AbstractsEngineering

The scope of this work is to theoretically and experimentally analyze the potential of gallium nitride (GaN) based monolithic broadband high power amplifiers with more than an octave bandwidth in the microwave domain up to 40 GHz with unprecedented power levels. The most fundamental theoretical limitation for reactively matched amplifiers is imposed by the Bode-Fano limit. This technology related figure of merit quantifies the attainable reflection coefficient for a matching network compensating the reactance of a single transistor in a given frequency band. A detailed analysis of the Bode-Fano criterion is performed for the input and output of GaN based HEMTs. The challenge in designing broadband amplifiers is to present the optimum complex load to the input and output of each transistor in an arbitrary interconnection in such a way that they deliver maximum output power or efficiency at all frequencies within a designated band. Because of the high output resistance of GaN transistors, the Bode-Fano limit at the output is aggravated dramatically as compared to lower voltage technologies such as gallium arsenide (GaAs). However, as calculations show, this theoretical limitation does not pose the dominating difficulty. The main limiting factor is the large impedance transformation ratio to be dealt with in the output matching network, attributable to large gate width devices. Since this limitation is not addressed by the Bode-Fano criterion, it needs special consideration and is addressed by reference to filter theory which allows to quantify the filter order of a matching network in terms of bandwidth and transformation ratio. In contrast to the output, the Bode-Fano criterion for the input is dependent on center frequency and device size. Calculations show that for reasonable device sizes, it poses a severe theoretical limitation in obtaining an octave bandwidth. Since in a multistage design two complex impedances face at the interstage, the problem becomes even more severe. In order to overcome this limitation, a novel power amplifier architecture is proposed, which evades the aggravated matching aspects introduced by designing multistage reactively-matched amplifiers. A dual-stage semi-reactively-matched amplifier (SRMA) which comprises a distributed active power splitter acting as the driver stage is introduced. In doing so, a purely real interstage impedance is obtained and therefore the proposed architecture allows wider bandwidth operation as compared to the conventional reactively-matched multistage topology. A 4.5 W 6 GHz to 20 GHz high power SRMA is designed and realized. The bandwidth ratio is the largest ever reported for a reactively matched multistage monolithic GaN power amplifier at the given frequency and output power. A very attractive way to enhance the gain of an amplifier is to reduce the Miller effect by using dual-gate active devices. A method to accurately describe dual-gate structures is demonstrated up to 18 GHz using a distributed modeling approach. A scalable nonlinear model with varying…