A Differential Combiner for Quasi-Complete Cancellation of Output Capacitance in mm-Wave Power Amplifiers With High- $Q$ Devices

This article proposes a new and simpler methodology to combine two transistors for high-frequency applications. By using a $\lambda/4$ transmission line to connect the drain pads of the identical transistors, the line characteristic impedance can be chosen to almost perfectly cancel out the output capacitance at the design frequency $f_0$ . When the combined network is loaded with a real impedance, the real part of the impedance seen at each of the intrinsic devices is twice the value of that load, while the imaginary part is exactly zero for one of the transistors and depends on $f_0$ for the other. This imaginary part is inversely proportional to $f_0$ and becomes relatively small with high $Q$ -factor (defined as product between optimum intrinsic load and output capacitance susceptance) devices at the frequency of operation ( $f_0$ ). To demonstrate the effectiveness of this methodology, a 28-GHz Doherty power amplifier (DPA) based on the NP15 microwave monolithic integrated circuit (MMIC) process from WIN Semiconductors, Taoyuan, Taiwan, has been designed using the proposed combining method for the main and auxiliary branches. The continuous wave (CW) characterization of this amplifier shows competitive results, which are comparable with the state-of-the-art in terms of efficiency, output power, gain, and bandwidth. Over a band between 27.5 and 29.5 GHz, the obtained saturated output power is higher than 32 dBm. The power added efficiency (PAE) at 6-dB output back-off (OBO) is in the range of 21%–24%, while it is from 24% to 34% at saturation. The saturated gain is between 8 and 12 dB over the abovementioned band.