A common mode feedback structure for differential OpAmps using NMOS depletion transistors

A fully differential amplifier featuring a novel common mode feedback (CMFB) structure is presented. Measurement results for the circuit made in a 0.8 m process prove that the new structure provides rail-to-rail common and differential mode input range that is difficult to handle for all active CMFB structures reported so far. This improvement is accomplished by the use of a nMOS depletion input pair transistors that can easily be processed in standard CMOS technologies. The differential OpAmp is designed for 3 V power supply and provides 61 dB amplification and a bandwidth of 24 MHz.