, and the current through transistors M1-M8 and resistors Rl and R2 remains substantially constant. [34] In a further embodiment of the present invention, the ratio of the current through transistor Ml 1 with respect to the current through transistors M9/M10 may be selected to be any suitable value. For example, MOS transistor Ml 1 may have a W/L that is 20 times the W/L of MOS transistors M9 and M10. In this embodiment, the current through Ml 1 is 20 times the current through M9 and M10. [35] When Vcc decreases, the current output of bias current source 11 decreases. The current through Ml 1 decreases by the same amount that the current through current source 11 decreases. Substantially all of the current drop through current source 11 is subtracted from the current through Ml 1, and the current through transistors M1-M8 and resistors Rl and R2 again remain substantially constant. Therefore, transistor Ml 1 is a feedback circuit that regulates its current so that the current through R1/R2 and Q4-Q9 are substantially constant. This is advantageous, because the feedback circuit causes the voltage drop across resistors Rl and R2 to remain substantially constant (e.g., 162 mV), the base voltages of Ql and Q2 to remain substantially equal to each other, and the output voltage V_RE_F to remain substantially constant despite small, first order changes in the current through current source 11. Thus, circuit 10 is desensitized from first order variations in V_Cc.

[36] With respect to base currents of Q4 and Q5, the base current of Q4 tends to cancel some but not all of the base current of Q5. Therefore, the base current of Q5 does introduce an error term into the circuit 10 with respect to reaching the balance point at which a zero temperature coefficient is achieved. However, the error term introduced by the base current of Q5 is relatively small and does effect the zero temperature coefficient much. To further ensure that the error term introduced by the base of Q5 is small, the impedances of Rl and R2 should be low relative to the base current of Q5, as is the case in the embodiment of FIG. 1. Also, Q5 can be a vertical PNP BJT, which has a relatively high base-to-collector current gain (β), which further reduces the error term introduced by the base current of Q5. [37] Parasitic capacitance in the feedback loop of circuit 10 provide sufficient compensation for the loop such that additional frequency compensation need not be added. However, an additional capacitor may be from the base of Q2 to ground to provide additional noise rejection in V_REF- [38] Band gap reference circuit 40 shown in FIG. 4 is a further embodiment of the present invention. Circuit 40 includes p-channel MOSFETs M4-M6, n-channel MOSFETs M9-M11, resistors Rl and R2, current source 11, and PNP BJTs Ql and Q2, as with the embodiment of FIG. 1. Circuit 40 also includes diodes 41-46 in place of BJTs Q4- Q9. Current source 11 outputs a current equal to 71. Current source 11 provides a current I to each of MOSFETs M4-M6. A current substantially equal to I flows through diodes 41-43 and resistors Rl and R2. A current substantially equal to I also flows through diodes 44-46 and resistor R2. A total current of 21 flows through R2.

[39] In circuit 40, diodes 41-43 have P-N junction areas that are eight times the P-N junction areas of diodes 44-46. Also, Ql has a base-emitter junction area that is eight times the base emitter junction area of Q2. Therefore, Rl should be selected to have a voltage drop of 54 mV ^• 4 = 216 mV to compensate for the fact that the voltage drop across Ql and diodes 41-43 is 216 mV greater than the voltage drop across Q2 and diodes 44-46. [40] A current of 41 flows through transistor Ml 1. Transistor Ml 1 has a W/L ratio that is eight times the W/L ratio of each of transistors M9 and M10. The feedback circuit comprising Ml 1 and Q3 ensure that a current equal to 1/2 flows through each of transistors M9 and M10.

[41] The resistance at R2 may be selected achieve a desired value at VREF- R2 may be trimmed to achieve a zero temperature coefficient at which point output signal V_REF remains constant over a range of temperature as discussed above with respect to FIG. 1. [42] In a further embodiment of the present invention, diodes 43 and 46 in circuit 40 may be eliminated, so that the base of Ql is coupled directly to diode 42 and the base of Q2 is coupled directly to diode 45. In another further embodiment of the present invention, diodes 42-43 and diodes 45-46 may be eliminated, so that the base of Ql is coupled directly to diode 41, and the base of Q2 is coupled directly to the diode 44. In still a further embodiment of the present invention, transistor Q4 in circuit 10 may be replaced with diode 41, eliminating transistor Ml, and transistor Q5 in circuit 10 may be replaced with diode 44.

[43] In still a further embodiment of the present invention, PNP BJTs Ql- Q2 and BJTs Q4-Q9 may be replaced with NPN bipolar junction transistors. PNP BJT Q3 may also be replaced with a NPN BJT.

[44] While the present invention has been described herein with reference to particular embodiments thereof, a latitude of modification, various changes and substitutions are intended in the foregoing disclosure, and it will be appreciated that in some instances some features of the invention will be employed without a corresponding use of other features without departing from the scope of the invention as set forth. Therefore, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope and spirit of the present invention. It is intended that the invention not be limited to the particular embodiments disclosed, but that the invention will include all embodiments and equivalents falling within the scope of the claims.

Claims

WHAT IS CLAIMED IS:

L A band gap reference circuit coupled to a source of bias current, the band gap reference circuit comprising: an amplifier comprising a differential pair of first and second bipolar junction transistors; a first circuit comprising a first P-N junction coupled to the first bipolar junction transistor; a second circuit comprising a second P-N junction coupled to the second bipolar junction transistor; a first resistor coupled to the first circuit; a second resistor coupled to the first resistor and the second circuit; and a feedback circuit coupled to the amplifier and to the source of bias current.

2. The band gap reference circuit of claim 1 wherein the first and second bipolar junction transistors are PNP transistors.

3. The band gap reference circuit of claim 1 wherein the band gap reference circuit provides an output voltage at the base of the first bipolar junction transistor that is substantially constant over a range of temperatures.

4. The band gap reference circuit of claim 1 wherein the band gap reference circuit provides an output voltage at the base of the first bipolar junction transistor that is substantially constant over a range of the bias current.

5. The band gap reference circuit of claim 1 wherein the first circuit comprises two bipolar junction transistors coupled together; and the second circuit comprises two bipolar junction transistors coupled together.

6. The band gap reference circuit of claim 5 wherein the first circuit comprises three emitter follower coupled bipolar junction transistors; and the second circuit comprises three emitter follower coupled bipolar junction transistors.

7. The band gap reference circuit of claim 6 wherein the three emitter follower coupled bipolar junction transistors of the first circuit are PNP transistors; and the three emitter follower coupled bipolar junction transistors of the second circuit are PNP transistors.

8. The band gap reference circuit of claim 6 wherein each of the three emitter follower bipolar junction transistors of the first circuit have base-emitter junction areas that are eight times the base-emitter junction areas of the three emitter follower bipolar junction transistors of the second circuit.

9. The band gap reference circuit of claim 1 wherein the first circuit comprises three diodes coupled in series; and the second circuit comprises three diodes coupled in series.

10. The band gap reference circuit of claim 1 the first circuit comprises two emitter follower coupled bipolar junction transistors coupled to a first diode; and the second circuit comprises two emitter follower coupled bipolar junction transistors coupled to a second diode.

11. The band gap reference circuit of claim 1 wherein the feedback circuit comprises a fifth transistor coupled to the amplifier.

12. The band gap reference circuit of claim 11 wherein the feedback circuit comprises a diode coupled sixth bipolar junction transistor coupled to the fifth transistor. ©

13. The band gap reference circuit of claim 1 wherein the first circuit comprises three NPN bipolar junction transistors coupled together; and the second circuit comprises three NPN bipolar junction transistors coupled together.

14. The band gap reference circuit of claim 1 wherein first transistor has a base-emitter junction area that is eight times the base-emitter junction area of the second transistor.

15. The band gap reference circuit of claim 1 wherein the amplifier, the first and second circuits, the first and second resistors, and the feedback circuit all receive bias current from a current source.

16. A method for providing a output reference voltage, the method comprising: providing the output reference voltage using a first P-N junction and a first resistor; providing a second voltage using a second P-N junction and a second resistor coupled to the first resistor; comparing the output reference voltage to the second voltage using a differential pair comprising first and second bipolar junction transistors; and regulating the current through a feedback circuit coupled to the differential pair to compensate for variations in current through the first and second resistors.

17. The method of claim 16 wherein the first and second bipolar junction transistors are PNP transistors.

18. The method of claim 16 wherein providing the output reference voltage using the first P-N junction and the first resistor further comprises providing the output reference voltage across the first resistor and third and fourth emitter follower coupled bipolar j unction transistors ; and providing the second voltage using the second P-N junction and the second resistor further comprises providing the second voltage across the second resistor and fifth and sixth emitter follower coupled bipolar junction transistors.

19. The method of claim 18 wherein providing the output reference voltage using the first P-N junction and the first resistor further comprises providing the output reference voltage across the first resistor and the third, the fourth, and a seventh emitter follower coupled bipolar junction transistors; and providing the second voltage using the second P-N junction and the second resistor further comprises providing the second voltage across the second resistor and the fifth, the sixth, and an eighth emitter follower coupled bipolar junction transistors.

20. The method of claim 19 wherein each of the third, fourth, and seventh bipolar junction transistors have base-emitter junction areas that are eight times the base- emitter junction areas of each of the fifth, sixth, and eighth bipolar junction transistors

21. The method of claim 16 wherein providing the output reference voltage using the first P-N junction and the first resistor further comprises providing the output reference voltage across the first resistor and first and second diodes coupled in series; and providing the second voltage using the second P-N junction and the second resistor further comprises providing the second voltage across the second resistor and third and fourth diodes coupled in series.

22. The method of claim 21 wherein providing the output reference voltage using the first P-N junction and the first resistor further comprises providing the output reference voltage across the first resistor and the first, the second^' and a fifth diodes coupled in series; and providing the second voltage using the second P-N junction and the second resistor further comprises providing the second voltage across the second resistor and the third, the fourth, and a sixth diodes coupled in series.

23. The method of claim 16 wherein the feedback circuit comprises a MOSFET that regulates its drain-source current.

24. The method of claim 23 wherein the MOSFET is coupled to a diode coupled fifth bipolar junction transistor.

25. The method of claim 16 wherein the differential pair is coupled to a current mirror circuit.

26. The method of claim 16 wherein the output reference voltage is substantially constant over a range of temperature.

27. The method of claim 16 wherein regulating the current through the feedback circuit further comprises regulating the current through the feedback circuit so that the current through the first and second resistors remains substantially constant over a range of current from a bias current source.

28. The method of claim 16 wherein comparing the output reference voltage to the second voltage using a differential pair further comprises supplying current to the differential pair from a current source.

29. A band gap reference circuit coupled to a source of bias current, the band gap reference circuit comprising: an amplifier comprising a differential pair of first and second bipolar junction transistors; a first circuit comprising a first plurality of emitter follower coupled bipolar junction transistors; a second circuit comprising a second plurality of emitter follower coupled bipolar junction transistors; a first resistor coupled to the first circuit; a second resistor coupled to the first resistor and the second circuit; and a feedback circuit coupled to the amplifier and to the source of bias current.