The amplifier is the key element in wireless hardware, be it a receiver or a transmitter. In the receiver, a low noise receiver is needed to "screen" the desired signal out of background noise before the signal reaches the down-converter. Although a futuristic direct conversion receiver is supposed to eliminate the conventional heterodyne system, the increasing operating RF frequency makes it difficult for direct conversion to be adopted. Good low noise reception can be accomplished routinely by the microwave industry with HEMT technology.
The high-power final-stage (or output) amplifier is the key element in the transmitter. Since this amplifier is the most power consuming, high efficiency amplifiers have drawn considerable research attention in the past several years. On the other hand, for the spectrally efficient modulation scheme used in CDMA, amplifiers will need to deal with a modulated RF carrier with a non-constant envelope, hence, amplifier linearity is important. In order to obtain higher power-added efficiency, a higher class of amplifiers such as Class AB, Class B or even Class C or switched mode Class E or F is often used. However, due to the V-I curves of devices operated in these classes, the amplifier becomes nonlinear. Therefore, often high efficiency and high linearity are contradictory objectives. There are several techniques to combat this problem.
One method tried at NEC is the use of adaptively controlled bias with a DC-DC converter for W-CDMA applications (see Fig. 5.20). A similar approach has been demonstrated at the University of California at San Diego to increase the average PAE while the amplifier is kept in Class A operation. Since the PAE is extremely low in most instances of normal operation, the DC supply voltage is reduced in such instances to reduce the DC power consumption.
Fig. 5.20. PAE improvement with DC-DC converter by NEC.
Somewhat more routine approaches are first to design a high PAE amplifier with a nonlinear mode and then to provide a linearity-improvement scheme, such as feedback, feed-forward and predistortion. These approaches tend to make the overall circuit configuration complex and the PAE lower. Matsushita Research Institute Tokyo reported a new approach called the Hybrid Adaptive Predistortion method shown in Fig. 5.21.
Fig. 5.21. Hybrid adaptive predistortion method by Matsushita.
A recent trend is to cope with the linearity-PAE issue from the point of "transmitter unit" rather than a single amplifier. The above examples include a control or signal processing circuit in the configuration.