Low-voltage reference uses the ∆VBE circuit
By Ron Mancini, Texas Instruments -- EDN, October 28, 2004
Some of my recent columns discuss the functions and advantages of zener-diode references (references 1, 2, and 3). Although zener diodes are stable and precise voltage references, they require high bias voltage (usually 8V minimum). The bias-voltage requirement precludes most zener-diode references from circuits with supply voltages of 5V or less. Low-voltage zener diodes are available, but their temperature drift renders them useless in precision applications. All semiconductor structures yield basic circuits that are temperature-sensitive; thus, any semiconductor reference must have some form of temperature compensation built in. Zener-diode references use an internal series diode for temperature compensation. Base-emitter-offset-voltage circuits have the same temperature-drift problem and solution, but because they do not depend on a zener junction, they function at lower voltage.
The difference equations for two transistors connected as diodes follow (Figure 1):
EQUATION 1
where ∆VBE is the base-emitter offset voltage, k is Boltzmann's constant, q is the electron charge, T is the absolute temperature, ln is the natural log, and JE is the emitter current density. When the units of current density are equal, they cancel each other out, and Equation 1 reduces to Equation 2, which expresses the temperature coefficient as a function of the collector currents.
EQUATION 2
You obtain the temperature coefficient by differentiating Equation 2.
EQUATION 3
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You use a diode-base-emitter junction to cancel out the drift that Equation 3 calculates, so the total drift must equal a diode drift of 2.18 mV/°C; the corresponding offset is 650 mV. You can't achieve the area ratio you need to meet these criteria through dissimilar junction areas because this ratio is too large; thus, use a smaller ratio, such as 12-to-1, and the resulting offset voltage is 63.9 mV. An amplifier with a gain of 10.17 yields an output voltage of 650 mV (Figure 2). Multiplying the 214-µV/°C result of Equation 3 by the 10.17 amplifier gain yields a drift of 2.18 mV/°C.
The complete bandgap reference uses two transistors for the diodes; these transistors feed current in a 12-to-1 ratio to generate the basic offset voltage and temperature coefficient. An amplifier that increases the offset voltage to 650 mV follows this stage. The output voltage drives the base of a pnp transistor, which has a current source at its emitter. The output voltage is the sum of the pnp base-emitter voltage and the multiplied differential-diode voltage: 1.25V. The final result is that the pnp base-emitter temperature coefficient cancels out the differential-diode temperature coefficient, yielding a stable, low-drift voltage reference. The output stage buffers the voltage reference and provides gain or attenuation to generate different reference voltages. This voltage reference functions in a low-voltage environment with temperature stability and low drift. In schematics, base-emitter-offset-voltage references often look similar to voltage regulators, such as the one in Figure 3, but don't confuse them with voltage regulators, because the references have limited current capacity.
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| Author Information |
 Ron Mancini is staff scientist at Texas Instruments. You can reach him at 1-352-569-9401, rmancini@ti.com. |
