Class-AB output stage design for high-speed three-stage op-amps

A new design approach for three-stage operational amplifiers (op-amps) is proposed. It allows to actually implement a symmetrical push-pull class-AB amplifier output stage for well-established three-stage amplifiers using a feedforward transconductance stage. Compared with the conventional design practice, the proposed approach leads to a significant improvement of the symmetry between the positive and the negative op-amp step response, resulting in similar values of the positive/negative settling time. The new approach proves to be very useful in order to fully exploit the potentiality allowed by the op-amp in terms of speed performances. Design examples in a commercial 0.35-µm CMOS prove the effectiveness of the proposed strategy.