Driving Power Op Amps Closer To Their Rails

摘要

Due to the typical circuit topologies found in the output stages of power operational amplifiers, the output voltage swing may be limited to as much as 12 V less than the applied power supply voltage. The discussion that follows illustrates one of several practical techniques that can significantly increasing efficiency by driving the output closer to the rail voltages using power op amps with special characteristics. Using these techniques can enhance power output by as much as 50%. Some power op amps have two sets of power supply terminals, enabling the user to apply a separate higher-voltage supply to the input stages of the amplifier, while the output stage is set to a lower voltage. For example, the power op amp depicted in Fig. 1 uses this arrangement to achieve a "close-to-rail" voltage swing since the output devices, usually enhancement-mode MOSFETs, are driven close to saturation by the higher voltage (V_s) available to drive the gates.