What is the difference between a bandgap reference and a Zener reference?

Bandgap references operate at low voltage (1.25 V typical), consume microamps of current, integrate on-chip in CMOS or BiCMOS processes, and achieve 5 to 50 ppm per degree Celsius temperature coefficients. Zener references use reverse breakdown of a junction (typically 5 to 7 V) and can achieve sub-1 ppm per degree Celsius stability in buried Zener designs (like the LTZ1000), but require higher operating voltage, dissipate milliwatts of power, and are only available as discrete components. For RF ICs, bandgap references are the standard because they integrate alongside the RF circuitry at supply voltages of 1.2 to 3.3 V.

Circuit Design

Bandgap Reference

Q: How does a bandgap voltage reference work?

A bandgap reference sums two voltages with opposite temperature coefficients to cancel the first-order temperature dependence. The base-emitter voltage V_BE of a bipolar transistor decreases at approximately -2 mV per degree Celsius (CTAT behavior). The difference in V_BE between two transistors running at different current densities (delta V_BE = kT/q times ln(N), where N is the current density ratio) increases with temperature (PTAT behavior). By scaling the PTAT voltage with a resistor ratio and adding it to V_BE, the output settles at approximately 1.25 V, which corresponds to the extrapolated bandgap voltage of silicon at 0 K. First-order temperature coefficients of 10 to 50 ppm per degree Celsius are typical; curvature-corrected designs achieve below 5 ppm per degree Celsius.

Q: Why are bandgap references critical in RF systems?

RF data converters (ADCs and DACs) require a voltage reference with stability better than half an LSB over temperature to maintain ENOB. A 14-bit DAC with a 1 V full scale needs reference stability better than 61 microvolts, or approximately 50 ppm over the operating range. RF synthesizers use bandgap references to set PLL charge pump current, directly affecting phase noise. LNA bias circuits depend on stable references to maintain consistent gain and noise figure across temperature. In phased array systems with hundreds of channels, reference drift causes beam pointing errors because each channel's phase shifter bias shifts differently.

/band-gap ref-er-ents/

A precision voltage reference circuit that produces a temperature-stable output of approximately 1.25 V by summing a CTAT (complementary to absolute temperature) base-emitter voltage with a PTAT (proportional to absolute temperature) thermal voltage difference. The 1.25 V output corresponds to the extrapolated bandgap energy of silicon at 0 K. First-order temperature coefficients of 10 to 50 ppm/°C are standard; curvature-corrected topologies achieve below 5 ppm/°C across -40 to +125°C, making them essential for biasing RF ADCs, DACs, LNAs, and PLL charge pumps.

Output: ~1.25 V

TC: 5-50 ppm/°C

Process: CMOS / BiCMOS

Understanding Bandgap References

Every RF integrated circuit needs stable bias voltages that remain constant across temperature, supply variation, and process corners. The bandgap reference solves this by exploiting two complementary temperature behaviors of bipolar junctions. A single transistor's V_BE drops at roughly -2 mV/°C (CTAT). The difference in V_BE between two transistors at different current densities (ΔV_BE = V_T × ln(N)) rises at +0.085 mV/°C per unit of ln(N) (PTAT). By scaling the PTAT voltage with a resistor ratio K and summing it with V_BE, the linear temperature terms cancel.

In modern RF transceiver SoCs, the bandgap reference is typically shared among multiple subsystems. The PLL charge pump draws a reference current to set loop bandwidth. The ADC and DAC rely on the reference for full-scale accuracy. LNA and PA bias DACs use it to maintain consistent gain. Any drift in the reference propagates to all these blocks, so post-layout simulation including package parasitics and substrate noise coupling is essential for production-grade designs.

Core Equations

Bandgap output voltage:
V_REF = V_BE + K × ΔV_BE
V_REF = V_BE + K × (kT/q) × ln(N)

At the zero-TC point:
∂V_REF/∂T = 0
K × (k/q) × ln(N) = |∂V_BE/∂T|
K × ln(N) ≈ 23.2 (for ∂V_BE/∂T = -2 mV/°C)

Typical values:
N = 8: K = ln(8) × correction ≈ 11.2
V_REF ≈ 0.6 V + 11.2 × 0.054 V ≈ 1.21 V

Reference Type Comparison

Type	Output (V)	TC (ppm/°C)	PSRR (dB)	Current	Integration
Basic BGR	1.25	20-50	-40	5-50 μA	On-chip CMOS
Curvature-corrected	1.25	2-10	-60	20-100 μA	On-chip BiCMOS
Sub-1V BGR	0.5-0.9	10-30	-50	1-10 μA	On-chip deep CMOS
Buried Zener	7.0	0.05-1	N/A	1-5 mA	Discrete only
FET-based	Varies	50-200	-30	0.5-5 μA	On-chip CMOS

Common Questions

Frequently Asked Questions

How does a bandgap voltage reference work?

It sums two voltages with opposite temperature coefficients. V_BE drops at -2 mV/°C (CTAT), while ΔV_BE between two transistors at different current densities rises proportionally to temperature (PTAT). A resistor ratio K scales the PTAT term so the linear temperature dependence cancels, yielding a stable output near 1.25 V. Curvature-corrected designs add a second-order compensation term to reduce residual parabolic drift below 5 ppm/°C.

Why are bandgap references critical in RF systems?

RF data converters need reference stability better than half an LSB over temperature. A 14-bit DAC with 1 V full scale requires better than 61 μV stability (50 ppm). PLL charge pump current, set by the reference, directly affects phase noise. LNA bias drift changes gain and noise figure. In phased arrays with hundreds of channels, reference drift causes beam pointing errors as each channel's phase shifter bias shifts differently.

How does a bandgap reference compare to a Zener reference?

Bandgap references operate at 1.25 V, consume microamps, integrate on-chip in CMOS/BiCMOS, and achieve 5-50 ppm/°C. Zener references use reverse breakdown at 5-7 V, achieve sub-1 ppm/°C in buried designs (LTZ1000), but require higher voltage, dissipate milliwatts, and remain discrete-only. For RF ICs operating at 1.2-3.3 V supply, bandgap references are the standard choice.

Related Terms

← Bandgap Energy Band n1 →