A Novel 1$\times$4 Coupler for Compact and High-Gain Power Amplifier MMICs Around 250 GHz

For application in radar and communication systems operating in the frequency range around 250 GHz, a power amplifier (PA) millimeter-wave monolithic integrated circuit (MMIC) has been developed. The amplifier makes use of a novel coupler approach allowing for in-phase and equal-magnitude distribution of an incoming signal to four parallel output ports. This is possible due to a subtle use of grounded coplanar waveguide and air-bridge microstrip transmission lines, providing different propagation constants. With a size of quarter-wavelength, the coupler is very compact and is the key to the presented PA topology. The amplifier exploits the virtual open occurring due to the identical signals for transistor biasing and matching. The amplifier topology is applied to develop an amplifier MMIC. When biased for maximum gain, it shows a 17-GHz 3-dB bandwidth around its center frequency at 248 GHz with a maximum gain of 32.8 dB. When biased for maximum output power, it provides an output power of 10 and 8.5 dBm at 250 and 270 GHz, respectively. At 250 GHz, it demonstrates a state-of-the-art power density of 125 mW/mm. Due to the amplifier topology, the MMIC is very compact $({\hbox{1.25}}\times {\hbox{0.75}}~{\hbox{mm}}^{2})$ .