Slew rate boost circuitry and method

Abstract

An operational amplifier includes a differential input stage ( ) having first ( ) and second ( ) input conductors, a class AB output stage ( ) coupled to an output of the differential input stage ( ) and including a pull-up transistor (M ) having a source coupled to a first supply voltage (V ), a drain coupled to an output conductor ( ), and a gate coupled to a first terminal ( ) of a class AB control circuit ( ), and a pull-down transistor (M ) having a source coupled to a second supply voltage (GND), a drain coupled to the output conductor ( ), and a gate coupled to a second terminal ( ) of the class AB control circuit ( ). A differential input signal is applied between the first ( ) and second ( ) input conductors, and simultaneously also is applied between first and second inputs of a first unbalanced differential amplifier ( ) and between first and second input to the second unbalanced differential amplifier ( ). If the differential input signal is of a first polarity and is of a magnitude substantially greater than a threshold voltage of the first unbalanced differential amplifier ( ), the magnitude of a turn-on voltage of the pull-down transistor (M ) is decreased and the magnitude of a turn-on voltage of the pull-up transistor (M ) is increased in response to an output voltage produced by the first unbalanced differential amplifier ( ). However, if the differential input signal is of a second polarity and is of a magnitude substantially greater than a threshold voltage of the second unbalanced differential amplifier ( ), then the magnitude of a turn-on voltage of the pull-up transistor (M ) is increased and the magnitude of a turn-on voltage of the pull-down transistor (M ) is simultaneously decreased, in response to an output voltage produced by the second unbalanced differential amplifier ( ).