Patent ID: 7288992

Claim:
A bias circuit for a bipolar transistor (Tr 1 ), comprising a signal input connection arranged to communicate an input signal to a base terminal of the bipolar transistor (Tr 1 ); a bias current supply arranged to supply a current to a collector terminal of the bipolar transistor (Tr 1 ); and a ground connection arranged for coupling to an emitter terminal of the bipolar transistor (Tr 1 ); the bias circuit further comprising: a reference current generation circuit for generating a reference current indicative of a required current through the bipolar transistor (Tr 1 ); a current sensing circuit for providing a sense output signal indicative of an actual current flowing through the bipolar transistor (Tr 1 ); a comparator and an error amplifier arranged to receive the reference current and the sense output signal, and to generate an error signal indicative of a difference between the required current and the actual current, and to apply a corresponding correction to the voltage of the base of the bipolar transistor (Tr 1 ), thereby to cause the actual current to approach the required current, wherein the bias circuit is arranged for connection to both a regulated power supply and an unregulated power supply, whereby the reference current is provided by the regulated power supply, and substantially the remainder of current drawn by the bias circuit in operation is provided by the unregulated power supply.