Low power output stage circuitry in an amplifier

摘要

A linear power amplifier having differential push-pull outputs, in which each output consists of an upper and a lower output transistor, includes cross-coupling transistors coupled between the gate of each of the output transistors and a reference voltage such that when the cross-coupled transistor is made conductive, it acts to turn off its associated output transistor. Each of the cross-coupled transistors is controlled by the voltage at the gate of the corresponding output transistor at the other of the differential outputs. Thus, the cross-coupling transistors insure that only one of the upper output transistors is on at one time and only one of the lower output transistors is on at any one time. These cross-coupling transistors operate in conjection with output stage shutoff circuitry to control the current wasted in the output stages of the linear power amplifier by ensuring that the output transistors at each output are not both conducting a significant amount of current at any one time. The output stage shutoff circuitry is activated by an external signal which is active when the differential input voltage to the amplifier is zero. During this time the output stage shutoff circuitry isolates the output transistors from the input stages, and forces the output transistors to be nonconductive. The cross-coupled control technique is also applied to single output stages in which cross-coupling transistors cause their respective output transistor to turn off when the other output transistor is conducting significant current.