Theoretical and experimental study of amplifier linearization based on predistorted signal injection technique

Modern wireless communication systems rely exclusively on digital modulation technologies. Sophisticated modulation schemes are adopted to increase spectral efficiency. Unfortunately, undesirable side-lobe is generated at the output of amplifiers as a result of the nonlinearity of the transistors. Design of ultra linear amplifier is of prime interest to microwave amplifier designers. Various techniques such as feedforward, feedback and predistortion have been developed in the past. Recently several methods based on the second-order signal injection have also been reported. Intermodulation distortion (IMD) suppression is achieved by exploiting the second-order nonlinearity and second-order distortion signal at either the second-harmonic or baseband (difference) frequencies. In comparison with the classical methods, this technique tends to be simple and low cost. However, the second-harmonic approach requires both precise gain and phase adjustment for perfect cancellation of the distorted signal, whereas the baseband method only gives limited IMD suppression. This thesis presents three novel linearization schemes for RF amplifiers based on harmonic and baseband signal injection.; In the first method, second-harmonic and baseband (difference) frequency components generated by predistortion circuits are simultaneously fed to the input of the main amplifier to mix with the fundamental signal for third-order intermodulation distortion cancellation. A general and rigorous analytical formulation is presented. From this derived expressions, the optimum conditions for IMD suppression are developed. The results also reveal the practical limitation of the proposed method subject to gain and phase error associated with the RF and baseband circuitry. Reduction of 30 dB and 25 dB are achieved for two-tone test and vector signal test respectively. This method eliminates the need of precise phase control of the RF path. Phase uncertainty of greater than ±60° is verified.; In the second method, the conventional second-harmonic injection method is extended for the suppression of fifth-order IMD. Two predistorters are used to generate distorted signals with different spectral profile at the second-harmonic. Signal extraction circuitry allows forth-order and second-order signal to be independently adjusted. Optimal adjustment of magnitude and phase of these signals results in simultaneous suppression of third-order IMD (IMD3) and fifth-order IMD (IMD5). IMD suppression of 30 dB and 20 dB is demonstrated experimentally for IMD3 and IMD5 respectively.; Finally, a third method is proposed to overcome the precise RF phase control required by the second method. Second- and forth-order distortion signal at both the second-harmonic and baseband are fed to mix with the fundamental signal for simultaneous third- and fifth order intermodulation distortion suppression. Experimental result shows that a reduction factor of 40 dB and 25 dB is achieved for IMD3 and IMD5 respectively. For vector signal test, the first side-lobe and second side-lobe are suppressed by 30 dB and 15 dB respectively. Besides, IMD cancellation is verified for RF phase variation up to ±80°.