The noise performance of Miller CMOS operational amplifiers with current-buffer frequency compensation

The noise of cascode transistors is usually considered to be negligible. It is demonstrated in this paper that this is not the case for an operational transconductance amplifiers exploiting Miller compensation via an embedded current buffer. Analysis, validated through simulations, shows that the noise power spectral density of the cascode transistor implementing the buffer, can be dominant at high frequencies. Its contribution can be of the same order of magnitude as that of the transistors in the first gain stage. A design equation is also given to limit the increase of this output noise contribution.