Myo Min Thant – Post-graduate Student,  National Research University of Electronic Technology (Moscow) E-mail: myominthant5129@gmail.com

V.A. Romanjuk – Ph.D. (Eng.), Associate Professor, National Research University of Electronic Technology (Moscow) E-mail: v.a.romanjuk@gmail.com

When developing new power amplifiers monolithic microwave integrated circuits (MMIC), special attention is given to increase their effectiveness. This is explained by the desire to alleviate the problem of heat removal from power amplifiers, as well as the necessary for long-term operation of radio devices from autonomous power sources. In this review, information is systematized on how to increase the efficiency of amplifiers, with a glance of the operating frequency, parameters and materials of the MMIC. Power added efficiency was taken as an assessment of the effectiveness. A well-known idea is used to develop amplifiers with increased efficiency- synthesize such time dependencies of the output current and voltage of the transistor, at which the minimum power is dissipated in the transistor. Since the efficiency of linear amplifiers is substantially less than 50%, in order to increase it, they are developing amplifiers, in which the transistor operates in a nonlinear mode.

The types of non-linear power amplifiers are divided into three groups: 1) amplifiers, in which the transistor operates with cut-off output current and harmonic output voltage (classes AB, B and C); 2) switch amplifiers that require abrupt voltage changes at the input of the transistor (classes D, E); 3) polyharmonic amplifiers containing complex multi-resonator output circuits (classes F, inverse class F).

The efficiency of amplifiers with harmonic output voltage is highest in the threshold mode of the transistor at a current cut-off angle less than 90º (class C), and approach to a value close to 100% when the cut-off angle is reduced. However, working at small cut-off angles reduces coefficient of power gain and requires transistors with high breakdown voltage between input electrodes. Class B (current cut-off angle 90º) is often used, convenient for increasing efficiency in push-pull circuits, but the maximum theoretical efficiency of a class B amplifier is 75,5%. The switch amplifiers have a very high efficiency (> 80%), but at relatively low frequencies (up to 1 GHz), which is associated with the difficulties of creating sharp voltage drops. 

The largest number of works is dedicated to the problem of increasing the efficiency at high frequencies are related to polyharmonic amplifiers. In Class F amplifiers, the ideal output voltage wave form is the meander, while the output current is in the form of a cosine curve and flows to that part of the oscillation period, when no voltage. Such voltage and current forms allow to achieve increasing efficiency in the presence of only the first harmonic of the input oscillations at the output of the transistor.

To create an ideal rectangular voltage shape, the input impedance module of the output circuit of the amplifier has maximum at an infinite number of odd harmonics of the input oscillation frequency. In practice, they are limited to approximate forms of voltage; for this, the output circuit contains 2–3 consecutively connected oscillatory circuits or a segment of the transmission line. The pulse form of the output current is provided by the operation of the transistor with a cut-off of the output current at a cut-off angle of 90º. The efficiency of class F amplifiers reaches more than 90% theoretically at frequencies of 1…3 GHz.

In the inverse class F amplifiers, there is a reverse form: the output current is the meander, and the voltage is the cosine wave sections. To form such a voltage, the input impedance module of the output circuit has maximum at even harmonics of frequency, but the close to a rectangular shape of the current is obtained when a voltage of sufficiently large amplitude is applied to the input of the transistor. Power added efficiency of inverse class F amplifiers reaches more than 90% theoretically at frequencies of 1…3 GHz. The possibility of increasing the efficiency is shown when amplifiers operate at different frequencies, including the millimeter wavelength range. The creation of highly efficient amplifiers does not depend on the output power, gain and supply voltage. The effective MMIC amplifiers are performed on different semiconductor materials: GaAs, GaN, InP, SiC, SiGe.

If the amplifier is intended for radio signal amplifiers, in which the information is enclosed in amplitude changes, then for the absence of nonlinear distortions, the mode of operation of the transistor must be linear and no tensional, and only at the moments of maximum amplitude becomes boundary. To increase the efficiency and prevent non-linear distortion of information, methods for amplifiers with harmonic output voltage have been proposed, in which the intensity mode is change with the changes of amplitude. One of these methods is the automatic changes of the supply voltage, as a result of which the operation of the transistor in overvoltage mode is excluded. A common way to increase the efficiency of a radio amplifier with varying amplitude of oscillations is to use a Doherty amplifier. In this amplifier, the transistor does not go into overstressed operation due to the connection of an auxiliary transistor at the moments of increasing amplitude.

In order to assess the obtaining of high efficiency at the greatest possible frequency, the parameter χ is used, equal to the product of the efficiency of the added power by the oscillation frequency. The largest values of χ obtained in the MMIC based on GaAs  (χ> 40 GHz at 68 GHz) and InP (χ> 25 GHz at 60 GHz).

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