Temperature-compensated low-voltage bandgap reference

Abstract

A low-voltage bandgap reference circuit includes a current source supplying a reference voltage rail. A first BJT has a collector coupled to the voltage rail via a resistor, a base coupled directly to the voltage rail, and an emitter coupled to ground via an emitter resistance. A second BJT has a collector coupled to the voltage rail via a resistor, a base coupled to voltage rail by a first base resistance and to ground via a second base resistance, and a collector coupled to the emitter resistance via an intermediate resistance. A third BJT has a collector driven by a current source, a base coupled to a node between the first and second base resistances, and an emitter coupled to ground. A feedback amplifier regulates the reference voltage rail to equalize collector voltages of the first and second BJTs.

Description

BACKGROUND

A voltage reference is typically provided by electronic circuitry that outputs a constant voltage despite variations in temperature or power supply that might normally or otherwise cause voltage fluctuations. As a result, the desired behavior is that the voltage reference remains constant even as conditions in the system vary. Such voltage references may be used in power supply voltage regulators, analog-to-digital converters, digital-to-analog converters, and the like as well as many other measurement and control systems.

Almost all integrated circuit devices require a precise voltage reference. One implementation is known as the Brokaw voltage reference, which generally provides a voltage reference between 1.2 and 1.3 V (i.e., about 1.25 V) and consequently necessitates a slightly higher input voltage (e.g., about 1.4 V). However, integrated circuit devices that require voltage references lower than 1.2 V, such as those in mobile applications, are not compatible with the Brokaw voltage reference.

Previous attempts have been made to provide suitable low voltage references such as the depletion NMOS voltage reference. However, such low voltage references have much higher spread due to manufacturing variations, and trimming is required to obtain the desired precision. Trimming is expensive in terms of die area, equipment, and test time.

SUMMARY

Accordingly, there is provided herein bandgap reference circuits and methods for providing a temperature-compensated low-voltage reference. One illustrative low-voltage bandgap reference circuit includes: a first current source (I2) coupled to supply current to a reference voltage rail; a first bipolar junction transistor (Q1) having a collector coupled to the reference voltage rail via a first collector resistance (RC2), a base coupled directly to the reference voltage rail, and an emitter coupled to a ground node via an emitter resistance (R2); a second bipolar junction transistor (Q0) having a collector coupled to the reference voltage rail via a second collector resistance (RC1), a base coupled to the reference voltage rail by a first base resistance (R4) and coupled to the ground node via a second base resistance (R3), and an emitter coupled to the emitter resistance by an intermediate resistance (R1); a third bipolar junction transistor (Q2) having a collector driven by a second current source (I1), a base coupled to a node between the first and second base resistances, and an emitter coupled to the ground node; and a feedback amplifier (S) that regulates the reference voltage rail to equalize collector voltages of the first and second bipolar junction transistors.

An illustrative method of providing a low-voltage bandgap reference includes: driving a reference voltage rail with a current from a first current source (I2); providing a first base emitter voltage (Vbe1) with a first bipolar junction transistor (Q1) having a collector coupled to the reference voltage rail via a first collector resistance (RC2), a base coupled directly to the reference voltage rail, and an emitter coupled to a ground node via an emitter resistance (R2); providing a second base emitter voltage (Vbe0) with a second bipolar junction transistor (Q0) having a collector coupled to the reference voltage rail via a second collector resistance (RC1), a base coupled to the reference voltage rail by a first base resistance (R4) and coupled to the ground node via a second base resistance (R3), and an emitter coupled to the emitter resistance by an intermediate resistance (R1); providing a third base emitter voltage (Vbe2) with a third bipolar junction transistor (Q2) having a collector driven by a second current source (I1), a base coupled to a node between the first and second base resistances, and an emitter coupled to the ground node; and regulating the reference voltage rail with a feedback amplifier (S) that operates to equalize collector voltages of the first and second bipolar junction transistors.

Another illustrative method providing a low-voltage bandgap reference includes: manufacturing an integrated circuit having the low-voltage bandgap reference circuit set out above; and packaging the integrated circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following detailed description of the various disclosed embodiments, reference will be made to the accompanying drawings in which:

FIG. 1 is a circuit diagram of a prior art circuit;

FIG. 2 is a circuit diagram of an illustrative circuit that regulates temperature-compensated output voltage;

FIG. 3 is a circuit diagram of another illustrative circuit that regulates temperature-compensated output voltage;

FIG. 4 is a top-view of an illustrative semiconductor apparatus including a semiconductor wafer; and

FIG. 5 is a perspective view of an illustrative integrated circuit device including a package and pins.

It should be understood, however, that the specific embodiments given in the drawings and detailed description thereto do not limit the disclosure. On the contrary, they provide the foundation for one of ordinary skill to discern the alternative forms, equivalents, and modifications that are encompassed together with one or more of the given embodiments in the scope of the appended claims.

NOTATION AND NOMENCLATURE

Certain terms are used throughout the following description and claims to refer to particular system components and configurations. As one of ordinary skill will appreciate, companies may refer to a component by different names. This document does not intend to distinguish between components that differ in name but not function. In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . . ”. Also, the term “couple” or “couples” is intended to mean either an indirect or a direct electrical or physical connection. Thus, if a first device couples to a second device, that connection may be through a direct electrical connection, through an indirect electrical connection via other devices and connections, through a direct physical connection, or through an indirect physical connection via other devices and connections in various embodiments.

DETAILED DESCRIPTION

The issues identified in the background are at least partly addressed by circuits and devices that regulate temperature-compensated output voltage. The circuits and devices proposed herein are improvements on the Brokaw reference circuits, such as the Brokaw reference circuit 100 illustrated in FIG. 1. The circuit 100 includes two transistors, Q0 and Q1; four resistors, R1, R2, RC1, and RC2; and a feedback amplifier, S. Here, Q0 has an emitter area eight times larger than Q1 as noted by the labels A and 8A. In other embodiments, Q0 has an emitter area N times larger than Q1 where N is any natural number bigger than 1. RC1 and RC2 are matched, and the bases of Q0 and Q1 receive a common voltage. When the voltage at their common base is small, such that the voltage drop across R1 is small, the larger area of Q0 causes Q0 to conduct more of the total current available through R2. As such, Q0 requires a smaller base-emitter voltage for the same current. The base-emitter voltage for each transistor, Vbe0 and Vbe1, has a negative temperature coefficient (i.e., it decreases with temperature). The difference between the two base-emitter voltages, ΔVbe, has a positive temperature coefficient (i.e., it increases with temperature).

The amplifier S uses negative feedback to supply a common base voltage to the two transistors, Q0 and Q1, causing each to draw current through their respective collector resistors RC1 and RC2. At a low base voltage, Q0 draws more current than Q1, and the resulting imbalance in collector voltages drives the amplifier S, which raises the base voltage. Alternatively, if the base voltage is high, forcing a large current through R2, the voltage across R1 will limit the current through Q0 so that the current through Q0 will be less than the current through Q1. Accordingly, the collector voltage imbalance will be reversed, causing the amplifier S to reduce the base voltage. Between these two extreme conditions is a base voltage at which the two collector currents match, toward which the amplifier S drives from any other condition. The two collector currents match when the emitter current densities are in the ratio 8-to-1, the emitter area ratio.

When this difference in current density has been produced by the amplifier S, ΔVbe will appear across R1. This difference is given by:

$\begin{array}{cc}\Delta \mathrm{Vbe}=\frac{\mathrm{kT}}{q}\ln \frac{J_1}{J_0}.& \left(1\right)\end{array}$ where k is the Boltzmann constant (1.38e−23 J*K⁻¹), q is the electron charge (1.602e⁻¹⁹ C), and T is the absolute temperature (Kelvin). Because the current through Q1 is equal to the current through Q0, the current through R2 is twice that through R1 and the voltage across R2 is given by:

$\begin{array}{cc}{V}_{R2}=2\frac{R_2}{R_1}\frac{\mathrm{kT}}{q}\ln \frac{J_1}{J_0}.& \left(2\right)\end{array}$

Assuming the resistor ratio and current density ratio are invariant, the voltage across R2 varies directly with T, the absolute temperature. The voltage at the base of Q1 is the sum of Vbe1 and the temperature-dependent voltage across R2. Accordingly, the circuit 100 output, VouT, is the sum of: 1) a value proportional to the base-emitter voltage difference (ΔVbe) and 2) one of the base-emitter voltages (Vbe1 or Vbe2), enabling temperature compensation to be achieved with an appropriate ratio of R1 and R2.

In this circuit 100 and other Brokaw reference circuits, V_OUT is regulated to about 1.25 V (i.e., anywhere from 1.2 V to 1.3 V). However, integrated circuit devices increasingly require voltage references lower than 1.2 V, which cannot be provided by the circuit 100, but which can be provided by the circuits illustrated in FIGS. 2 and 3.

FIG. 2 illustrates a circuit 202 that regulates temperature-compensated output voltage, Vref, to less than 1.2 V. The circuit 202 may be part of a larger circuit, part of an integrated circuit device, formed on a semiconductor wafer, and the like as represented by dashed rectangle 200. The circuit 202 includes three bipolar junction transistors (“BJTs”), Q0, Q1, and Q2; two metal-oxide semiconductor field-effect transistors (“MOSFETs”), M0 and M1; six resistors, R1, R2, R3, R4, RC1, and RC2; two current sources, I1 and I2, and a feedback amplifier, S. The amplifier S keeps identical current through transistors Q0 and Q1 by sensing voltages on bottom terminals of resistors RC1 and RC2. The amplifier sets zero voltage between its inputs using the feedback loop through M0. Because the upper terminals of RC1 and RC2 are tied together, there are identical voltages across RC1 and RC2 resulting in identical currents through RC1 and RC2 (and consequently identical current through Q0 and Q1). In at least one embodiment, M1 is a depletion negative MOSFET (“NMOS”) transistor or low Vth NMOS, and M0 is an NMOS or BJT.

The current source I2 supplies a reference voltage rail 204, and the circuit 202 includes a loop branch 206 coupled to the reference voltage rail 204. This branch 206 obtains the base-emitter voltage of Q1, Vbe1, which has a negative temperature coefficient. The circuit also includes a ΔVbe loop branch 208. This branch obtains a voltage including the voltage difference from the base-emitter voltages of Q1 and Q2 as described above, but also including a fractional base-emitter voltage of Q2, Vbe2. This fractional voltage enables a reduced positive temperature-coefficient. The fractional Vbe2 voltage may be created on resistor R4. While resistances may be sensitive to process variation, their ratios generally remain quite precise. As such, the circuit 200 employs a resistor ratio of R4 to R3 to set the fraction of Vbe2 that is incorporated into the ΔVbe loop. In this way, temperature compensation for output voltages lower than 1.25 V and/or 1.2 V may be achieved. Specifically, the feedback amplifier S sets identical voltages from the loop branches on inputs of the amplifier to regulate an output voltage of the circuit on the reference voltage rail at a temperature-compensated value below 1.2V. For example, the feedback amplifier S combines the Vbe1 voltage with the reduced ΔVbe voltage to regulate the output voltage Vref at a temperature-compensated value below 1.25 V and/or 1.2 V. Such regulation may be performed without trimming and with an accuracy better than ±1%. Specifically, the output voltage may be given by:

$\begin{array}{cc}\mathrm{Vref}=2\frac{R_2}{R_1}\frac{\mathrm{kT}}{q}\ln N+V_{\mathrm{be}1}-2V_{\mathrm{be}}\frac{R_4}{R_3}\frac{R_2}{R_1}& \left(3\right)\end{array}$ where V_T=kT/q.

As indicated by equation (3), the output voltage may be set by balancing four resistors, R1, R2, R3, and R4. The input voltage of the circuit may be higher than the output voltage by less than 10 millivolts.)

FIG. 3 illustrates a circuit 302 that regulates temperature-compensated output voltage, Vref, to less than 1.2 V. The circuit 302 may be part of a larger circuit, part of an integrated circuit device, formed on a semiconductor wafer, and the like as represented by dashed rectangle 300. The circuit 302 includes five BJTs, Q0, Q1, Q2, Q3, and Q4; fifteen MOSFETs, M0, M1, M2, M3, M4, M5, M6, M7, M8, M9, M10, M11, M12, M13, and M14; eight resistors, R1, R2, R3, R4, R5, R6, RC1, and RC2; and one capacitor, Cc. The feedback amplifier is implemented by R6, Q3, Q4, M4, M6, M7, M8, M9, M11, M12, M13, M14, M0, and Cc.

The circuit 302 includes a loop branch 306 coupled to a reference voltage rail 304. This branch 306 obtains a voltage, Vbe1, with a negative temperature coefficient as described above. The circuit also includes a ΔVbe loop branch 308. This branch obtains a ΔVbe voltage as described above, using a fractional Vbe2 voltage to provide a reduced, positive temperature-coefficient. The fractional Vbe2 voltage may be created on resistor R4, using the resistor ratio R4 to R3 as described above. Specifically,ΔVbe=V_T*ln(N)−Vbe2(R4/R3)  (4)where N is the ratio of emitter areas between Q0 and Q1. Accordingly, the output voltage is given by:

$\begin{array}{cc}\mathrm{Vref}=2\left[V_T*\ln \left(N\right)-\mathrm{Vbe}2\left(R4/R3\right)\right]\left(R2/R1\right)+\mathrm{Vbe}1\mathrm{or}& \left(5\right)\\ \mathrm{Vref}=2\left(\frac{R_2}{R_1}\right)V_T*\ln \left(N\right)+V_{\mathrm{be}1}-2V_{\mathrm{be}2}\left(\frac{R_4}{R_3}\right)\left(\frac{R_2}{R_1}\right)& \left(6\right)\end{array}$

As indicated by equations (5) and (6), the output voltage may be set by balancing four resistors, R1, R2, R3, and R4. The input voltage of the circuit may be higher than the output voltage by less than 10 millivolts.

FIG. 4 is a top-view of an illustrative semiconductor apparatus 400 including a semiconductor wafer 402. The wafer 402, also called a slice or substrate, is a thin slice of semiconductor material, such as a crystalline silicon, used in electronics for the fabrication of integrated circuits. The wafer 402 serves as the substrate for circuits 404 built in and over the wafer 402 and undergoes many microfabrication process steps such as doping or ion implantation, etching, deposition of various materials, and photolithographic patterning. The circuits 404 may be the circuits 202, 302 discussed above with respect to FIGS. 2 and 3, and the wafer 402 may be represented by the dashed rectangles 200, 300. After such processes, the individual circuits 404 are separated and packaged as illustrate in FIG. 5.

FIG. 5 is a perspective view of an illustrative integrated circuit device 500 including a package 502 and pins 504 coupled to the package 502. The package 502 may house circuits 202, 302 discussed above with respect to FIGS. 2 and 3, and the package 502 may be represented by the dashed rectangles 200, 300. Packaging is the final stage of semiconductor device fabrication, in which the circuit is encapsulated in a supporting package 502 that prevents physical damage and corrosion. The package 502 supports the pins 504, which connect the device 500 to a circuit board. Packages may be single in-line packages (“SIPs”), dual in-line packages (“DIPs”), ceramic DIPs, glass sealed DIPs, quadruple in-line packages (“QIPs”), skinny DIPs, zig-zag in-line packages (“ZIPs”), molded DIPs, plastic DIPs, and the like.

In some aspects systems, devices, and methods for regulating temperature-compensated output voltage are provided according to one or more of the following examples:

Example 1

A low-voltage bandgap reference circuit includes a current source supplying a reference voltage rail. The circuit further includes a Vbe loop branch coupled to the reference voltage rail to obtain a Vbe voltage with a negative temperature coefficient. The circuit further includes a ΔVbe loop branch to obtain a ΔVbe voltage, the ΔVbe loop branch employing a fractional Vbe voltage, to provide a reduced, positive temperature coefficient. The circuit further includes a feedback amplifier that sets identical voltages from the loop branches on inputs of the amplifier to regulate an output voltage of the circuit on the reference voltage rail at a temperature-compensated value below 1.2V.

Example 2

An integrated circuit device includes a package and pins coupled to the package. The device further includes a low-voltage bandgap reference circuit, housed by the package, including a Vbe loop branch coupled to a reference voltage rail to obtain a Vbe voltage with a negative temperature coefficient. The circuit further includes a ΔVbe loop branch to obtain a ΔVbe voltage, the ΔVbe loop branch employing a fractional Vbe voltage, to provide a reduced, positive temperature coefficient. The circuit further includes a feedback amplifier that sets identical voltages from the loop branches on inputs of the amplifier to regulate an output voltage of the circuit on the reference voltage rail at a temperature-compensated value below 1.2V.

Example 3

A semiconductor apparatus includes a semiconductor wafer and circuits formed in or on the wafer. Each circuit includes a Vbe loop branch coupled to a reference voltage rail to obtain a Vbe voltage with a negative temperature coefficient. Each circuit further includes a ΔVbe loop branch to obtain a ΔVbe voltage, the ΔVbe loop branch employing a fractional Vbe voltage, to provide a reduced, positive temperature coefficient. Each circuit further includes a feedback amplifier that sets identical voltages from the loop branches on inputs of the amplifier to regulate an output voltage of the circuit on the reference voltage rail at a temperature-compensated value below 1.2V.

The following features may be incorporated into the various embodiments described above, such features incorporated either individually in or conjunction with one or more of the other features. The output voltage may be regulated on the reference voltage rail at the temperature-compensated value below 1.2V without trimming. The output voltage may be regulated on the reference voltage rail at the temperature-compensated value below 1.2V with an accuracy better than ±1%. The ΔVbe voltage may be a difference in base-emitter voltages of two transistors reduced by the fractional Vbe voltage. The fractional Vbe voltage may be created on a resistor, and the value of the fractional Vbe voltage may be given by a ratio of the resistor and another resistor. The output voltage may be set by balancing four resistors. The output voltage may be given by

$\mathrm{Vref}=2\left(\frac{R_2}{R_1}\right)V_T*\ln \left(N\right)+V_{\mathrm{be}1}-2V_{\mathrm{be}2}\left(\frac{R_4}{R_3}\right)\left(\frac{R_2}{R_1}\right).$ An input voltage may be higher than an output voltage by less than 10 millivolts.

Numerous other modifications, equivalents, and alternatives, will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such modifications, equivalents, and alternatives where applicable.

Claims

1. A low-voltage bandgap reference circuit comprising: a first current source (I2) coupled to supply current to a reference voltage rail;a first bipolar junction transistor (Q1) having a collector coupled to the reference voltage rail via a first collector resistance (RC2), a base coupled directly to the reference voltage rail, and an emitter coupled to a ground node via an emitter resistance (R2);a second bipolar junction transistor (Q0) having a collector coupled to the reference voltage rail via a second collector resistance (RC1), a base coupled to the reference voltage rail by a first base resistance (R4) and coupled to the ground node via a second base resistance (R3), and an emitter coupled to the emitter resistance by an intermediate resistance (R1);a third bipolar junction transistor (Q2) having a collector driven by a second current source (I1), a base coupled to a node between the first and second base resistances, and an emitter coupled to the ground node; anda feedback amplifier (S) that regulates the reference voltage rail to equalize collector voltages of the first and second bipolar junction transistors.

2. The circuit of claim 1, wherein the first bipolar junction transistor provides a first base emitter voltage (Vbe1) having a negative temperature coefficient, wherein the second bipolar junction transistor provides a second base emitter voltage (Vbe0) that yields a differential voltage (ΔVbe) when subtracted from the first base emitter voltage, the differential voltage having a positive temperature coefficient, and wherein the third bipolar junction transistor provides a third base emitter voltage (Vbe2) to fractionally reduce the differential voltage.

3. The circuit of claim 2, wherein the first and second collector resistances are equal, wherein a first ratio of the emitter resistance to the intermediate resistance (R2/R1) and a second ratio of the first base resistance to the second base resistance (R4/R3) balance contributions from the positive and negative temperature coefficients to ensure that the reference voltage rail is temperature compensated and maintained below 1.2 volts.

4. The circuit of claim 3, wherein the first current source supplies said current from a voltage that does not exceed the reference voltage rail by more than 10 millivolts.

5. The circuit of claim 3, wherein the second bipolar junction transistor has an emitter area N times larger than an emitter area of the first bipolar junction transistor.

6. The circuit of claim 5, wherein the reference voltage rail has a regulated voltage of

7. The circuit of claim 3, wherein to regulate the reference voltage rail, the feedback amplifier drives a gate voltage of a MOSFET coupled between the reference voltage rail and the ground node.

8. The circuit of claim 3, further comprising an n-channel MOSFET having a drain coupled to the base of the second bipolar junction transistor, a gate coupled to the collector of the third bipolar junction transistor, and a source coupled to the base of the third bipolar junction transistor.

9. A method of providing a low-voltage bandgap reference, the method comprising: driving a reference voltage rail with a current from a first current source (I2);providing a first base emitter voltage (Vbe1) with a first bipolar junction transistor (Q1) having a collector coupled to the reference voltage rail via a first collector resistance (RC2), a base coupled directly to the reference voltage rail, and an emitter coupled to a ground node via an emitter resistance (R2);providing a second base emitter voltage (Vbe0) with a second bipolar junction transistor (Q0) having a collector coupled to the reference voltage rail via a second collector resistance (RC1), a base coupled to the reference voltage rail by a first base resistance (R4) and coupled to the ground node via a second base resistance (R3), and an emitter coupled to the emitter resistance by an intermediate resistance (R1);providing a third base emitter voltage (Vbe2) with a third bipolar junction transistor (Q2) having a collector driven by a second current source (I1), a base coupled to a node between the first and second base resistances, and an emitter coupled to the ground node; andregulating the reference voltage rail with a feedback amplifier (S) that operates to equalize collector voltages of the first and second bipolar junction transistors.

10. The method of claim 9, wherein the first base emitter voltage has a negative temperature coefficient, wherein the intermediate resistance sustains a differential voltage (ΔVbe) between the first and second base emitter voltages, reduced by a fraction of the third base emitter voltage, the reduced differential voltage having a positive temperature coefficient.

11. The method of claim 10, wherein the first and second collector resistances are equal, wherein a first ratio of the emitter resistance to the intermediate resistance (R2/R1) and a second ratio of the first base resistance to the second base resistance (R4/R3) balance contributions from the positive and negative temperature coefficients to ensure that the reference voltage rail is temperature compensated and maintained below 1.2 volts.

12. The method of claim 11, wherein the first current source supplies said current from a voltage that does not exceed the reference voltage rail by more than 10 millivolts.

13. The method of claim 11, wherein the second bipolar junction transistor has an emitter area N times larger than an emitter area of the first bipolar junction transistor.

14. The method of claim 13, wherein the reference voltage rail has a regulated voltage of

15. The method of claim 11, wherein to regulate the reference voltage rail, the feedback amplifier drives a gate voltage of a MOSFET coupled between the reference voltage rail and the ground node.

16. A method of providing a low-voltage bandgap reference, the method comprising: manufacturing an integrated circuit having: a first current source (I2) coupled to supply current to a reference voltage rail;a first bipolar junction transistor (Q1) having a collector coupled to the reference voltage rail via a first collector resistance (RC2), a base coupled directly to the reference voltage rail, and an emitter coupled to a ground node via an emitter resistance (R2);a second bipolar junction transistor (Q0) having a collector coupled to the reference voltage rail via a second collector resistance (RC1), a base coupled to the reference voltage rail by a first base resistance (R4) and coupled to the ground node via a second base resistance (R3), and an emitter coupled to the emitter resistance by an intermediate resistance (R1);a third bipolar junction transistor (Q2) having a collector driven by a second current source (I1), a base coupled to a node between the first and second base resistances, and an emitter coupled to the ground node; anda feedback amplifier (S) that regulates the reference voltage rail to equalize collector voltages of the first and second bipolar junction transistors; andpackaging the integrated circuit.

17. The method of claim 16, wherein the first bipolar junction transistor provides a first base emitter voltage (Vbe1) having a negative temperature coefficient, wherein the second bipolar junction transistor provides a second base emitter voltage (Vbe0) that yields a differential voltage (ΔVbe) when subtracted from the first base emitter voltage, the differential voltage having a positive temperature coefficient, and wherein the third bipolar junction transistor provides a third base emitter voltage (Vbe2) to fractionally reduce the differential voltage.

18. The method of claim 17, wherein the first and second collector resistances are equal, wherein a first ratio of the emitter resistance to the intermediate resistance (R2/R1) and a second ratio of the first base resistance to the second base resistance (R4/R3) balance contributions from the positive and negative temperature coefficients to ensure that the reference voltage rail is temperature compensated and maintained below 1.2 volts.

19. The method of claim 18, wherein the second bipolar junction transistor has an emitter area N times larger than an emitter area of the first bipolar junction transistor, and wherein the reference voltage rail has a regulated voltage of

20. The method of claim 18, wherein to regulate the reference voltage rail, the feedback amplifier drives a gate voltage of a MOSFET coupled between the reference voltage rail and the ground node.