An analytical model for the transient response of CMOS class AB operational amplifiers

An analytical study of the transient response of a CMOS class AB opamp operating in a voltage follower configuration is presented. As with class A opamps, we identify nonlinear and linear regions of operation corresponding to slewing and settling periods in the transient response. But in contrast with class A opamps, it is shown that the feedback configuration should be considered for the entire duration of the transient response. It is shown that doublets (pole-zero pairs) have significant impact on the transient response of the class AB opamps in both nonlinear and linear regions of operation. One result is that in order to prevent overshoot in the transient response due to the doublets, the pole of the double should be located at a frequency higher than about four times the unity-gain bandwidth. The proposed analytical model is valid for any location of the doublets. It agrees well with the results of HSPICE computer simulations, and has the advantage over the latter of providing circuit designers with a clear relationship between the design goals and the device parameters