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Bandgap voltage reference
Bandgap voltage reference
Bandgap voltage reference
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A bandgap voltage reference is a voltage reference circuit widely used in integrated circuits. It produces an almost constant voltage corresponding to the particular semiconductor's theoretical band gap, with very little fluctuations from variations of power supply, electrical load, time, temperature (as of 1999, they typically have an initial error of 0.5–1.0% and a temperature coefficient of 25–50 ppm/°C).[1]

David Hilbiber of Fairchild Semiconductor filed a patent in 1963[2] and published this circuit concept in 1964.[3] Bob Widlar,[4] Paul Brokaw[5] and others[6] followed up with other commercially-successful versions.

Operation

[edit]
Circuit of a Brokaw bandgap reference
Characteristic and balance point of T1 and T2

The voltage difference between two p–n junctions (e.g. diodes), operated at different current densities, is used to generate a current that is proportional to absolute temperature (PTAT) in a resistor. This current is used to generate a voltage in a second resistor. This voltage in turn is added to the voltage of one of the junctions (or a third one, in some implementations). The voltage across a diode operated at constant current is complementary to absolute temperature (CTAT), with a temperature coefficient of approximately −2 mV/K. If the ratio between the first and second resistor is chosen properly, the first order effects of the temperature dependency of the diode and the PTAT current will cancel out.

Although silicon's (Si) band gap at 0 K is technically 1.165 eV, the circuit essentially linearly extrapolates the bandgap–temperature curve[7] to determine a slightly higher but precise reference around 1.2–1.3 V (the specific value depends on the particular technology and circuit design); the remaining voltage change over the operating temperature of typical integrated circuits is on the order of a few millivolts. This temperature dependency has a typical parabolic residual behavior since the linear (first order) effects are chosen to cancel.

Because the output voltage is by definition fixed around 1.25 V for typical Si bandgap reference circuits, the minimum operating voltage is about 1.4 V, as in a CMOS circuit at least one drain-source voltage of a field-effect transistor (FET) has to be added. Therefore, recent work concentrates on finding alternative solutions, in which for example currents are summed instead of voltages, resulting in a lower theoretical limit for the operating voltage.[6]

The first letter of the acronym, CTAT, is sometimes misconstrued to represent constant rather than complementary. The term, constant with temperature (CWT), exists to address this confusion, but is not in widespread use.

When summing a PTAT and a CTAT current, only the linear terms of current are compensated, while the higher-order terms are limiting the temperature drift (TD) of the bandgap reference at around 20 ppm/°C, over a temperature range of 100 °C. For this reason, in 2001, Malcovati[8] designed a circuit topology that can compensate high-order non-linearities, thus achieving an improved TD. This design used an improved version of Banba's [6] topology and an analysis of base-emitter temperature effects that was performed by Tsividis in 1980.[9] In 2012, Andreou[10][11] has further improved the high-order non-linear compensation by using a second operational amplifier along with an additional resistor leg at the point where the two currents are summed up. This method enhanced further the curvature correction and achieved superior TD performance over a wider temperature range. In addition it achieved improved line regulation and lower noise.

The other critical issue in design of bandgap references is power efficiency and size of circuit. As a bandgap reference is generally based on BJT devices and resistors, the total size of circuit could be large and therefore expensive for IC design. Moreover, this type of circuit might consume a lot of power to reach to the desired noise and precision specification.[12]

Despite these limitations, the band gap voltage reference is widely used in voltage regulators, covering the majority of 78xx, 79xx devices along with the TL431 and the complementary LM317 and LM337. Temperature coefficients as low as 1.5–2.0 ppm/°C can be obtained with bandgap references.[a] However, the parabolic characteristic of voltage versus temperature means that a single figure in ppm/°C does not adequately describe the behavior of the circuit. Manufacturers' data sheets show that the temperature at which the peak (or trough) of the voltage curve occurs is subject to normal sample variations in production. Bandgap references are also suited for low-power applications.[b]

Mixed-signal microcontrollers may provide an internal bandgap reference signal to be used as reference for any internal comparator(s) and analog-to-digital converter(s).

Patents

[edit]
  • 1966, US Patent 3271660, Reference voltage source, David Hilbiber.[13]
  • 1971, US Patent 3617859, Electrical regulator apparatus including a zero temperature coefficient voltage reference circuit, Robert Dobkin and Robert Widlar.[14]
  • 1981, US Patent 4249122, Temperature compensated bandgap IC voltage references, Robert Widlar.[15]
  • 1984, US Patent 4447784, Temperature compensated bandgap voltage reference circuit, Robert Dobkin.[16]

Notes

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See also

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References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Bandgap voltage reference

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A is an designed to produce a fixed, stable output voltage that remains largely independent of temperature, supply voltage, and process variations, typically around 1.25 volts, by leveraging the physics of bandgaps in to compensate for thermal effects through the summation of complementary temperature-dependent voltage components. This reference voltage is fundamental in analog and mixed-signal integrated circuits, serving as a precise benchmark for applications requiring high accuracy, such as analog-to-digital converters (ADCs), digital-to-analog converters (DACs), systems, and precision measurement equipment. The core principle of operation relies on balancing the negative temperature coefficient (CTAT) of the base-emitter voltage (VBE) of a , which decreases by approximately 2 mV/°C, with the positive temperature coefficient (PTAT) of the difference in base-emitter voltages (ΔVBE) between two transistors biased at different current densities. This ΔVBE is given by (kT/q) ln(N), where k is Boltzmann's constant, T is absolute temperature, q is the electron charge, and N is the ratio of emitter areas or current densities, yielding a PTAT term of about 0.20 mV/°C for a decade ratio at . In a typical Brokaw-style bandgap cell, these components are amplified and summed via an and network to produce VREF = VBE + K · ΔVBE, where K is a scaling factor (often around 10–20) chosen to null the net at approximately 300 K, resulting in an output near 1.25 V based on the extrapolated bandgap voltage of 1.205 V. Bandgap references offer key advantages over alternatives like Zener diodes or buried Zener references, including simpler integration in and bipolar processes, lower power consumption (as low as 50 µA in micropower designs), reduced (typically 4–16 ppm RMS wideband), and scalability to higher voltages like 2.5 V or 5 V through amplification. Modern implementations achieve temperature coefficients (TCs) as low as 3 ppm/°C and initial accuracies from 0.02% to 1%, making them suitable for systems up to 12-bit resolution, though they may exhibit higher long-term drift (20–50 ppm/√kHr) compared to specialized Zener types. The concept traces its origins to the early 1960s, with David Hilbiber of describing a temperature-stable voltage using diode-connected transistors in a 1964 IEEE paper, followed by Bob Widlar's refinements in 1965 and 1971 that introduced current-density-based PTAT generation and the first commercial IC (LM109). Paul Brokaw's 1974 IEEE publication established the widely adopted three-terminal bandgap cell topology, which underpins most contemporary designs, including low-dropout variants and sub-1 V extensions for advanced nodes.

Fundamentals

Definition and Purpose

A bandgap voltage reference is an electronic circuit designed to produce a fixed output voltage, typically in the range of 1.2–1.3 V, that remains nearly independent of variations in temperature, supply voltage, load current, and manufacturing process parameters. This stability arises from the circuit's ability to compensate for temperature-induced changes in semiconductor device characteristics, making it a cornerstone of precision analog design. The primary purpose of a bandgap voltage reference is to serve as a precise and stable voltage source for critical components in electronic systems, including analog-to-digital converters (ADCs), digital-to-analog converters (DACs), voltage regulators, and comparators. In these applications, it ensures accurate and conversion by providing a consistent benchmark against which other voltages are measured or generated, thereby minimizing errors in and . Key performance characteristics include an initial accuracy of 0.5–1.0%, a of 25–50 ppm/°C, long-term stability on the order of 20–50 ppm/√khr, and a high (PSRR) that effectively filters supply noise. Historically, the design draws from the extrapolated silicon bandgap voltage of approximately 1.205 at 0 K. The reference voltage is typically about 1.25 at , enabling it to mimic this fundamental material property for enhanced thermal invariance. Compared to alternatives like zener diode references, which suffer from higher noise and poorer temperature stability, bandgap references offer superior performance in integrated circuits; buried zener references provide even better precision but require more complex fabrication.

Physical Basis

The physical basis of bandgap voltage references stems from the semiconductor bandgap energy, which represents the minimum energy required to excite electrons from the valence band to the conduction band in materials like silicon. In silicon, the bandgap energy EgE_g is approximately 1.17 eV at 0 K, but for voltage reference purposes, the relevant parameter is the extrapolated bandgap voltage Vg(0)V_g(0) at absolute zero, valued at about 1.205 V. This extrapolation accounts for the temperature-dependent narrowing of the bandgap and yields a stable reference voltage near 1.25 V at room temperature (300 K), which is temperature-independent when properly compensated. The bandgap energy directly influences the forward voltage drop across a p-n junction, VBEV_{BE}, making it the cornerstone for generating precise, stable voltages in integrated circuits. In a p-n junction under forward bias, VBEV_{BE} is primarily determined by the bandgap energy, as it governs the potential barrier that carriers must overcome for conduction. The dependence of VBEV_{BE} arises from the bandgap's variation with and the intrinsic properties of carrier . A key for this dependence is given by the formula: VBE(T)=Vg(0)−λT+TT0[VBE(T0)−Vg(0)+λT0],V_{BE}(T) = V_g(0) - \lambda T + \frac{T}{T_0} \left[ V_{BE}(T_0) - V_g(0) + \lambda T_0 \right], where Vg(0)V_g(0) is the extrapolated bandgap voltage at 0 , λ\lambda is the of the bandgap (approximately 3×10−43 \times 10^{-4} −1^{-1} for ), TT is the absolute , and T0T_0 is a reference temperature (often 300 ). This linear model captures the negative temperature coefficient of VBEV_{BE} (typically -1.5 to -2 mV/), which decreases as temperature rises due to the bandgap narrowing and increased thermal generation of carriers. The underlying mechanisms involve the , intrinsic carrier concentration, and processes in the p-n junction. In , the —a measure of the for electrons—remains constant across the junction, aligning the energy bands between p-type and n-type regions. The intrinsic carrier concentration nin_i, which quantifies thermally generated electron-hole pairs, is given by ni∝T3/2exp⁡(−Eg/2kT)n_i \propto T^{3/2} \exp(-E_g / 2kT), where kk is Boltzmann's constant; this exponential dependence on the bandgap drives the temperature sensitivity of junction currents. Under forward bias, dominates as minority carriers (electrons in p-type, holes in n-type) move down concentration gradients across the junction, injecting charge and enabling current flow proportional to exp⁡(qVBE/kT)\exp(qV_{BE}/kT), where qq is the electron charge. These processes link the bandgap physics to the stable voltage output in references. Silicon is the predominant material for bandgap voltage references due to its well-characterized bandgap properties, low cost, and compatibility with standard and bipolar fabrication processes, enabling reliable ~1.25 V outputs. In contrast, gallium arsenide (GaAs) features a wider bandgap of about 1.42 eV at 300 K, resulting in higher extrapolated reference voltages (around 1.4-1.5 V), but its higher cost and complexity limit it to specialized high-frequency or optoelectronic applications rather than general-purpose references.

Principles of Operation

Temperature Dependencies in Semiconductors

The base-emitter voltage VBEV_{BE} of a forward-biased exhibits a , typically around −2-2 mV/°C at under constant current density conditions, manifesting as complementary-to-absolute-temperature (CTAT) behavior. This dependence arises primarily from the temperature-induced decrease in the bandgap energy, which reduces the barrier for carrier injection across the , and from reductions in carrier mobility that diminish the with increasing temperature. Bandgap narrowing, particularly in heavily doped emitter regions, further exacerbates this effect by effectively lowering the bandgap and altering the intrinsic carrier concentration. A key aspect of this temperature dependence involves differences in VBEV_{BE} between transistors operating at different current densities, given by ΔVBE=kTqln⁡(I1I2)\Delta V_{BE} = \frac{kT}{q} \ln\left(\frac{I_1}{I_2}\right), where kk is Boltzmann's constant, TT is absolute , qq is the charge, and I1I_1, I2I_2 are the collector currents. This ΔVBE\Delta V_{BE} term is proportional to absolute (PTAT), providing a positive temperature coefficient of (k/q)ln⁡(n)≈0.2(k/q) \ln(n) \approx 0.2 mV/°C for a typical current density of n=10n=10 in devices. The overall temperature behavior of VBEV_{BE} can be approximated as VBE(T)=Vg−TTr(Vg−VBE(Tr))−αTV_{BE}(T) = V_g - \frac{T}{T_r} (V_g - V_{BE}(T_r)) - \alpha T, where VgV_g is the extrapolated bandgap voltage at 00 K (about 1.2051.205 V for ), TrT_r is a reference , VBE(Tr)V_{BE}(T_r) is the base-emitter voltage at TrT_r, and α\alpha is an empirical coefficient accounting for linear effects such as mobility variations (typically on the order of 10−510^{-5} to 10−410^{-4} V/). This model captures the first-order linear decline but overlooks higher-order nonlinearities. Process variations, such as fluctuations in doping levels during fabrication, significantly influence the of VBEV_{BE}, leading to spreads of up to ±24\pm 24 mV (3σ\sigma) in ΔVBE\Delta V_{BE} that propagate to errors in the overall temperature coefficient. Higher emitter doping, for instance, intensifies bandgap narrowing and alters recombination rates, resulting in more negative temperature coefficients and increased variability across devices. Second-order effects introduce curvature in the VBEV_{BE} versus temperature plot, stemming from nonlinear terms like the temperature dependence of the thermal voltage and logarithmic corrections in the saturation current expression, such as ηkTqln⁡(TTr)\eta \frac{kT}{q} \ln\left(\frac{T}{T_r}\right), where η\eta is a fitting parameter (often around 3-4 for silicon). These nonlinearities cause residual temperature dependence beyond the linear approximation, typically contributing a curvature of 1−101-10 μ\muV/°C², which must be considered for precision applications.

PTAT and CTAT Components

The proportional-to-absolute-temperature (PTAT) component in a bandgap voltage reference is generated by exploiting the difference in base-emitter voltages (VBEV_{BE}) across two bipolar junction transistors (BJTs) operated at different current densities. Specifically, if one transistor (Q1) carries a collector current IC1=II_{C1} = I and the other (Q2) carries IC2=nII_{C2} = nI where n>1n > 1, the voltage difference is ΔVBE=VTln⁡(n)\Delta V_{BE} = V_T \ln(n), with VT=kT/qV_T = kT/q being the thermal voltage, kk Boltzmann's constant, TT the absolute temperature, and qq the electron charge. This ΔVBE\Delta V_{BE} is inherently PTAT, increasing linearly with temperature, and is typically developed across a to produce a PTAT current or voltage term. The complementary-to-absolute-temperature (CTAT) component arises directly from the base-emitter voltage VBEV_{BE} of a single BJT, which exhibits a negative temperature coefficient of approximately -1.5 to -2 mV/°C at due to the temperature dependence of the bandgap energy and carrier concentrations in semiconductors. In the reference circuit, VBEV_{BE} serves as the CTAT term, providing a voltage that decreases with increasing and offsets the positive of the PTAT term. To achieve a temperature-independent output, the PTAT and CTAT terms are summed in a weighted manner: VREF=VBE+K⋅ΔVBEV_{REF} = V_{BE} + K \cdot \Delta V_{BE}, where KK is a dimensionless scaling factor (often implemented as a resistor ratio R2/R1R_2 / R_1). The value of KK is selected to null the first-order temperature coefficient, such that dVREF/dT≈0dV_{REF}/dT \approx 0, given by K≈(VG−VBE)/(ΔVBE)K \approx (V_G - V_{BE}) / (\Delta V_{BE}) at a reference temperature, where VGV_G is the extrapolated silicon bandgap voltage at 0 K (approximately 1.205 V). This cancellation balances the negative CTAT slope against the positive PTAT slope. The resulting reference voltage stabilizes near the bandgap voltage, derived as VREF≈VG+(T/Tr)(VBE(Tr)−VG)V_{REF} \approx V_G + (T / T_r) (V_{BE}(T_r) - V_G), where TrT_r is the reference temperature; for processes, this yields VREF≈1.25V_{REF} \approx 1.25 V with residual higher-order curvature. In basic implementations, operational amplifiers are employed to ensure equal collector currents in the PTAT-generating transistors via feedback (current mirroring) and to amplify or buffer the summed voltage for output stability without loading effects. A key limitation of this first-order approach is the minimum supply voltage required, typically around 1.4 V, arising from the need to support VBEV_{BE} (≈0.7 V) plus the saturation voltage VsatV_{sat} (≈0.2-0.3 V) of current sources or the operational amplifier output stage.

Circuit Designs

Brokaw Cell Topology

The Brokaw cell topology, introduced in 1974, represents a foundational design for bandgap voltage references using an operational amplifier to ensure balanced currents through two bipolar junction transistors (BJTs). In the classic schematic, two BJTs, Q1 and Q2, have their collectors connected through resistors to the power supply, with the collector nodes connected to the inputs of the operational amplifier, while their bases are driven by the output of the operational amplifier. The emitters of Q1 and Q2 are interconnected through a resistor R (often denoted as R1), creating a difference in base-emitter voltages (ΔV_BE) across this resistor due to differing current densities—typically achieved by making Q2's emitter area larger than Q1's (e.g., 8:1 or 10:1 ratio). Feedback is provided by two equal resistors (often labeled R2 for symmetry) connected from the amplifier's output to the bases of Q1 and Q2, which sets the gain for the PTAT term and, through the opamp action, ensures the amplifier inputs (at the collectors) are at the same potential, forcing equal collector currents in Q1 and Q2. Operationally, the circuit generates a proportional-to-absolute-temperature (PTAT) current through R, given by I_PTAT = ΔV_BE / R, where ΔV_BE arises from the logarithmic difference in current densities between Q1 and Q2. This PTAT current is mirrored and combined with the complementary-to-absolute-temperature (CTAT) component from the base-emitter voltage V_BE of Q1, producing a temperature-compensated output at the amplifier's output node. The key output voltage equation is: VOUT=2VBE+(2R2R1)ΔVBEV_{OUT} = 2 V_{BE} + \left( \frac{2 R_2}{R_1} \right) \Delta V_{BE} where the resistor ratio (2 R2 / R1) is tuned such that the positive of the ΔV_BE term cancels the negative coefficient of the V_BE terms, yielding a flat response near the silicon bandgap voltage of approximately 1.22 V extrapolated to 0 K, often resulting in an output around 1.25 V. The PTAT and CTAT principles ensure this compensation without requiring additional circuitry for basic operation. This topology offers several advantages, including simplicity with a low component count (typically seven transistors including mirrors), low power consumption, and good (PSRR) due to the feedback loop. Typical temperature coefficients (tempco) achieved are less than 50 ppm/°C over wide ranges like -55°C to +125°C, with residual drifts as low as 2-4 ppm/°C using matched thin-film resistors. However, it requires a bipolar process for the BJTs, as standard lacks high-quality vertical bipolars, and includes drawbacks such as the need for startup circuitry (e.g., a transient like a FET) to prevent the circuit from latching into a zero-current stable state. Variations include implementations using PNP BJTs for low-voltage operation in processes with parasitic bipolars or added bipolar layers, or NPN configurations for higher performance, adapting the design to constraints while maintaining the core feedback structure.

Widlar and Alternative Topologies

The Widlar bandgap voltage reference, introduced by Robert Widlar in 1971, employs a configuration with bipolar junction transistors (BJTs) and placed in the emitter paths to generate both proportional-to-absolute-temperature (PTAT) and complementary-to-absolute-temperature (CTAT) components. In this topology, two BJTs (Q1 and Q2) are biased with equal collector currents via a current mirror, but Q1 includes an emitter R that creates a voltage difference ΔV_BE across it, serving as the PTAT term derived from the thermal voltage V_T and the emitter area ratio n between Q2 and Q1. The CTAT component comes from the base-emitter voltage V_BE of a third transistor Q3, which sums with the amplified PTAT voltage at the output node through additional R1 and R2 to produce a temperature-stable reference voltage near the bandgap of 1.205 V. The output voltage for the Widlar topology is given by: VREF=VBE(1+2R2R1)+R2RVTln⁡(n)V_{REF} = V_{BE} \left(1 + \frac{2R_2}{R_1}\right) + \frac{R_2}{R} V_T \ln(n) where V_BE is the CTAT base-emitter voltage (approximately -2 mV/°C), V_T is the thermal voltage (kT/, approximately +0.086 mV/°C at ), and the resistor ratios and ln(n) term set the PTAT slope to cancel the CTAT nonlinearity, yielding minimal when optimized. Compared to the Brokaw cell, which relies on a voltage-feedback loop with an , the Widlar design offers advantages in low-power applications due to its current-mode operation, enabling smaller quiescent currents (often in the microamp range) and suitability for single-supply voltages as low as 2 V without requiring a high-gain . It also facilitates easier integration in processes by leveraging parasitic vertical PNP BJTs for the PTAT generation, reducing the need for specialized bipolar fabrication. Alternative topologies to the Widlar design include exponential current references that utilize logarithmic amplifiers to extract PTAT currents from transistor V_BE differences, converting them via anti-log stages to form the reference voltage, which can improve in certain sensor-integrated circuits. Another variation involves resistorless designs employing switched-capacitor techniques for PTAT voltage generation, where capacitors charge to ΔV_BE during clock phases and transfer the charge to mimic division, eliminating resistor-matching issues and while enabling sub-1 V operation in deep-submicron . In comparison to the Brokaw topology, the Widlar circuit exhibits higher (typically >1 MΩ), making it more ideal as a reference, though it is susceptible to offset voltages from current-mirror mismatches and tolerances, potentially requiring trimming for precision below 0.1%. Implementation challenges include ensuring accurate ratios (e.g., 10:1 for optimal TC cancellation) and area scaling for ln(n), as mismatches can introduce temperature drift up to 50 ppm/°C without calibration; additionally, thermal noise contributes to output ripple, often mitigated by filtering. This topology found early application in 1970s integrated circuits, such as National Semiconductor's LM109 and LM194 precision , where it provided stable referencing for op-amp biasing and regulator feedback loops in low-power analog systems.

Advanced Implementations

Low-Voltage and Sub-Bandgap References

Sub-bandgap voltage references achieve output voltages below the conventional 1.25 V silicon bandgap by adjusting the weighting factors in the summation of the complementary-to-absolute-temperature (CTAT) base-emitter voltage VBEV_{BE} and the proportional-to-absolute-temperature (PTAT) difference ΔVBE\Delta V_{BE}. The reference voltage is given by VREF=αVBE+βΔVBEV_{REF} = \alpha V_{BE} + \beta \Delta V_{BE}, where α<1\alpha < 1 and β>1\beta > 1 to yield VREFV_{REF} in the range of approximately 0.5–1.0 V. This approach modifies the standard PTAT/CTAT balance to prioritize lower output while maintaining temperature stability. Several techniques enable low-voltage operation below 1.25 V to meet the demands of modern low-power integrated circuits. Current-mode converts the PTAT and CTAT voltages into currents, which are then combined and mirrored to generate the , allowing operation with reduced supply headroom. Resistive division scales down the traditional bandgap output using a network, as pioneered in early sub-1 V designs, though it introduces sensitivity to resistor matching. Switched-capacitor techniques discretize the process to avoid continuous-time limitations, facilitating operation in deep-submicron processes with minimal static power. These methods face significant challenges in technologies, including reduced headroom for stacking, which limits minimum supply voltage to around 0.9 V in 0.18 μm processes due to drops. Additionally, altering the PTAT/CTAT weights exacerbates nonlinear curvature in the temperature dependence, increasing the compared to standard references. Representative examples include piecewise linear approximation, which segments the temperature curve for targeted compensation in sub-bandgap designs, and beta-multiplier references adapted for (IoT) devices, achieving quiescent currents below 1 μA while delivering stable low voltages. Post-2012 advancements have focused on nano-power implementations in advanced nodes, such as 28 nm , realizing 0.6 V outputs with temperature coefficients around 10 ppm/°C and supply voltages as low as 0.6 V for ultra-low-power applications.

Curvature Compensation and High-Precision Techniques

The nonlinear temperature dependence of the base-emitter voltage VBE(T)V_{BE}(T) in bipolar junction transistors introduces curvature in bandgap references, primarily arising from quadratic and higher-order terms such as Tln⁡TT \ln T that deviate from the ideal linear complementary-to-absolute-temperature (CTAT) behavior after first-order compensation. These terms, which become prominent over wide temperature ranges, result in residual temperature coefficients (tempcos) on the order of 10-50 ppm/°C without further correction. Curvature compensation techniques address these nonlinearities through higher-order cancellation methods. Piecewise linear compensation employs networks or switched segments to approximate the with linear segments tailored to different ranges, effectively reducing tempco to below 5 ppm/°C in implementations. Exponential CTAT generation leverages the nonlinear current gain β\beta of bipolar transistors to produce a compensation current with an exponential profile that counters the VBEV_{BE} , achieving sub-3 ppm/°C performance in trimmable designs. Dynamic element matching, often integrated with these approaches, randomizes mismatch errors in current mirrors to suppress low-frequency noise contributions to . High-precision enhancements further minimize errors beyond curvature correction. Chopping stabilization modulates the in the reference circuit at a high frequency to shift 1/f noise to higher frequencies, where it can be filtered, enabling tempcos as low as 1 ppm/°C while maintaining low power. trimming adjusts resistor values post-fabrication to achieve initial output accuracy below 0.1%, compensating for process variations in CTAT and PTAT ratios. Digital calibration, common in system-on-chip (SoC) integrations, uses on-chip logic to periodically adjust the reference based on embedded sensors, supporting tempcos under 2 ppm/°C across industrial ranges. These techniques enable bandgap references with tempcos below 2 ppm/°C over -40°C to +125°C. For instance, the ADR4550 achieves a maximum tempco of 2 ppm/°C through laser-trimmed compensation and low-noise amplification. Similarly, the LT6657 delivers 1.5 ppm/°C drift via chopping and precise PTAT/CTAT balancing. Post-2012 evolutions have extended these methods to advanced nodes. All-MOS designs utilizing parasitic vertical PNP bipolar transistors in standard processes have realized tempcos of 0.5 ppm/°C by exploiting MOS weak-inversion characteristics for PTAT generation alongside correction. In FinFET technologies at 7 nm nodes, bandgap references incorporate programmable compensation and reduced parasitics to maintain tempcos around 6 ppm/°C, supporting high-density integration in low-power SoCs.

Applications

Role in Analog Integrated Circuits

Bandgap voltage references play a critical role in analog integrated circuits by providing a stable, temperature-independent voltage source essential for precision operations. In analog-to-digital converters (ADCs) and digital-to-analog converters (DACs), they serve as the reference voltage for accurate signal conversion, ensuring consistent thresholds across varying environmental conditions. For instance, in successive approximation register (SAR) ADCs achieving 16-bit or higher resolution, the bandgap reference maintains the full-scale range with minimal drift, enabling effective number of bits (ENOB) close to the theoretical maximum by compensating for thermal variations in semiconductor parameters. In operational amplifiers (op-amps) and comparators, bandgap references establish precise levels and threshold voltages, contributing to -stable performance in feedback loops and decision-making circuits. A representative example is the adjustable , where an internal bandgap reference generates a nominal 1.25 V output that remains stable over a wide range, allowing the regulator to deliver adjustable outputs with low coefficients. This stability is vital for maintaining gain accuracy and offset in op-amp-based chains. On-chip integration of bandgap references in mixed-signal system-on-chips (SoCs), such as those in microcontrollers, enables compact, self-contained precision without external components. Noise management is a key aspect, with flicker (1/f) noise in bandgap circuits often mitigated through capacitor-based filtering to achieve integrated noise densities below 1 μV rms in low-frequency bands (e.g., 0.1–10 Hz), preserving signal integrity in sensitive analog blocks. Practical implementations highlight these roles; the shunt reference, leveraging a bandgap core, is widely used in switch-mode feedback loops within analog ICs to regulate output voltages with high precision and low . Similarly, the MAX6009 provides a low-power (1 μA quiescent current) bandgap-derived 2.5 V reference suited for battery-operated analog circuits, such as portable interfaces, where space and energy constraints demand minimal footprint and dissipation.

Use in Power Management and Sensors

Bandgap voltage references are integral to voltage regulators, where they provide a stable reference voltage for feedback loops to ensure output stability across wide temperature ranges. In linear regulators like the series and low-dropout (LDO) regulators, the bandgap reference maintains precise regulation by compensating for thermal variations, typically operating reliably from -40°C to 125°C. For instance, in LDO designs such as those based on curvature-compensated circuits, the reference achieves a temperature drift of 8.11 ppm/°C with input voltages from 3 V to 5 V, enabling consistent performance in power supply circuits. In battery management systems (BMS) for lithium-ion batteries, particularly in electric vehicles (EVs), bandgap references deliver temperature-compensated voltage thresholds for monitoring charge and discharge cycles, preventing overvoltage or undervoltage conditions. Devices like the Renesas ISL78600 multi-cell Li-ion battery manager employ precision bandgap references to achieve cell reading errors as low as 1.2 mV over a decade of operation, factoring in thermal stresses. High-precision implementations, such as current-mode bandgap circuits fabricated in 180 nm CMOS, offer a temperature coefficient of 2.69 ppm/°C over -40°C to 125°C, supporting accurate voltage monitoring in EV battery packs with line sensitivity of 42 ppm/V. For sensor applications, bandgap references serve as stable baselines for and conversion. In PTAT-based sensors, the reference combines with proportional-to-absolute- elements to generate accurate outputs, with series resistance compensation reducing non-linearity to 0.44 ppm/°C from 0°C to 85°C. Similarly, in readout circuits for resistive transducers like sensors, bandgap-derived references ensure precise ratiometric measurements, as demonstrated in designs achieving low noise and high linearity. In automotive and industrial environments, AEC-Q100 qualified bandgap references withstand harsh conditions including vibration, electromagnetic compatibility (EMC) issues, and elevated temperatures up to 150°C. For example, the Texas Instruments LM61440-Q1 synchronous buck converter integrates a bandgap reference for regulation, qualified under AEC-Q100 Grade 1 with junction temperatures to 150°C, supporting automotive power distribution. The Infineon TLS835B2ELV linear regulator similarly uses an internal bandgap-based voltage reference, operating from -40°C to 150°C while meeting AEC-Q100 standards for reliability in vibration-prone settings. Recent evolutions in low-power bandgap references cater to wearables and (IoT) devices, enabling always-on sensing with quiescent currents below 1 μA. Ultralow-power designs, such as those using N+-doped PMOS transistors in 0.18 μm processes, consume about 50 nA at 1.5 V supply while maintaining a of 75 ppm/°C over -40°C to 85°C, ideal for battery-constrained applications like 2020s smartwatches. These references often incorporate low-voltage techniques to operate below 1 V, further extending battery life in portable nodes.

History and Development

Early Concepts and Inventions

The roots of bandgap voltage references trace back to the limitations of early voltage references in the , when Zener diodes emerged as the primary discrete devices for providing stable voltages through reverse breakdown mechanisms. Invented based on Clarence Zener's theory of and practically realized in by the late , these diodes offered breakdown voltages around 5-7 V with reasonable stability but exhibited significant temperature dependence, typically with coefficients of ±1 to ±2 mV/°C that varied by operating voltage, making them unsuitable for precision applications over wide temperature ranges. This instability spurred exploration into fundamental semiconductor physics, particularly the temperature behavior of p-n junctions as described in William Shockley's 1949 theory of p-n junctions, which derived the diode equation V=kTqln⁡(IIS)V = \frac{kT}{q} \ln \left( \frac{I}{I_S} \right), revealing the base-emitter voltage VBEV_{BE} in bipolar transistors decreases by about -2 mV/°C due to the temperature-sensitive saturation current ISI_S. Early experimental data from Bell Laboratories, documented in 1950s transistor handbooks and reports, quantified VBEV_{BE} variations across temperatures, highlighting its complementary-to-absolute-temperature (CTAT) characteristic relative to the silicon bandgap energy Eg≈1.12E_g \approx 1.12 eV, which increases slightly with decreasing temperature and inspired ideas for combining CTAT and proportional-to-absolute-temperature (PTAT) effects to cancel thermal drifts. In the early , as monolithic integrated circuits gained traction, Robert Widlar at pioneered temperature-stable biasing circuits for operational amplifiers, notably in the μA702 released in 1964, where he employed current mirrors and ratios to generate PTAT voltage differences that partially compensated VBEV_{BE} drifts, achieving improved thermal performance over discrete Zener-based designs without yet forming a full bandgap reference. These efforts built directly on the extrapolated insights from VBEV_{BE} measurements, setting the stage for integrated temperature-independent references. The conceptual breakthrough occurred in 1964 when David Hilbiber, also at Fairchild, proposed the first bandgap voltage reference in his ISSCC presentation, summing a PTAT term—derived from the ΔVBE\Delta V_{BE} between two bipolar transistors operated at different current densities—with a CTAT VBEV_{BE} to produce an output near the extrapolated bandgap voltage of 1.22 V at 0 K, resulting in a circuit with a temperature coefficient below 10 ppm/°C. This innovation marked the shift toward fully integrated solutions, with Widlar implementing the first commercial bandgap in National Semiconductor's LM113 voltage regulator in 1971 and incorporating similar references into linear ICs like the LM10 op-amp shortly thereafter, enabling compact, on-chip precision biasing in analog systems.

Key Patents and Modern Evolutions

The foundational for the bandgap voltage reference was US3271660, granted in 1966 to David F. Hilbiber, which described a basic circuit combining a proportional-to-absolute- (PTAT) voltage from differently biased diodes and a complementary-to-absolute- (CTAT) base-emitter voltage to achieve stability. A practical implementation followed with US3617859 in 1971, issued to Robert C. Dobkin and Robert J. Widlar, introducing a regulator with a zero-temperature-coefficient reference suitable for monolithic fabrication. Further refinement came in US4249122 (1981) by Widlar, which enhanced low-power operation through optimized current mirroring and resistor scaling for . Dobkin advanced precision with US4447784 (1984), incorporating nonlinear curvature compensation to minimize second-order errors in the reference output. In the post-1980s era, bandgap designs evolved toward compatibility, particularly in the , enabling cost-effective integration in standard digital processes without bipolar transistors, as exemplified by National Semiconductor's developments in low-voltage references for mixed-signal ICs. The saw sub-1 V references emerge to support scaled supply voltages, with Malcovati et al. demonstrating a curvature-compensated BiCMOS bandgap operating at 1 V supply, achieving 5 ppm/°C and 0.54 V output. Modern evolutions from 2012 to 2025 emphasize nano-power consumption for and IoT applications, including sub-1 V designs in advanced nodes. Adaptive trimming techniques have improved accuracy, such as auto-trimming circuits achieving 9.99 ppm/°C over -40°C to 125°C with 54 µW consumption for AIoT chips. Industry milestones include ' ADR45xx series in the , offering <1 ppm/°C and ±0.02% initial accuracy for precision . ' REF50xx family, updated in the 2020s for automotive use, provides 3 ppm/°C drift and AEC-Q100 qualification for harsh environments. To address high-voltage gaps in silicon-based designs, recent shifts incorporate wide-bandgap materials like and GaN; a 4H-SiC voltage reference supports depletion-mode GaN circuits up to 200°C, reducing thermal limitations in power electronics.

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