RF Characterization of SiGe HBT Power Amplifiers Under Load Mismatch

We present the RF characterization of silicon-germanium heterojunction bipolar transistor (SiGe HBT) power amplifiers (PA) under load mismatched conditions. Experimental results demonstrate a strong dependence of a PA´s RF performance on the phase of mismatched antenna loads. For a load mismatch of voltage standing-wave ratio (VSWR) of 10:1, the 1-dB compressed RF output power (P_{1 dB}), transducer gain (G_T), output third-order intercept point, and power-added efficiency differ by 7.5 dBm, 8.1 dB, 8.3 dBm, and 15%, respectively, between the optimal and worst phase conditions, for an SiGe HBT PA biased at V_CC=3.3 V, collector current I_CE =400 mA, and frequency of 1.88 GHz. At the optimal phase condition up to VSWR of 10:1, the SiGe HBT PA maintains its linearity, RF output power, gain, and efficiency close to that at a VSWR of 1:1. At all the nonoptimal phases, the deterioration in the RF performance increases with the magnitude of load mismatches. The nonlinear characteristic of a PA under load mismatches is due to amplitude and phase-distortion mechanisms