A Fully-Integrated 3.5GHz Single-Chip GaN Doherty PA for Sub-6GHz 5G

This video describes the design of an S-band, packaged Doherty PA MMIC for use in 5G base station transmitter applications. The design was realised on the GCS 0.4μm GaN process, and assembled by Filtronic into an 8 mm x 8 mm custom laminate QFN-36 package. 5G roll-out will take place in two distinct frequency ranges —the sub-6 GHz region (FR1) and the mmWave frequencies (FR2). It is expected that the first 5G networks will utilise sub-6GHz frequencies and comprise large cells in cities, towns, and rural areas.

The use of the sub-6GHz bands will make the transition from 4G LTE to 5G easier, since existing LTE/4G networks already use channels within bands between 600MHz and 3.5GHz, and will retain the benefit of superior long-distance propagation characteristics available at lower frequencies. One of the expected lower frequency 5G bands will be between 3.4 GHz and 3.8 GHz. Many conventional GaN PAs are designed to operate in Class AB, which can provide very good power-added efficiency (PAE) at compression. However, when backed off to improve linearity, PAE drops off rapidly. Use of Doherty amplifier topology allows for significant improvements in PAE when operated at back-off from compression.