| dc.description.abstract | By enhancing the power efficiency of RF power amplifiers (PAs), the heating problem of mobile devices can be alleviated and their talk time can be extended. Variable load technique has been proposed to enhance the efficiency of PAs at power back-off. In this work, a PA is designed at 1.95 GHz. Ferroelectric capacitors are used in the output matching network of the PA to provide a tunable load impedance, demonstrating efficiency enhancement.
A fabrication process for ferroelectric parallel-plate capacitors is developed. Measurement results show that, the capacitance of a ferroelectric varactor with an area of 10×10 μm2 is 2 pF. When biased at about 7 V, the capacitance of the ferroelectric varactor can be reduced to half of the capacitance at 0-V bias. At 1.95 GHz, the quality factor of the ferroelectric varactor is above 20.
A 1.95-GHz tunable PA is designed using a commercially available discrete pHEMT transistor. The drain and gate of the transistor are biased at 5 V and 0.45 V, respectively, with the quiescent current being 256 mA. The power amplifier operates in Class AB region. Load-pull simulations under different input power levels are performed using ADS. The simulated data is then processed using Matlab to find the optimum load impendences that maximize the power-added efficiency (PAE) at various output power levels. After that, we then proceed to design a ferroelectric-based tunable matching network, of which the impedance coverage must encompass the desired optimum load impendences.
Based on the fabrication process developed by us, the ferroelectric-based tunable matching network is on a sapphire substrate. On the same substrate, the pHEMT transistor and other passive components are mounted using epoxy, completing the fabrication of the tunable PA. On wafer measurement is performed. The measured impendence coverage of the tunable matching network is found to be close to the simulated impedance coverage. At 1.95 GHz, the maximum output P1dB of the tunable PA is approximately 13.1 dBm. Its small-signal gain is about 6 dB. By adjusting the bias voltages of the tunable matching network, the efficiency enhancement and the dc power consumption reduction are observed as long as the power back-off is within 3 dB. At 3-dB back-off, the dc power consumption is reduced from 660 mW to 570 mW.
In this work, a fabrication process for ferroelectric varactors is developed. A tunable matching network is designed based on the ferroelectric varactors. Finally, by changing the bias voltages of the tunable matching network, it is demonstrated that the efficiency of the PA at power back-off is indeed enhanced. | en_US |