A sub-0.5V, 1.5µW rail-to-rail constant gm opamp and its filter application

A low voltage, low power opamp utilizing normal NMOS and native transistor differential pairs as the input stage was implemented in a conventional 0.18μm CMOS process. The undesired conduction path of the native transistor differential pair at high input common-mode voltages was eliminated using an improved current control stage such that a supply voltage of <;0.5V was achieved. The g_m variations of the opamp input stage were measured to be <;6.9% for V_DD ranging from 0.45V to 1V with a total current consumption of ~3μA. A Sallen-Key low-pass biquad filter with a cut-off frequency at 7.3kHz was designed based on the proposed opamp. For a 0.5V supply, a THD of <;-57dB at 1.25kHz was measured for a 400mV_P-P output swing.