Broadband Outphasing Transmitter Using Class-E Power Amplifiers
An outphasing transmitter using true-transient class-E power amplifiers shows limited frequency bandwidth due to the high sensitivity of the amplifiers to frequency-dependent varying load impedances when performing into outphasing conditions. This paper describes a design technique for broadband operation of an outphasing transmitter using class-E amplifiers. Proper loading network tuning and asymmetric combiner phasing allow optimum impedances to be presented to the amplifiers across a frequency band and as a function of input-drive phase angle. The concept is verified with a prototype at 10-MHz center frequency showing 40% bandwidth. The measured efficiency curves resemble those of conventional class-E outphasing at frequencies from 8 to 12-MHz achieving efficiencies greater than 80% or so at 10-dB output power back-off. On the other hand, the output power varies around 1.5-dB within the same frequency range which is almost half of the variation of a single-ended broadband class-E amplifier.