Millimeter-Wave Series Power Combining Using Sub-Quarter-Wavelength Baluns

We present a new millimeter-wave power-combining technique using transmission-line baluns which both connect transistor outputs in series and inductively tune the transistor output capacitances. The baluns are much shorter than a quarter-wavelength (λ/4), hence are more compact and have less insertion loss than a λ/4 balun. We introduce one topology providing an even number of series connections, including 2:1 and 4:1, and a second topology providing either an even or odd number of series connections. We then analyze segmented transformer power-combiners as a set of multi-conductor transmission-lines, and explore the relationship between transformer and transmission-line balun power-combiners. We demonstrate the technique with 2:1 and 4:1 series-connected designs implemented in a 0.25 μm InP HBT process. At 86 GHz, a single-stage power amplifier (PA) using the 2:1 baluns exhibits 30.4% peak PAE, 20.37 dBm output power (Pout) and 23 GHz 3-dB bandwidth from a 448 × 816 μm ² die. A two-stage PA using the 2:1 baluns exhibits 30.2% PAE, and 23.14 dBm Pout from an 824 × 816 μm ² die. At 81 GHz, a two-stage PA with 4:1 series output power-combining exhibits 23.4% PAE, and 26.7 dBm (470 mW) Pout from a 1,080 × 980 μm ² die.