Advanced Transmitter Architectures Based on Switch Mode Power Amplifiers
Doctoral thesis, 2014
Nowadays the main driving parameters for the research in radio transmitters in wireless infrastructure are energy efficiency, frequency agility, and integration.
This thesis presents new solutions at the device waveform-, circuit-, and transmitter level which exploit the inherent high efficiency potential of switch-mode power amplifiers (SMPAs) for realization of energy efficient, wide-band, highly integrated transmitters for wireless communication applications.
In the first part of the thesis, a continuum of novel high efficiency class-E power amplifier modes are derived, significantly extending the known SMPA design space. In contrast to conventional SMPA modes, the new continuum allows some variation for the switch impedances, providing important possibilities on wide-band SMPA designs. This is experimentally verified in a 1 W SiGe BiCMOS SMPA design having a drain efficiency of above 70% over a 1.3-2.2 GHz bandwidth.
In the second part a novel combiner synthesis technique is developed that enables realization of wide-band highly efficient outphasing transmitters. The technique is based on the calculation of the combiner network parameters from the boundary conditions required for highly efficient switch-mode operation of the transistors in each branch. The approach is validated in a CMOS-GaN outphasing transmitter design providing a peak output power of 44 dBm and a 7.5 dB output power back-off efficiency exceeding 52% over a 750-1050 MHz bandwidth. It is further shown that the same theoretical approach can also be used for design of Doherty PA combiner networks. A 28 W 3.5 GHz Doherty PA is designed and manufactured for experimental verification providing a record-high power added efficiency of 51% at an adjacent channel leakage ratio (ACLR) of -50 dBc with carrier-aggregated 100 MHz LTE test signals.
In the third part a new SMPA topology particularly suitable for amplification of RF pulse-width modulation (RF-PWM) signals is presented. In classical pulse width modulated SMPAs the varying pulse width leads to switching losses and hence efficiency degradation. We present an electronically tunable load output network that alleviates this problem.
A 10 W 2 GHz CMOS-GaN RF-PWM transmitter demonstrator is constructed and characterized to demonstrate the feasibility of the proposed technique. ACLR after digital pre-distortion linearization is -45 dBc at a drain efficiency of 67% with W-CDMA communication signals.
The solutions presented in this thesis will facilitate realizations of frequency agile, energy efficient and highly integrated/digitalized radio transmitters for future wireless communication systems.