The pole-zero doublet: A cascode operational amplifier with cross coupled capacitor

A symbolic analysis is presented to investigate the effect of cross coupled capacitor on a cascode operational amplifier. A complete transfer function of the amplifier with cross coupled capacitor is derived and verified through circuit simulations. The modeled transfer function shows presence of a pole-zero doublet in the amplifier's frequency response when the cross-coupled capacitor is connected across it. The analysis further results in a closed form equations to optimize the amplifier based on user defined specifications such as, an open loop dc gain, a unity gain bandwidth and a phase margin. To check the validity of the model, a 50 dB open loop dc gain and 255 MHz unity gain bandwidth cascode amplifier is designed in a standard 90 nm CMOS technology with the supply voltage of 1.5 V. The results obtained from simulation and modeled transfer function show good agreement with each other and have an acceptable average relative error of 3 %.