The Efficiency Improvement of a Compact Inverted Doherty Amplifier Using Bias Line Adjustment

We describe our proposal of an efficiency improvement method for a compact inverted Doherty amplifier using a bias line adjustment. In order to compensate for the capacitive parasitic components of the high-power transistors, we adjusted a lambda/4 bias line to have an inductive impedance value. The inductive bias line made parallel resonance with the capacitive internal component. The following matching network and offset line rotated the output reflection coefficient (Gamma_OUT) to almost -1, which was desirable for the compact inverted Doherty amplifier design. The optimum length of the bias line was found after some adjustments. Compared to the Doherty amplifier using a lambda/4 bias line, the proposed Doherty amplifier using an adjusted bias line provided improved power-added efficiencies (PAEs) of 6% and 4% points for a two-tone signal at an output power of 45 dBm and for the pi/4-DQPSK signal at an output power of 47 dBm, respectively.