Indirect compensation techniques for three-stage CMOS 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 vertically stacking (i.e. cascoding) transistors becomes 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 new design techniques for the realization of three-stage op-amps. The proposed and experimentally verified op-amps, fabricated in 500 nm CMOS, typically exhibit 30 MHz unity-gain frequency, near 100ns transient settling and 72° phase-margin for 500pF load. This results in significantly higher op-amp performance metrics over the traditional op-amp designs while at the same time having smaller layout area.