| High linearity RF power amplifiers (PA) are required in wireless mobile systems based on CDMA, W-CDMA standards, which use non-constant envelope and multi-carrier modulation schemes. These wireless mobile systems often encounter impedance mismatches at an antenna due to signal reflections from the surrounding objects. RF power is reflected back into a power amplifier due to antenna mismatch, and degrades the power amplifier's linearity, RF output power and efficiency.; We present the RF characterization of SiGe and GaAs HBT power amplifiers under load mismatched conditions, and demonstrate the strong dependence of a PA's RF performance on the phase of mismatched loads. Experimental results show that at an optimal phase condition up to a load VSWR of 6:1, SiGe and GaAs PAs maintain their RF output power, and linearity close to that of a load VSWR of 1:1. At all non-optimal phases, the RF output power, linearity, transducer gain and power added efficiency of a PA degrades swiftly. RF output power for SiGe and GaAs PAs varies by ∼6--8 dB, while adjacent channel power ratio changes by ∼23 dBc over the phase of load reflection coefficient at a load VSWR of 4:1.; We propose an adaptive phase tuning circuit technique to preserve the linearity of a power amplifier under load impedance mismatches. It dynamically tunes the phase of reflection coefficient at the output of a PA to provide optimal impedance match, and maintain RF performance close to that at 50 O termination for all load conditions. A power amplifier module integrates an adaptive phase control loop consisting of a phase shifter, directional couplers, phase detector and control circuitry, at the output of a PA. Using this technique, a 900 MHz GaAs PA in low power mode delivers a constant 1-dB compressed RF output power of 19.2 dBm +/-0.5 dB at all the phases of load mismatch of VSWR of 4:1. The measured ACPR for the module under reverse link CDMA IS95 signal at an offset of 1.98 MHz is less than -40 dBc over all load phases at VSWR of 4:1, and an RF output power of 18 dBm. |
