Indirect compensation techniques for three-stage fully-differential op-amps

As CMOS technology continues to evolve, the supply voltages are decreasing while at the same time the transistor threshold voltages are remaining relatively constant. Making matters worse, the inherent gain available from the nano-CMOS transistors is dropping. Traditional techniques for achieving high-gain by cascoding become less useful in nano-scale CMOS processes. Horizontal cascading (multi-stage) must be used in order to realize high-gain op-amps in low supply voltage processes. This paper discusses indirect compensation techniques for op-amps using split-length devices. A reversed-nested indirect compensated (RNIC) topology, employing double pole-zero cancellation, is illustrated for the design of three-stage op-amps. The RNIC topology is then extended to the design of three-stage fully-differential op-amps. Novel three-stage fully-differential gain-stage cascade structures are presented with efficient common mode feedback (CMFB) stabilization. Simulation results are presented for the designed RNIC fully-differential three-stage op-amps. The fully-differential three-stage op-amps, designed in 0.5 μm CMOS, typically exhibit 18 MHz unity-gain frequency, 82 dB open-loop DC gain, nearly 300 ns transient settling and 72° phase-margin for a 500 pF load.