Research And Design Of High-power Linear Feedforward Amplifier For VHF Application

The communication band has become more and more crowded due to the vigorousdevelopment of the broadband communications business as well as a sharp rise of wirelesscommunications subscribers. Linear modulation technique with non-constant envelopemodulation and relatively high spectral efficiency such as QPSK and QAM is widelyutilized. Therefore, more stringent requirements of the linear indicators are proposed. Thispaper first introduces the basic design theory of the power amplifier and analyzes thenonlinear characteristics of it, including the AM-AM nonlinear AM-PM nonlinear.Secondly, the commonly used nonlinear indicators of amplifier are introduced. What’smore, this paper analyzes and compares technology principles, advantages anddisadvantages of four linear technology: the power back-off technology, negative feedbacktechnology, predistortion technology and feedforward technology. Finally, the mostpromising feed-forward technology combines high reliability, wide bandwidth andimproved linearity becomes the most promising linearization techniques.This design uses a cascade of MRF6V2010and MRF6V4300to constitute the mainPA and they both uses the structure of balanced amplifier.This article described in detailthe design steps of the main PA meanwhile shown the corresponding simulation results. Inaddition, designed and simulated the feedforward PA system modules, including theauxiliary amplifier, error amplifier, couplers, delay timer and phase shifter. Finally, built acomplete feedforward PA system in ADS and carried out feasibility verification.The simulation results show that the system can achieve120W output power, morethan50dB gain and29.4%efficiency when frequency changes between57.4MHz and63.6MHz. What’s more, through controlment of amplitude and phase accuracy offundamental cancellation loop and distortion cancellation loop,when two-tone frequencyspacing ranges from0Hz to3MHz, the IMD3of the system is less than-50dBc,compared to the main PA linearity improved more than8dB, the main PA intermodulationdistortion has been very well inhibited, indicating that the program meets therequirements of system indicators.