K-band MMIC power amplifier based on a 3-stacked GaAS pHEMT device

The microwave backhaul represents a key factor in determining cost and service quality of mobile communication systems. In a microwave radio, the Power Amplifier (PA) is one of the most demanding components, strongly affecting overall power consumption, hence the need of high efficiency architectures. Considering the ongoing trend towards an increased bandwidth and modulation complexity, also the PA linearity is becoming crucial. The frequency bands adopted for microwave backhaul are spread over the full microwave range, however, a significant number of systems work in the K-band. While at lower frequencies the market is dominated by LDMOS and GaN technologies, capable to provide high output power, in this frequency range, PAs are usually realized in GaAs Microwave Monolithic Integrated Circuit (MMIC) technology. Being the power density of GaAs devices of the order of 0.5-1 W/mm, to achieve peak power levels around 2-10 W typical of backhaul applications specific combining strategies needs to be adopted. In this framework, a very promising solution is the stacked topology where two or more devices are series coupled forming an equivalent macro-device with enhanced breakdown voltage. Moreover, compared to more conventional combining methods, the stacked architecture reduces losses and allow increasing the bandwidth, as it increases the output impedance, thus mitigating output matching issues. The stacked configuration is already widely adopted in CMOS PA, while its application to GaAs PAs is becoming of interest only more recently. Here we report the characterization results of a 3-stacked multiple common gate amplifier implemented in 0.15 um MMIC GaAs technology by TriQuint foundry. CW characterization results in the 20–23 GHz range show early compression spots, negatively affecting the saturated output efficiency, which is lower than expected. Despite this, the amplifier performance are still satisfying and comparable to existent literature: saturated output power and efficiency in excess of 31.8 dBm and 27%, respectively, and gain higher than 9.5 dB in the whole bandwidth. To verify the stacked macro-device in absence of high-frequency effects, the same basic cell has been manufactured for low frequency (2–6 GHz) operation. With output power in excess of 33 dBm the CW and load-pull characterization results on this module confirmed the potential of the staked topology and its equivalence with a single device. At 4 GHz the presence of early compression spots similar to what experienced in K-band in case of output mismatch, suggest that the issues encountered at high frequency are due to the impact of process variability and inaccuracy of the large-signal device model on output matching.