Bias circuit for stacked power amplifiers with a variable power supply

Abstract

Stacked amplifier architectures that have bias circuits that are tolerant of supply voltage variations, which provide good reliability, are fast to follow Vchanges, and exhibit small variations in gain as Vvaries. One embodiment encompasses a bias circuit for an amplifier stack, including a source follower circuit configured to output a bias voltage to a transistor within the amplifier stack and configured to be coupled to a variable voltage, and a controller coupled to the source follower circuit, configured to be coupled to a constant voltage, and configured to apply a constant voltage to the source follower circuit. The source follower circuit generates a reference voltage and (1) outputs a constant bias voltage when the variable voltage is above the reference voltage, and (2) outputs a bias voltage proportional to the variable voltage when the variable voltage is below the reference voltage.