Subthreshold MOSFET transistor amplifier operation

Due to their ultra low-power consumption and the high demand of portable applications, subthreshold MOS-FET transistor operation (where the power-supply voltage is lower than the threshold voltage, V_DD<V_th) took considerable interest in the last decade. However, most of the subthreshold transistor applications are in the logic circuits domain. In this paper, we will investigate the application of the MOSFET transistor in the subthreshold region as an amplifier, develop the ac small-small equivalent circuit, derive expressions for the voltage gain in a common-source configuration, and compare this amplifier with that using the MOSFET transistor operating in the superthreshold region (where V_DD>V_th).