RF (radio frequency) power architectures within the telecommunications field focus on achieving high DC-to-RF efficiency at significant power back off from Psat (the average output power when the amplifier is driven deep into saturation). This is due to the high peak to average ratio (PAR) of the transmitted digital signals such as W-CDMA (wideband code division multiple access), LTE (long term evolution) and WiMAX (worldwide interoperability for microwave access). The most popular power amplifier architecture currently employed is the Doherty amplifier. The Doherty amplifier employs a class AB main amplifier and a class C peaking amplifier, and efficiency is enhanced through load modulation of the main amplifier from the peaking amplifier. However, if high efficiency at a high output backoff (OBO) is required, a highly asymmetric ratio between the main and peaking amplifiers is required.
The Doherty architecture has an inherent degradation in the efficiency between the peak OBO point and the peak power point. To overcome this, a three way Doherty architecture can be used, in which the main class AB amplifier is replaced with a Doherty amplifier and load modulation is provided to the first peaking amplifier between the peak OBO point and the peak power point. However, the main amplifier is connected to the external load impedance (typically 50 Ohms) through a series of three ¼λ (quarter wavelength) transmission lines prior to any device impedance matching. This can lead to the amplifier being narrow band in nature due to the band-limiting characteristics of the ¼λ transmission lines. As such, three way Doherty amplifiers are typically designed for a specific band of operation used for wireless communication applications like WCDMA, LTE, WiMAX, etc. Such bands of operation are 1805-1880 MHz, 1930-1990 MHz, etc.