A 68.5∼90 GHz High-Gain Power Amplifier With Capacitive Stability Enhancement Technique in 0.13 μm SiGe BiCMOS

This article presents a wideband millimeter-wave power amplifier (PA) with a capacitive stability enhancement technique. It is composed of three differential cascode stages to attain high-power gain and large output power. To ensure the stabilization of the amplifier, a robust technique based on a series base capacitor is proposed and applied in each stage. Each capacitor creates a low-frequency zero to adjust the phase and amplitude responses of the differential cascode structure. As a result, the phase margin of the loop gain is increased remarkably. By adjusting the series base capacitors, the low-frequency zeros and gains of the three stages can be optimized. Therefore, the operating bandwidth is also extended. Meanwhile, a transformer-based four-path power combination network with a large current capacity is developed to further enlarge the output power. The circuit is demonstrated in a commercial 0.13 mu m SiGe BiCMOS process. According to the measurement results, the PA obtains a saturated output power (P-Sat) of 18.5 dBm and output referred 1-dB compression point (OP1dB) of 16.32 dBm at 77 GHz. The tested small-signal gain is 26.7 dB at 77 GHz. Its 3 dB gain bandwidth is beyond 21.5 GHz, which ranges from 68.5 to 90 GHz. The P-Sat variation is less than 0.57 dB at the frequency range from 70 to 85 GHz. The measurement also shows the PA with the proposed technique is unconditionally stable over the entire frequency band. The chip area is only 0.684 mm(2).