Bias boosting technique for a 1.9 GHz class AB RF amplifier

Abstract

A bias boosting technique for a 3.2 V, 1.9 GHz class AB RF amplifier designed in a 30 GHz BiCMOS process is presented in this paper. In a class AB amplifier, the average current drawn from the supply depends on the input signal level. As the output power increases so does the average currents in both the emitter and the base of the power transistor. The increased average current causes an increased voltage drop in the biasing circuitry and the ballast resistor. This reduces the conduction angle in the amplifier, pushing it deep into class B and even class C operation, reducing the maximum output power by 25%. To avoid the power reduction, the amplifier should have a larger bias which inevitably has a larger power dissipation at low output power levels. The proposed bias boosting circuitry dynamically increases the bias of the power transistor as the output power increases. The amplifier has less power dissipation at low power levels with an increased maximum output power.