This invention relates to voltage reference circuits, and more particularly, to a bandgap voltage reference circuit for providing a stable output voltage operating independent of temperature and power supply variations.
Voltage reference circuits are common in many modern electronic designs for providing a stable reference signal. The bandgap voltage reference circuit is well suited for this niche due to its temperature independent characteristics as discussed in an article entitled "A SIMPLE THREE-TERMINAL IC BANDGAP REFERENCE" by A. Paul Brokaw, IEEE Journal of Solid State Circuits, Vol. SC-9, No. 6, December, 1974. Briefly, the Brokaw article discloses a two transistor configuration conducting equal currents, but having dissimilar emitter areas, say eight-to-one, creating different current densities and base-emitter junction potentials (V.sub.be). The first transistor typically possesses the larger emitter area and, correspondingly, the lower current density and the lesser V.sub.be. By connecting two resistors in series with the emitter path of the first transistor and coupling the emitter of the second transistor to the interconnection thereof, a delta V.sub.be having a positive temperature coefficient is developed across the upper resistor. If the currents flowing through the first and second transistors are made of appropriate and constant magnitude and equal in value, the positive temperature coefficient of the voltage across the upper resistor tends to cancel the inherent negative temperature coefficient of the base-emitter junction of the first transistor thereby providing an output voltage at the collector of the second transistor which is insensitive to temperature variation, as is understood.
The current flowing through the first and second transistors is typically provided by a PNP transistor current mirror configuration having the emitters thereof coupled to the positive power supply conductor. Any transients appearing on the positive power supply are reflected in the current flowing through the first and second transistors, inducing variation in the V.sub.be 's thereof and the potential developed across the emitter resistors. This translates to movement in the collector potential of the second transistor, thus, the output voltage is dependent upon the power supply voltage. The fluctuation in the circuit signal levels attributed to power supply variation is commonly known as the Early voltage effect and is an undesirable condition which adversely influences the regulated output signal.
Hence, there is a need for an improved voltage reference circuit having an output voltage operating independent of temperature and power supply variations.