A bias circuit for a transistor amplifier, the bias circuit comprising a low-pass filter block, a reference transistor, a sum node, a reference current source, and a current difference block, wherein the low-pass filter block is configured to sense a DC bias voltage at a control terminal of the transistor amplifier and provide the DC bias voltage to a control terminal of the reference transistor; the reference transistor is configured to output a bias current in response to the DC bias voltage and provide the bias current to the sum node; the sum node is configured to receive a reference current from the reference current source and combine the reference current with the bias current from the reference transistor to provide a difference current; and the current difference block is configured to receive the difference current from the sum node and provide the difference current to the control terminal of the transistor amplifier.