Class-E Power Amplifiers for Pulsed Transmitters
Licentiate thesis, 2012
Nowadays the main driving parameters for the radio transmitter research are: energy efficiency, frequency re-configurability and integration. Pulsed transmitter architectures have attracted large interest in the recent years due to their potential to meet these demands. In pulsed transmitters, a highly efficient switch mode power amplifier (SMPA) is used in conjunction with a pulse modulator to achieve high efficiency linear amplification. Digital implementation of the modulator enables wide bandwidth for the signal generation path and improves the level integration. The bandwidth in a pulsed transmitter is however typically limited by the load matching network of the SMPA. This thesis presents two main contributions in the field of pulsed transmitter architectures.
In the first part focus is given to bandwidth improvement of class-E switch mode PAs. A continuum of novel closed-form class-E modes is derived extending the traditional class-E design space. The extended design space provides important possibilities for wide band design of the class-E load networks and for further efficiency optimization of class-E PAs. A wide band design methodology is thus developed based on the analytical design equations.
In the second part, focus is given to efficiency improvement of RF pulse width modulation (RF-PWM) based transmitters. A new SMPA topology particularly suitable for energy efficient amplification of RF-PWM signals is derived. It is analytically shown that high efficiency can be maintained over a wide power dynamic range if the imaginary part of the class-E load impedance is varied along with the pulse width. Using in-house (Chalmers University) SiC varactor diodes to implement the tunable load impedance, a 2 GHz 10 W peak output power CMOS-GaN HEMT RF-PWM transmitter demonstrator is realized. The static measurements show that a drain efficiency >70% can be obtained over a 6.5 dB dynamic range. A digital pre-distortion based linearization scheme is proposed to enhance the linearity of the transmitter. An adjacent power ratio of -45 dBc and average drain efficiency of 67% is achieved using a realistic W-CDMA communication signal. These results clearly demonstrate the feasibility of the proposed pulsed transmitter topology for high efficiency linear amplification.