A multistage amplifier topology with nested Gm-C compensation for low-voltage application

To design an operational amplifier for low-voltage applications, cascoding is no longer a suitable technique for achieving high DC gain. Instead, multiple cascaded stages must he used, with each stage a simple (noncascode) inverting or noninverting amplifier. In designing a multistage opamp with multiple feedback loops, special care must be taken to ensure stability. A well-known compensation technique is nested Miller compensation. The complexity of the transfer function of the NMC based multistage amplifiers is reflected on its stability conditions. This makes it difficult to devise a systematic design procedure that yields stable NMC-based amplifiers. A topology using nested transconductance (G/sub m/)-capacitance compensation (NGCC) has a simple transfer function that yields simple stability conditions. These conditions can be exploited to simplify design. The authors show an n-stage NGCC amplifier topology consisting of n nested modules and describe a four-stage implementation using a 2 /spl mu/m CMOS process.