Converting a Three-Stage Pseudoclass-AB Amplifier to a True-Class-AB Amplifier

We convert a low-voltage low-transistor-count wide-swing multistage pseudoclass-AB amplifier proposed by Mita et al. to a true-class-AB amplifier. The conversion is made possible using gate-drain feedback to combine two inverting common- source amplifiers to form a single noninverting stage. Both the pseudoclassand true-class-AB amplifiers were fabricated in a 0.5-μm CMOS 2P3M process. They are designed to operate from ±1.25-V supplies at a nominal quiescent current of 175 μA and a minimum phase margin of 45° when driving capacitive loads from 1 to 200 pF and resistive loads from 500 Ω to 1 MΩ. The total com- pensation capacitance of the proposed class-AB amplifier is 12 pF, which is 50% less than the pseudoclass-AB amplifier. The simu- lated unity-gain frequency of the class-AB amplifier is 4.9 MHz at a load of 25 pF||1kΩ, which is 88% higher than that of the pseudoclass-AB amplifier. Experimental measurements show that the proposed amplifier has a maximum total bias current of 175 μA, as compared with 1.05 mA for the pseudoclass-AB am- plifier. Measured slew rates of the proposed amplifier are 2.7 and 3.3 V/μs, twice as much as those of its pseudoclass-AB counterpart.

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