Input transmission line adjustment for efficiency enhancement in Doherty amplifiers

Conventional Doherty power amplifiers need a λ/4 transmission line at the input of the peak amplifier. This provides a 90 degree phase shift of the input signal to allow a phase correct combination at the output. In this paper a discrete Doherty power amplifier for 2GHz LTE band is introduced that comes with a phase shifter at the peak amplifier's input with 110 degree. It shows a 2.5% points higher value for power added efficiency at 1 dB compression point compared to 90 degree phase shift. Furthermore, the degradation of the power added efficiency in the backoff could be enhanced from 27% to 16 %. This amplifier delivers an output power of 31.5 dBm, a gain of 9.5 dB and a power added efficiency (PAE) at the 1 dB compression point of 41% and 34.6% at 6 dB backoff. The maximum efficiency is about 44.3%at 3 dB backoff. It also offers a high linearty with an output referred intercept point of the third order intermodulation (OIP3) of 46 dBm. Common FR4 as substrate and standard electronic components are used. As core element of main and peak amplifier a HEMT FET of the company Avagotech is used. Both amplifiers are equally designed including bias input networks as well as output networks. The only difference of both amplifiers is the operating point whereas main amplifier is working in classAB and peak amplifier in classC mode.