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A voltage reference is not only a convenience, but is also a necessity for many electronic devices, for example a voltage DAC which converts a digital input to an output voltage. This result is the product of the digital word, which is scalar with no dimension, and some voltage to which the scaled output is referred. The DAC needs to “know” how big a certain voltage is, in order to set its full scale value. We need a component which “knows” how big a volt is. It turns out that the silicon all around us can be persuaded to give up its secret knowledge of the Volt if we will accommodate it. The base emitter voltage of a bipolar transistor versus temperature can, in theory, be extrapolated to equal a known physical constant which has the dimension Volt at a temperature of zero Kelvin. This constant is called the extrapolated bandgap of silicon. Unfortunately, this voltage is temperature sensitive,
but predictably so. However, silicon has a second property which also relates temperature and voltage and this can be combined with the change in Vbe to almost cancel the temperature effects and make a voltage approximating the bandgap at all temperatures, or at least the temperatures most of us require