Evaluation of transformer and capacitor coupling in W-band broadband CMOS power amplifiers

This paper evaluates the transformer and capacitor coupling used in W-band broadband CMOS power amplifiers. Therefore two four-stage differential power amplifiers were implemented in 40 nm CMOS technology one with transformer coupling and one with capcitor coupling. In order to achieve broadband performance a complex output matching network consisting either of transmission lines and transformer or transmission lines and capacitor is employed. The power amplifier core and output matching network are optimized for the best compromise between bandwidth, output power and power-added-efficiency. Both implementations are measured and compared to the state-of-the-art. For both cases a bandwith nearly over the whole W-band is realized. The transformer-coupled power amplifier achieves output power of 10 dBm with 10 % power-added-efficiency while the capacitor-coupled power amplifier shows output power of 8 dBm with 8 % power-added-efficiency.