Bias circuitry

A radio frequency amplifier system comprising:

• a power transistor having an amplifier control terminal coupled to a bias output node;
• and
• bias circuitry comprising:
• a bias drive device having a first current terminal coupled to a first voltage supply node, a bias control terminal coupled to a control node, and a second current terminal coupled to the bias output node;
• an impedance control device having a third current terminal and an impedance control terminal that are coupled together and a fourth current terminal coupled to a fixed voltage node;
• an output impedance resistor coupled between the third current terminal and the bias output node;
• and
• a pull-down device coupled between the bias output node and the fixed voltage node, wherein a higher voltage applied to the control node sets an output impedance at the bias output node to approximately equal to a lower impedance of the pull-down device in parallel with the bias drive device and a lower voltage applied to the control node sets the output impedance at the bias output node to approximately equal to resistance of the output impedance resistor.