A microwave power amplifier with independently biased InGaP/GaAs HBTs is proposed, and its superior performance is confirmed. Using harmonic balance simulation, the optimal bias conditions for an amplifier with two independently biased InGaP/GaAs HBTs were investigated with the aim of achieving high-efficiency low-distortion performance. A 1.9-GHz-band cascode power amplifier was designed and fabricated. Power efficiencies and third-order intermodulation distortions (IMD3) for the fabricated amplifier were estimated. The collector bias voltage of the first stage transistor mainly affects power-added efficiency (PAE). The base bias current of the first-stage HBT mainly affects IMD3 characteristics, and that of the second-stage HBT mainly affects PAE. The proposed amplifier shows superior performance when compared to a conventional cascode amplifier. The amplifier achieved a maximum PAE of 68.0% with an output power of 14.8dBm, and IMD3 better than -35dBc with a PAE of 25.1%, for a maximum output power of 10.25dBm at 1.9GHz. A PAE of more than 60% was achieved from 1.87 to 1.98GHz.

We present an analytical nonlinear adiabatic theory of the microwave electron device that we call the Autophase Microwave Tube (AMT). In contrast to the well-known Traveling Wave Tube (TWT), the AMT exploits a highly efficient non-synchronous beam-wave interaction for the amplification (or generation) of the HF electromagnetic waves, and, differently from klystron and such hybrid devices as twystron, it employs a continuous beam-wave interaction. Because of these distinctive features, the AMT presents a special class of microwave electron devices, which feature very high electronic efficiency (which tends to 100%) and large bandwidth. Here, we develop the theory that allows one to find the profiles of static longitudinal electric or magnetic field (or both) over the device length, which yield negligible de-bunching together with highly efficient amplification (generation) of the HF electromagnetic wave. The analysis of electron motion in the bunch is performed by means of Lyapunov stability theory. The numerical example illustrates the possibility of achieving the electronic efficiency of AMT as high as 92%. We compare different autophase regimes in the AMT and show that the profiling of the longitudinal static magnetic focusing field in the helix AMT with the non-azimuthally symmetric wave has many advantages with respect to other regimes.

A new harmonic termination that controls the waveform of the drain current to be rectangular is developed for high-efficiency power amplifier modules. Its harmonic termination is a short circuit at the third harmonic and a non-short circuit at the second harmonic. It is found experimentally and confirmed by simulation that the load-matching condition at the third-order harmonic improves the efficiency of a transistor by more than 13%. By using this tuning, 57.7% power-added efficiency of the module is achieved at the output power of 29.9 dBm with ACP of -50 dBc, NACP of -65 dBc at 925 MHz and Vdd of 3.5 V.

Microwave power transistors for high efficiency applications are surveyed briefly. Methodologies for microwave transistor power amplifier circuit design are discussed. Microwave transistor power amplifiers are categorized according to their operation into classes A, AB, B, C, and F, and some preliminary results on output power, power efficiency, and power gain for the amplifiers in various classes are provided by an analysis using an ideal transistor model. Circuit conditions controlling the voltage and current waveforms and device parameters such as the knee voltage in the device current-voltage characteristics are discussed for viewpoint of realizing high-efficiency power amplifier operation. A practical power amplifier design is considered with respect to the device characteristics and circuit conditions.