Design and Analysis of Ka-Band Power Amplifier With Sandwiched-Coupler-Balun and Folded-T-Line Power Combiner

This article presents a two-way current-combining Ka-band power amplifier (PA) in a 130-nm SiGe BiCMOS process. A two-way power-combining network composed of sandwiched-coupler-balun (SCB) and folded-T-line (also known as meander line) is utilized to realize low-loss broadband large-signal impedance matching under a high impedance transformation ratio. Parallel peaking inductance is also employed in the cascode amplifiers to improve the power gain and efficiency, and the quality factor (Q factor) of the peaking inductance is considered during the design process. The symmetrical interstage matching network (ISMN) between the driver and two output stages (OAs) is utilized to improve large-signal performance under broadband operation. The measurement result shows that the proposed PA has a peak small-signal gain of 30.5 dB at 34.8 GHz and a 3-dB bandwidth of 10 GHz (31–41 GHz). At 35 GHz, the PA achieves a 23.5-dBm Psat with 33.9% peak power added efficiency (PAE) and 22.2-dBm OP1dB. Across 31–39 GHz, the Psat and peak PAE of the PA remain 22.7–23.8 dBm and 24.5%–33.9%, respectively. For modulated signal tests, this PA demonstrates −25.4-/ −26.1-/−25.4-dB rms error vector magnitude (EVM) and −25.3-/−27.5-/−25.3-dBc adjacent-channel power leakage ratio (ACLR) at 35/37/39 GHz with a 250-MSym/s 64-quadratic-amplitude modulation (QAM) signal, and it also demonstrates −25.3-/−25.1-/−25.3-dB rms EVM and −26.3-/−27.7-/−27.1-dBc ACLR at 35/37/39 GHz with a 400-MSym/s 64-QAM signal.