A CMOS bandgap reference without resistors

Bandgap references add the forward bias voltage across a pn diode with a voltage that is proportional to absolute temperature (PTAT) to produce an output that is insensitive to changes in temperature. The relative weighting of the voltages added is usually adjusted by trimming the ratio of two resistors. Although inexpensive resistors of suitable values are available in analog CMOS processes, the area of such resistors is increased in standard digital processes because silicide is often used to reduce the sheet resistance of the polysilicon and diffusion layers. As a result, the length and area of the required resistors is increased, increasing not only the cost, but also the susceptibility of the reference operation to substrate noise coupling. One way to overcome this problem is to use an extra mask to selectively block the silicide, but this mask also increases the cost. This paper presents a circuit solution to the above problem: a bandgap reference without resistors. It uses only MOS transistors biased in saturation or cutoff. The devices biased in saturation operate in strong inversion, for which accurate device models are usually available, simplifying the design process, especially in digital CMOS technologies.