Bias circuit for stacked power amplifiers with a variable power supply

A bias circuit for an amplifier stack, the bias circuit including:

• (a) a source follower circuit configured to output a bias voltage when coupled to a gate of a transistor within the amplifier stack, the source follower circuit configured to be coupled to a variable voltage source for the amplifier stack, wherein the source follower circuit includes:
• (1) a source follower transistor having a conduction channel configured to be coupled to the variable voltage source and having a gate;
• (2) a first resistor coupled between the conduction channel of the source follower transistor and a reference potential;
• and
• (3) an output between the source follower transistor and the resistor, the output configured to be coupled to the gate of the transistor within the amplifier stack;
• and
• (b) a controller coupled to the source follower circuit, configured to be coupled to a constant voltage source, and configured to apply a constant voltage to the source follower circuit, wherein the controller includes:
• (1) a transistor having a conduction channel configured to be coupled to the constant voltage source and having a gate;
• (2) a second resistor coupled to the conduction channel of the transistor;
• (3) a third resistor coupled between the second resistor and the reference potential;
• (4) an operational amplifier having a first input coupled to a node between the second and third resistors, a second input coupled to a reference voltage, and an output coupled to the gate of the transistor and to the gate of the source follower transistor.

An amplifier, including:

• (a) an amplifier stack comprising at least two series-connected FETs and configured to be coupled to a variable voltage source;
• (b) a bias circuit for the amplifier stack, the bias circuit including, for at least one FET within the amplifier stack:
• (1) a source follower circuit including an input terminal, an output terminal coupled to a gate of the at least one FET, and a control input, the source follower circuit configured to have the input terminal be coupled to the variable voltage source for the amplifier stack and to output a bias voltage on the output terminal to the gate of the at least one FET;
• and
• (2) a controller coupled to the control input of the source follower circuit, configured to be coupled to a constant voltage source, and configured to apply a constant voltage to the control input of the source follower circuit.