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A high-speed driving scheme and a compact high-speed low-power rail-to-rail class-B buffer amplifier, which are suitable for small- and large-size liquid crystal display applications, are proposed. The driving scheme incorporates two output driving stages in which the output of the first output driving stage is connected to the inverting input and that of the second driving stage is connected to the capacitive load. A compensation resistor is connected between the two output stages for stability. The second output stage is used to improve the slew rate and the settling time. The buffer draws little current while static but has a large driving capability while transient. The circuit achieves the large driving capability by employing simple comparators to sense the transients of the input to turn on the output stages, which are statically off in the stable state. This increases the speed of the circuit without increasing static power consumption too much. A rail-to-rail folded-cascode differential amplifier is used to amplify the input signal difference and supply the bias voltages for the second stage. An experimental prototype output buffer implemented in a 0.35-μm CMOS technology demonstrates that the circuit draws only 7-μA static current and exhibits the settling times of 2.7 μs for rising and 2.9 μs for falling edges for a voltage swing of 3.3 V under a 600-pF capacitance load with a power supply of 3.3 V. The active area of this buffer is only 46.5×57μm2.
Date of Publication: Nov. 2004