Class-AB Amplifier with Third Order Nonlinearity Cancellation

Class-AB amplifiers have better power efficiency than class-A amplifiers, which makes them a suitable choice for pipeline ADCs where low power, high performance opamps are desired. However, class-AB amplifiers have significantly lower linearity compared to class-A amplifiers due to the problem of cross-over distortion. In this work, the problem of designing power efficient, high linearity class-AB amplifiers is addressed. The Monticelli bias scheme, which is the most popular class-AB bias scheme today, is analyzed. A new class-AB bias scheme, which is highly robust to process variations and achieves better linearity than the Monticelli bias scheme, is proposed. The problem of “cross-over distortion” in the class-AB amplifier is addressed and a solution is proposed. The idea behind this method is that the third order nonlinearity in the class-AB amplifier can be reduced or eliminated by matching the output stage NMOS and PMOS transistor transconductance across the input signal range, ensuring a constant transconductance at the output, improving linearity performance. To ensure good matching across process, voltage and temperature variations (P.V.T), a feedback calibration scheme is proposed