Active Bias Circuit used for Linearity Improvement in an HBT Power Amplifier

This work presents a method for linearising a Power Amplifier by adjusting the operating point as a function of input power. The goal is to achieve a linear response for higher output power by decreasing the gain for lower power levels and keep constant gain at higher power levels by increasing the base current. A very simple circuit utilizes that the base-emitter voltage drops at high output levels and boosts the base current at these levels. One amplifier is optimized for minimum gain ripple and output power at P1-dB is increased by 1,7 dB. The other is optimized for minimum phase compression and output power at P1-dB is increased by 2,2 dB. Both amplifiers are compared to a class A amplifier. Third order IMD products are smaller for output power larger than 23,5 dBm for the case of minimum gain ripple, while the IMD performance is better for all output powers for the case of minimum phase compression. Both amplifiers achieve higher efficiency than the class A amplifier, especially at low output power.