1Student, M. Sc. [Engg.], Department of Electronics and Computer Engineering, M.S. Ramaiah School of Advanced Studies, Bangalore, 560 054
2Senior Lecturer, Department of Electronics and Computer Engineering, M.S. Ramaiah School of Advanced Studies, Bangalore, 560 054
3Asstiant Professor, Department of Electronics and Computer Engineering, M.S. Ramaiah School of Advanced Studies, Bangalore, 560 054
4Professor and Head, Department of Electronics and Computer Engineering, M.S. Ramaiah School of Advanced Studies, Bangalore, 560 054
*Contact Author e-mail: timm@msrsas.org
Online published on 18 February, 2020.
An ideal amplifier operates from DC to infinite frequencies. Its output is a perfectly amplified version of its input and its does not produce any harmonic frequencies. A practical RF power amplifier is composed of RF transistors which are non-linear devices. It is the nonlinear behaviour of transistors that gives rise to harmoncis at the output. In a RF transmitter or receiver there are two phenomena that we observe which make the RF amplifier non-linear. The amplifier transfer function is highly dependent on frequency of operation in both magnitude and phase.
The amplifier gain is a non-linear function of the input voltage. Due to non-linear behviour of amplifiers, the input information in two input frequencies is output as the sum and difference of the two input frequencies. Hence, if two frequencies are applied at the amplifier input the amplifier output contains other unwanted frequencies which are linear combinations of the original input frequencies
Linearization refers to a multiplicity of techniques, which mitigate the extent of non-linearity exhibited by a power amplifier. Linearization techniques reduce Intermodulation Distortion, Total Harmonic Distortion (THD) and increase the usable linear range of an amplifier. In this research we describe the Doherty technique. The Doherty techqniue requires spiltting of an incoming signal and processing through a main amplifier and a peaking amplifier. The Doherty technique has been demonstrated for 3 separate classes of amplifier models-Saleh, Ghorbani and Rapp. The aim of this work is to achieve reduction in THD for each of the 3 classes of RF power amplifiers. The 3 classes of power amplifiers were modeled in the 900 MHz to 1300 MHz range. Physical effects of nonlinear amplifiers like AM/AM distortion and AM/PM distortion are taken into account in the models. The THD is measured with and without the application of Doherty technique. The Doherty technqiue results in an improvement of THD of upto 3 dB.
Power Amplifiers, Linear/Non-Linear, Saleh, Ghorbani and Rapp Amplifiers, Doherty Technique
