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The complexity of a digital circuit produced on a single chip is usually described in terms of the number of transistors involved, or sometimes of the number of logic gates. These may be combined on the chip to make a wide variety of digital and analog circuits. The improvement in the fabrication technology of integrated circuits has made possible the construction of a huge number of components on a single chip. They are also less simple to fabricate than MOS circuits. Compared with MOS circuits, bipolar circuits have higher operating speeds but have the disadvantages of high power consumption and low packing density. In bipolar integrated circuits the components are bipolar transistors and other devices that are fabricated using the p-n junction properties of semiconductors. The development of MOS technology has allowed extremely complex functions to be performed on a single chip. MOS circuits have a high packing density. A very large number of MOSFETs can be packed together on one silicon chip, i.e.
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In MOS integrated circuits the active devices are MOSFETs, which operate at low currents and high frequencies. The individual devices normally consist of semiconductor diodes, transistors, and resistors. For more information about this group of bandgap references, please refer to the JSSC paper, " A CMOS bandgap reference circuit with sub-1-V operation".An implementation of a particular electronic-circuit function in which all the individual devices required to realize the function are fabricated on a single chip of semiconductor, usually silicon. Later, researchers developed bandgap references called fractional bandgaps that could output voltages as low as a few millivolts. This was the first precision bandgap-based voltage reference. These issues were later resolved by a groundbreaking design introduced by A. Moreover, it could not produce useful voltage levels such as 2.5 V and 5 V. Despite being a great achievement, it had current drive sensitivity limitations. Widlar’s voltage reference, which was published in 1971, laid the foundation for today’s bandgap references. Typical bandgap references can achieve temperature coefficients as low as 20 ppm/☌. This is much less than the temperature coefficient of a base-emitter voltage. The temperature coefficient of the thermal voltage is $$\fracC$$ In this equation, k is the Boltzmann constant, q is the charge carried by a single electron, and T is temperature in Kelvin. One way of generating the 2 mV/☌ temperature coefficient is by noting that the thermal voltage ( V T) given by the following equation is a linear function of the absolute temperature: For example, if we can generate a voltage that’s a linear function of the absolute temperature and has a positive temperature coefficient of 2 mV/☌, then we may be able to compensate for the variations introduced by the base-emitter junction. If we don’t have access to a better device, we’ll have to somehow compensate for the temperature-induced variations. Such device limitations will affect the circuit output. Unfortunately, the ambient temperature can affect the properties of the different components within a circuit.įor example, the base-emitter voltage of a BJT transistor is a linear function of the absolute temperature and exhibits a temperature coefficient of about -2 mV/☌. The goal of a voltage reference is to generate a stable voltage that is ideally independent of changes in temperature and other external factors. Additionally, there is a group of bandgap references, called fractional bandgaps, that can create output voltages as low as a few millivolts. We will see that a normal bandgap voltage reference can generate reference voltages as low as about 1.23 V. In addition to these drawbacks, the Zener diode based method required supply levels larger than 5 V. Before that, the only semiconductor solution relied on using noisy, temperature-sensitive Zener diodes. The bandgap reference technique is one of the most commonly used methods for creating a temperature-independent reference voltage.īob Widlar, the legendary electronics engineer, laid the foundation for today’s bandgap voltage references in the late 1960s. This article presents some basic information about bandgap circuits, which are widely used to generate temperature independent reference voltages.