Analysis and design of a GHz bandwidth adaptive bias circuit for an mmW Doherty amplifier

This paper derives theoretical results for adaptive bias in Doherty amplifiers and presents the design and measurements of an integrated adaptive bias circuit tailored for high peak-to-average high bandwidth signals. Fundamental equations for output power, impedance, and efficiency of the complete Doherty amplifier are derived. Even with ideal transistor models, the Doherty amplifier is fundamentally nonlinear due to saturation of the main amplifier and class-C nonlinearity of the auxiliary. Increasing the transconductance of the auxiliary amplifier mitigates the distortion. Adaptive bias offers the possibility to control the output current characteristic of the auxiliary amplifier. This means that adaptive bias linearises and mitigates the need for an oversized auxiliary amplifier. Both methods, transconductance scaling and adaptive bias, are analysed and compared as well as having a band limited adaptive bias signal. The design of a multiple GHz bandwidth adaptive bias circuit is presented. To verify the circuit design and the theoretical predictions, an mmW Doherty amplifier in 22 nm CMOS-FD-SOI, utilizing the presented adaptive bias circuit, is measured and compared with and without adaptive bias. Comparison is conducted both using continuous-wave and modulated high bandwidth signals. Measured results confirm the predicted improvements by the adaptive bias as derived by the theoretical analysis.