Brokaw bandgap reference is a voltage reference circuit widely used in

integrated circuit An integrated circuit or monolithic integrated circuit (also referred to as an IC, a chip, or a microchip) is a set of electronic circuits on one small flat piece (or "chip") of semiconductor material, usually silicon. Large numbers of tiny ...

s, with an output voltage around 1.25 V with low temperature dependence. This particular circuit is one type of a

bandgap voltage reference A bandgap voltage reference is a temperature independent voltage reference circuit widely used in integrated circuits. It produces a fixed (constant) voltage regardless of power supply variations, temperature changes, or circuit loading from a devi ...

, named after

Paul Brokaw Paul Brokaw is an expert on integrated circuit design who has spent most of his career at Analog Devices, where he holds the position of Analog Fellow. He is the inventor of many analog IC circuits, including the Brokaw bandgap reference Brokaw band ...

, the author of its first publication. Brokaw, P., "A simple three-terminal IC bandgap reference", ''IEEE Journal of Solid-State Circuits'', vol. 9, pp. 388–393, December 1974. Like all temperature-independent bandgap references, the circuit maintains an internal voltage source that has a positive temperature coefficient and another internal voltage source that has a negative temperature coefficient. By summing the two together, the temperature dependence can be canceled. Additionally, either of the two internal sources can be used as a

temperature sensor Mechanical temperature sensors * Thermometer * Therm Electrical temperature sensors * Thermistor- Thermistors are thermally sensitive resistors whose prime function is to exhibit a large, predictable and precise change in electrical resistance whe ...

. In the Brokaw bandgap reference, the circuit uses

negative feedback Negative feedback (or balancing feedback) occurs when some function (Mathematics), function of the output of a system, process, or mechanism is feedback, fed back in a manner that tends to reduce the fluctuations in the output, whether caused by ...

(by means of an

operational amplifier An operational amplifier (often op amp or opamp) is a DC-coupled high-gain electronic voltage amplifier with a differential input and, usually, a single-ended output. In this configuration, an op amp produces an output potential (relative to c ...

) to force a constant current through two

bipolar transistor A bipolar junction transistor (BJT) is a type of transistor that uses both electrons and electron holes as charge carriers. In contrast, a unipolar transistor, such as a field-effect transistor, uses only one kind of charge carrier. A bipolar t ...

s with different emitter areas. By the

Ebers–Moll model A bipolar junction transistor (BJT) is a type of transistor that uses both electrons and electron holes as charge carriers. In contrast, a unipolar transistor, such as a field-effect transistor, uses only one kind of charge carrier. A bipolar t ...

of a transistor, * The transistor with the larger emitter area requires a smaller base–emitter voltage for the same current. * The ''difference'' between the two base–emitter voltages has a positive temperature coefficient (i.e., it increases with temperature). * The base–emitter voltage for each transistor has a negative temperature coefficient (i.e., it decreases with temperature). The circuit output is the sum of one of the base–emitter voltages with a multiple of the base–emitter voltage differences. With appropriate component choices, the two opposing temperature coefficients will cancel each other exactly and the output will have no temperature dependence. In the example circuit shown, the opamp ensures that its inverting and non-inverting inputs are at the same voltage. This means that the currents in each collector resistor are identical, so the collector currents of Q1 and Q2 are also identical. If Q2 has an emitter area that is ''N'' times larger than Q1, its base-emitter voltage will be lower than that of Q1 by a magnitude of kT/q*ln(N). This voltage is generated across R2 and so defines the current ''I'' in each leg as kT/q*ln(N)/R2. The output voltage (at the opamp output) is therefore VBE(Q1) + 2*I*R1, or VBE(Q1)+2*kT/q*ln(N)*R1/R2. The first (VBE) term has a negative temperature coefficient; the second term has a positive temperature coefficient (from the ''T'' term). By an appropriate choice of N and R1 and R2, these temperature coefficients can be made to cancel, giving an output voltage that is nearly independent of temperature. The magnitude of this output voltage can be shown to be approximately equal to the bandgap voltage (EG0) of Silicon extrapolated to 0 K.

References

External links

Original IEEE paper(pdf)
nbsp;— This is the 1974 paper describing the circuit.
A Transistor Voltage Reference, and What the Band-Gap Has To Do With It
nbsp;— This 1989 video features Paul Brokaw explaining his bandgap voltage reference.
How to make a Bandgap Voltage Reference in One Easy Lesson
by A. Paul Brokaw of IDT
ELEN 689-602: Introduction to Bandgap Reference Generators
nbsp;— Includes detailed description and analysis of Brokaw bandgap reference.
The Design of Band-Gap Reference Circuits: Trials and Tribulations
nbsp;— Robert Pease, National Semiconductor (In "The Best of Bob Pease", page 286, shows Brokaw cell in Figure 3)
ECE 327: LM317 Bandgap Voltage Reference Example
nbsp;— Brief explanation of the temperature-independent bandgap reference circuit within the LM317. The circuit is nearly identical, but the document discusses how the circuit allows different currents through matched transistors (rather than a single current through different transistors) can set up the same voltages with opposing temperature coefficients. {{DEFAULTSORT:Brokaw Bandgap Reference Electronic circuits Analog circuits

See also

References

External links