Step-response optimisation techniques for low-power, high-load, three-stage operational amplifiers driving large capacitive loads

Two simple efficient techniques to optimise the closed-loop transient response of three-stage amplifiers for large capacitive load applications are proposed and developed. The proposed approaches exploit a current comparator in the inner amplifier nodes to sense the input voltage transients and to switch on an auxiliary driving device providing slew-rate enhancement and settling time improvement without extra static power dissipation. SPECTRE simulations are carried out on a three-stage amplifier adopting a recently proposed reversed-nested Miller compensation strategy with a voltage follower and two nulling resistors, for which a novel design methodology is provided as well. Simulation results confirm the effectiveness of the two proposed techniques, showing a symmetrical step response with a significant improvement in large-signal speed performance. Both proposed solutions are suitable for any particular three-stage amplifier topology and are also independent of the adopted compensation network.