Power amplifier bias modulation for multi-level supply envelope tracking

An envelope tracking system comprising:

• a power amplifier configured to receive a radio frequency signal at a signal input and to provide an amplified radio frequency signal at a signal output, the power amplifier including an amplifier transistor configured to receive the radio frequency signal at a base, and a cascode transistor having an emitter connected to a collector of the amplifier transistor and a collector connected to the signal output;
• a multi-level supply envelope tracker configured to generate a power amplifier supply voltage of the power amplifier based on an envelope signal indicating an envelope of the radio frequency signal;
• a programmable supply voltage filter operable to filter the power amplifier supply voltage to generate a filtered power amplifier supply voltage;
• and
• a bias modulation circuit configured to modulate a bias of a base of the cascode transistor of the power amplifier based on a voltage level of the filtered power amplifier supply voltage.

A method of envelope tracking, the method comprising:

• receiving a radio frequency signal at a signal input of a power amplifier and providing an amplified radio frequency signal at a signal output of the power amplifier, the power amplifier including an amplifier transistor having a base receiving the radio frequency signal and a cascode transistor having an emitter connected to a collector of the amplifier transistor and a collector connected to the signal output;
• generating a power amplifier supply voltage of the power amplifier based on an envelope signal using a multi-level supply envelope tracker, the envelope signal indicating an envelope of the radio frequency signal;
• filtering the power amplifier supply voltage to generate a filtered power amplifier supply voltage using a programmable supply voltage filter;
• and
• modulating a bias of a base of the cascode transistor of the power amplifier based on a voltage level of the filtered power amplifier supply voltage using a bias modulation circuit.