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Nonlinear signal distortions in output power amplifiers

DOI 10.18127/j00338486-201908(11)-04


Yu.A. Bryukhanov – Dr.Sc.(Eng.), Professor, Head of Department of Infocommunication and Radiophysics, P.G. Demidov Yaroslavl State University
K.S. Krasavin – Post-graduate Student, P.G. Demidov Yaroslavl State University

Nonlinear radio transmitter effects usually occur in output power amplifier, which is connected to the transmitting antenna. The output amplifier must have high efficiency. Most of the characteristic of the electronic device must be used for this. In this case, input signal waveform is different from output signal waveform, that is, the nonlinear distortions are appeared.
The goal is to analyze effects of nonlinearity on the distortions of the unmodulated harmonic signal and the modulating signal of the amplitude-modulated (AM) signal for single-ended and push-pull output power amplifiers.
It was believed that the amplifier characteristic is preset by the cubic polynomial and demodulation of AM signal is performed using the synchronous quadrature amplitude detector. The distortions estimate based on Total Harmonic Distortion (THD). We consider memoryless amplifier and amplifier with memory (parallel RLC circuit), single-ended and push-pull schemes and two operation modes with and without cutoff.
The distortions of the harmonic signal at θ = 90° is 11.11% for a memoryless push-pull amplifier, which is 3.56 times less than for a single-ended amplifier with the same cutoff angle. This is due to the absence of an even-order harmonic signal at the output of the push-pull amplifier. In case of a push-pull amplifier with memory under the same conditions, the distortions is 2.1 and 3.8 times less than in a memoryless push-pull amplifier at Q is 5 and 10, respectively.
The THD of modulating signal is 24.35 and 16.16% for a memoryless push-pull amplifier under AM signal with the m factor is 0.5 and 1 and θ = 90°, which is 5.25 and 5.90 times less, respectively, than for a single-ended amplifier. Calculations show that the THD is 8.21 and 7.04% for a push-pull amplifier with memory at m = 1, θ = 90° as Q is 5 and 10, respectively, which is 2.40 and 2.27 times less than in a single-ended amplifier and the same cutoff angle.
The results of this paper can be used for the design of signal transmission systems.

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