The Influence of the Output Impedances of Peaking Power Amplifier on Broadband Doherty Amplifiers

The effect of the output impedance of peaking power amplifier (PA) on Doherty PAs (DPAs) is analyzed in this paper. In the design procedure of DPAs, the ideal case is that the output impedance of auxiliary PA is infinite at output power back-off (OPBO) level. However, it is almost impossible to realize this perfect condition in broadband DPAs. Therefore, when the output impedance of peaking path deviates from infinity, some potential effects on DPAs must be produced. In this paper, these effects are explained at the internal plane of transistors. The conclusion is that, at different normalized frequencies, there are different optimal impedance regions for the output impedance of peaking stage. This means that the noninfinite output impedances of peaking stage can enhance the performances of broadband DPAs so long as they are elaborately processed. A 1.65-2.7-GHz (48% bandwidth) broadband DPA is designed considering the effects of peaking PA. The experimental results show that this DPA obtains a drain efficiency of 41%-59.6% at 6-dB OPBO levels and a drain efficiency of 55.8%-72.2% at saturation power levels. The maximum output power across the entire operating band is 43.1-45.2 dBm with a gain of 9.0-10.2 dB. Furthermore, the designed DPA achieves an adjacent channel leakage ratio of -45.8 dBc with an output power of 36.1 dBm at 2.0 GHz after digital predistortion when it is excited by 5-MHz WCDMA signal with a peaking-to-average power ratio of 8.6 dB.