A Fully Integrated 3.3–3.8-GHz Power Amplifier With Autotransformer Balun

This paper demonstrates the usage of a differential autotransformer as an output balun for an integrated power amplifier (PA) operating at low-gigahertz frequencies. In comparison with a conventional transformer balun, an autotransformer balun offers lower power losses, thereby increasing the saturated output power and reducing the gain compression at the edge of target power range. A theoretical analysis of an integrated autotransformer is given, comparison with a magnetic transformer is performed. The concept was experimentally verified in a fully integrated PA for a 3.3-3.8-GHz WiMAX band fabricated in SiGe : C bipolar technology. The active part of the amplifier implements the derivative superposition method aimed at linearizing the power transfer characteristic. Measured PA delivers saturated output power above 29 dBm. The maximum achieved power-added efficiency exceeds 40% at 3.4 GHz. At 3.5 GHz, 1-dB gain compression occurs for
P

_out
= 24.6 dBm.