**Outphasing or Chiriex technique**

Also known as linear amplification using nonlinear components (abbr. LINC), this technique uses two nonlinear high efficiency power amplifier to boost up two signals with differently controllable phases. The two amplified signals are then combined with vector addition and the phase difference between the two signals defines the power level of the resulting signal. Compared to EER and ET, phase is the dynamically changing quantity and not the supply voltage of the RF amplifier (Helaoui et al, 2007).

#### **Doherty technique**

The concept of Doherty power amplifier utilizes two power devices which are operated as main and auxiliary amplifiers. As soon as a certain level of input power is reached, main amplifier—normally class B — is running into saturation providing its maximum efficiency. As the main amplifier starts to saturate, the auxiliary amplifier starts to conduct current. The saturation condition of the main amplifier is maintained by load modulation caused by the current from the auxiliary amplifier, so that the main power device acts like a voltage source. At peak output power, the auxiliary amplifier just begins to saturate and high efficiency is ensured for both amplifiers. Block diagram of a Doherty power amplifier is depicted in Fig. 4.. Details about Doherty amplifier can be found in the literature (Raab, 1987).

Compared to other efficiency enhancement techniques, Doherty concept has gained its popularity due to simple architecture which deals with RF circuit design issues only, whereas other techniques make use of digital signal processing to improve average efficiency. Thus, it is more straightforward to design a Doherty power amplifier to cope with new peak to average power ratio value where high average efficiency is desirable. This can be simply achieved by modifying the input power division ratio between main and auxiliary amplifier. If necessary, three amplifiers can also be used in order to maintain high average efficiency over a high dynamic range.

Fig. 4. Block diagram of a Doherty amplifier.
