An extended Doherty amplifier with high efficiency over a wide power range

TLDR

The paper presents an extended Doherty amplifier that sustains high efficiency over a wide output power range (>6 dB). The design is experimentally validated using InGaP/GaAs heterojunction bipolar transistors at 950 MHz, tested with CDMA signals under varied bias and matching, and supported by generalized design equations that analyze the impact of finite device output impedance on gain and linearity. Measured power‑added efficiencies reach 46 % at P1dB (27.5 dBm), 45 % at 9 dB back‑off, and at least 39 % over a 12 dB range, outperforming classical Doherty amplifiers (which drop after 5–6 dB) and exceeding a comparable class‑B amplifier by 2.6× at 10 dB and 3× at 20 dB back‑off, while achieving 45 % PAE at 25 dBm with an adjacent‑channel power ratio of –42 dBc.

Abstract

An extension of the Doherty amplifier, which maintains high efficiency over a wide range of output power (>6 dB), is presented in this paper. This extended Doherty amplifier is demonstrated experimentally with InGaP/GaAs heterojunction bipolar transistors at 950 MHz. Power-added efficiency (PAE) of 46% is measured at P/sub 1dB/ of 27.5 dBm and 45% is measured at 9 dB backed off from P/sub 1dB/. Additionally, PAE of at least 39% is maintained for over an output power range of 12 dB backed off from P/sub 1dB/. This is an improvement over the classical Doherty amplifier, where high efficiency is typically obtained up to 5-6 dB backed off from P/sub 1dB/. Compared to a single transistor class-B amplifier with similar gain and P/sub 1dB/, the extended Doherty amplifier has PAE 2.6 /spl times/ higher at 10 dB back off and 3 /spl times/ higher at 20 dB back off from P/sub 1dB/. Under different bias and output matching conditions, the amplifier was also evaluated with CDMA signals. At the highest measured power of 25 dBm, the extended Doherty amplifier achieves a PAE of 45% with an adjacent channel power ratio of -42 dBc. Generalized design equations are also derived and the consequences of finite device output impedance on amplifier gain and linearity are explored.