Adaptive body biasing in CMOS circuits to extend the input common mode operating range

Abstract

In order to get the best of both high and low common mode ranges, an adaptive body biasing method using a pair of replica devices is implemented. Each replica device corresponds to a NMOS (or PMOS) device that constitutes the input pair used in a logic circuit or other type of integrated circuits. This configuration helps to increase the threshold voltage of the device, utilizing body effect, at high input common mode voltage, as desired for NMOS, and at low input common mode voltage, as desired for PMOS. At the same time, this configuration scales the threshold back to normal at low input common mode voltages, thereby countering the negative impact of body effect. In short, the body bias applied to the NMOS (or PMOS) device helps in adapting the threshold voltage to the operating condition.

Description

BACKGROUNDTechnical Field[0001]Described are methods and apparatus for biasing integrated circuit devices to extract desired performance range.Brief Description of the Prior Art[0002]In a typical Complementary Metal Oxide Semiconductor (CMOS) circuit having an input stage comprising negative-channel MOS (NMOS) devices or positive-channel MOS (PMOS) devices, the maximum or minimum input common mode voltage is determined by the threshold voltage of the NMOS or PMOS.[0003]In case of an NMOS input pair, to achieve a high common mode input range, the body of the NMOS input pair is typically connected to Ground (GND) or the lowest voltage (VSS) in order to raise the threshold voltage through body effect. The body effect is the change in threshold voltage of a transistor due to the voltage difference between the source and bulk (substrate) of the transistor device. Connecting the body of the input pair NMOS devices to Ground has a negative impact in that the low common mode input range is ...

AI Technical Summary

Benefits of technology

The patent text describes a biasing scheme for NMOS and PMOS input devices that allows them to operate in a wider range of common mode voltage. The biasing scheme should also prevent the small signal source-body voltage from affecting the input device's gain. An adaptive body biasing method using a pair of replica devices is implemented to achieve this. The replica device has the same parameters and dimensions as the corresponding NMOS or PMOS device. The device is built in an isolated well structure of the process. This method helps in adapting the threshold voltage to the operating condition. Embodiments of this disclosure increase the threshold voltage of the device at high input common mode voltage for NMOS and at low input common mode voltage for PMOS while scaling the threshold back to normal at low input common mode voltages.

Problems solved by technology

Connecting the body of the input pair NMOS devices to Ground has a negative impact in that the low common mode input range is sacrificed and the NMOS devices will turn off earlier as the input common mode voltage is decreased.

This has a negative impact in that the high common mode input range is sacrificed and the PMOS devices will turn off earlier as the input common mode voltage is increased.

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