A bulk-driven CMOS OTA with 68 dB DC gain

An ultra-low voltage rail-to-rail operational transconductance amplifier (OTA) based on a standard digital 0.18 mu m CMOS process is described in this paper. Techniques for designing a 0.8 volt fully differential OTA are discussed including bias and reference current generator circuits. To achieve rail-to-rail operation, complementary input differential pairs are used, where the bulk-driven technique is applied to reduce the threshold limitation of the MOSFET transistors. The OTA gain is increased by using auxiliary gain boosting amplifiers. This additional circuitry enables the OTA to operate at 0.8 volts, achieving an open loop gain of 68 dB while consuming 94 mu W. The DC gain of the amplifier is the highest gain achieved to date in bulk-driven amplifiers.