Measurement of load current in a multi-phase power amplifier

Abstract

A power amplifier supplies substantial load currents that must be accurately monitored to provide feedback so that the load currents can be properly controlled. A sense current is generated from a load current that is scaled such that the sense current is an accurate representation of the load current but having a substantially smaller magnitude. The scaled sense current is generated by coupling a power sense resistor to a pilot sense resistor by a voltage follower. The power sense resistor is in series with the load current and develops a load voltage thereacross. A sense voltage, being substantially equal to the load voltage is impressed at the pilot sense resistor by the voltage follower. The pilot sense resistor is some predetermined ratio of the power sense resistor so that absolute values are not critical. The sense current flowing through the pilot sense resistor is therefore scaled according to the ratios of the power and pilot sense resistors. A current mirror supplies current to the voltage follower and a cascode current mirror is further connected to the current mirror to diminish an undesired Early effect in the transistors making up the voltage follower and the current mirror. The sense current accurately follows the load current even over high frequencies.