Abstract:
In this paper, we propose MOSFET model in region II (saturation) of a subthreshold region (where
current becomes nearly constant with respect to drain-source voltage for a fixed gate-source voltage as shown in
figure 1). We also propose designs of current mirrors operating in region II of subthreshold region for biomedical
applications circuits like pacemakers, retinal implants, neural recording systems which are to be implanted within
chest, eye and skull respectively. These circuits also find use in emerging electronics devices such as palmtops, laptops
etc. The most important property of these circuits is extremely low power consumption in order to increase battery
life time yet it has structural simplicity. The proposed current mirror has a power dissipation ranging from 4nW to
1000nW for an input current ranging from 1nA to 250nA. Its output resistance is found to be of the order of 108 to
1010 . The minimum source voltage (V_dd or V_source) required to bias the proposed mirror varies between 550
mV to 875mV for the input current varying between 1nA to 250nA with W/L RATIO of all MOS transistors being
kept at 1. The minimum source voltage required further decreases if we increase W/L RATIO of MOS transistors.
The current mirror shows extremely low temperature sensitivity of 0.031ppm/degree C and it has a very high
resistance to source voltage fluctuations in comparison to the best current mirror circuits reported till date. The
operations of the current mirror have been validated through simulations in Cadence using 180nm TAIWAN
SEMICONDUCTOR MANUFACTURING COMPANY (TSMC) technology. The purpose of this paper is to model a
MOSFET in region II of the subthreshold region and to use that model to show that a topologically simple current
mirror designed in the subthreshold region works better than highly complex, saturation region current mirrors in
terms of output resistance, matching accuracy and variation with temperature.