In many portable transceivers, the power amplifier (PA) is the most power consuming block. In general, the maximum power efficiency can be achieved only when the PA is transmitting peak output power. The efficiency worsens as the output power decreases. Under typical operating conditions, the PA transmits less than peak output power, therefore effective power efficiency is much lower than the maximum value.
One of the efficiency enhancement techniques under investigation is the Doherty amplifier. The merit of this technique is that it allows a power amplifier to achieve a maximum or close-to-maximum efficiency over a wider range of the output power. In the Doherty amplifier, an auxiliary amplifier is introduced. The auxiliary amplifier turns on when the output power is high and, by means of a passive impedance inverter, effectively lowers the impedance seen by the main amplifier, thus allowing higher output power while maintaining high efficiency. However, since the Doherty amplifier consists of two amplifiers, it is subject to phase mismatch between the two paths. Also, the main amplifier has to experience different load impedance due to the on and off state of the auxiliary amplifier. These can lead to a significant degradation in the linearity. Different linearization techniques that can be applied to Doherty amplifier will also be investigated.