Abstract: A common base amplifier (29) has an input (31) and an output (32). A transistor (33) has an emitter coupled to the input (31) of the amplifier (29), a collector coupled to the output (32) of the amplifier (29), and a base coupled to a voltage reference (34) provides low input impedance and unity current gain. A control circuit (38) controls a first bias circuit (36) and a second bias circuit (37). The second bias circuit (37) is coupled to the collector of the transistor (33) and provides a bias current for the transistor (33) while transistor (33) outputs the bias current which is received by the first bias circuit (36). Control circuit (38) determines the current magnitude for both the first bias circuit (36) and the second bias circuit (37) and ensures that the current magnitudes are maintained at a fixed ratio.