A Ka-Band Doherty-Like LMBA for High-Speed Wireless Communication in 28-nm CMOS

This article presents a millimeter-wave (mm-wave) Doherty-like load modulated balanced amplifier (LMBA) for high-speed wireless communication in a 28-nm bulk CMOS process. The presented architecture is based on a load modulation technique that injects an auxiliary signal into the isolation port a 90 degrees power-combining hybrid coupler at the output stage. The injected signal thereby modulates the load impedance at the balanced ports that is seen by the two main power amplifiers (PAs). In contrast to a Doherty PA, both impedances are modulated in the same manner, either upward or downward, both resistively and reactively, without the need for a bandwidth limiting impedance inverter. Furthermore, there is no need for complex Doherty output matching structures, and hence, a low-loss hybrid can be used to modulate the load impedance. The auxiliary signal is generated using a Class-C auxiliary amplifier, producing an overall Doherty-like response. The presented architecture, which uses a single RF input and requires no additional baseband input signal or control signals, is able to achieve multi-Gb/s data rates up to 18 Gb/s. From the continuous-wave measurements, the amplifier demonstrates the power back-off (PBO) efficiency enhancement. At 36 GHz, the PA achieves a measured maximum power added efficiency (PAE) of 32% with 22.6-dBm saturated output power (P-sat) and 30.5% PAE at a 19.6-dBm output referred 1-dB compression point (OP1dB). At 6-dB PBO from P-sat, the PA achieves a PAE of 24.2%. Modulated signal measurements were carried out using a single-carrier 64-quadrature amplitude modulation (QAM) signal at the center frequency of 36 GHz without digital pre-distortion (DPD). For a 64 QAM signal with a baud rate of 2 GSym/s (12 Gb/s), the PA achieves an average output power (P-avg) of 16 dBm with a PAE(avg) of 22% with an EVMrms of -25 dB. Similar measurements were done using a 3 GSym/s signal, giving a data rate of 18 Gb/s, where the PA achieves a P-avg of 15.5 dBm and PAE avg of 20% with an EVMrms of -25.1 dB.