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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Linear microwave transistors for power amplifiers. Reference data
Encyclopedia of radio electronics and electrical engineering / Reference materials The current level of development of REA and its elemental base currently makes it possible to create completely solid-state VHF FM and television transmitters with an output power of up to 5 kW [1,2]. Amplifying paths based on broadband transistor amplifiers have a number of advantages over tube ones. Solid state transmitters are more reliable, electrically safe, easy to use and easier to manufacture. With a block-modular design of the transmitter, the failure of one of the blocks of the final amplifier does not lead to disruption of the broadcast, since the transmission will continue until the block is replaced, only with reduced power. In addition, the broadband path of the transistor amplifier does not require additional tuning to a specific channel within the operating frequency band [3]. It is generally accepted that the reliability of a transmitter depends primarily on the reliability of the active components used. Thanks to the use of modern high-power linear microwave transistors, the design features and manufacturing technology of which provide a significant increase in their time between failures, the issue of increasing the reliability of solid-state transmitters has received a fundamental solution [4]. The growing requirements for the technical and economic indicators of VHF FM and high-power television transmitters, as well as the achieved level of domestic technology in the field of creating high-power silicon bipolar transistors, stimulated the development of a new class of devices - high-power linear microwave transistors. The Research Institute of Electronic Technology (Voronezh) has developed and produces a wide range of them for use in the meter and decimeter wavelength ranges. Transistors are specially designed for use in high-power television and radio broadcasting transmitters, repeaters, in particular, in television repeaters with joint amplification of sound and image signals, as well as in multichannel signal amplifiers of base stations of a cellular communication system [5]. These transistors meet extremely stringent requirements for linearity of the transfer characteristic, have a margin for power dissipation and, as a result, increased reliability. Structurally, such transistors are made in metal-ceramic cases. Their appearance is shown in Fig. 1 (cases of not all transistors mentioned in the article are shown; the missing ones can be seen in article [6]). High linear and frequency properties of transistor structures are realized due to the use of precision isoplanar technology. Diffusion layers have a submicron design rate. The width of the emitter elements of the topology is about 1,5 μm with their extremely developed perimeter.
In order to eliminate failures caused by secondary electrical and thermal breakdown, the transistor structure is formed on a silicon chip with a two-layer epitaxial collector and emitter stabilizing resistors. Transistors also owe long-term reliability to the use of gold-based multilayer metallization. Linear transistors with a power dissipation of more than 50 W (with the exception of KT9116A, KT9116B, KT9133A), as a rule, have a structurally built-in LC input matching circuit, made in the form of a microassembly based on a built-in MIS capacitor and a system of wire leads. Internal matching circuits make it possible to expand the operating frequency band, simplify the input and output matching, and also increase the power gain Cp in the frequency band. At the same time, these transistors are "balanced", which means that there are two identical transistor structures on one flange, united by a common emitter. Such a constructive and technical solution makes it possible to reduce the inductance of the output of the common electrode and also contributes to the expansion of the frequency band and simplification of matching. With push-pull switching on of balanced transistors, the potential of their midpoint is theoretically equal to zero, which corresponds to the artificial "ground" condition. Such inclusion actually provides approximately a fourfold increase in the output complex impedance compared to a single-cycle one at the same output signal level and effective suppression of even harmonic components in the spectrum of the useful signal. It is well known that the quality of television broadcasting primarily depends on how linear the transfer characteristic of the electronic path is. The issue of linearity is especially acute when designing nodes for joint amplification of image and sound signals due to the appearance of combinational components in the frequency spectrum. Therefore, the three-tone method proposed by foreign experts for estimating the linearity of the transfer characteristic of domestic transistors was adopted by the level of suppression of the combination component of the third order. The method is based on the analysis of a real television signal with a signal ratio of the image carrier frequency of -8 dB. -16 dB sideband frequency and -7 dB audio carrier frequency relative to the output power at the peak of the envelope. Transistors for joint amplification, depending on the frequency and power range, should provide the value of the coefficient of the combinational components of the MOH, as a rule, not more than -53 ... -60 dB. The considered class of microwave transistors with strict regulation of the suppression of combinational components was called superlinear transistors abroad [7]. It should be noted that such a high level of linearity is usually realized only in class A mode, where it is possible to carry out the maximum mode linearization of the transfer characteristic.
In the meter range, as can be seen from the table, there are a number of transistors, represented by the KT9116A, KT91166, KT9133A and KT9173A devices with an output peak power of Rvmx.peak, respectively, 5,15, 30 and 50 watts. In the decimeter wave range, such a series is represented by devices KT983A, KT983B, KT983V, KT9150A and POS with РВВ1Х, PIK equal to 0,5, 1,3,5, 8 and 25 W. Superlinear transistors are usually used in joint amplifiers (in class A mode) of television repeaters and power amplifier modules of transmitters with a power of up to 100 watts. However, the output stages of powerful transmitters require more powerful transistors that provide the necessary level of the upper limit of the linear dynamic range when operating in an advantageous energy mode. Acceptable harmonic distortion at a high signal level can be obtained by applying split amplification in class AB mode. Based on the analysis of the thermophysical conditions of the transistor operation and the features of the formation of the linearity of a single-tone signal, a series of microwave transistors was specially developed for the operation mode in class AB. The linearity of the characteristics of these devices according to a foreign method is estimated by the level of compression (compression) of the gain in terms of the power of a single-tone signal - the compression ratio Kszh or otherwise - determine the output power at a certain normalized Kszh. For use in the meter wave range in class AB mode, there are now KT9151A transistors with an output power of 200 W and KT9174A transistors - 300 W. For the decimeter range, transistors 2T9155A, KT9142A, 2T9155B, KT9152A, 2T9155V, KT9182A with an output power of 15 to 150 W have been developed. For the first time, the possibility of creating modular solid-state transmitters in the decimeter range with joint amplification of image and sound signals with a power of 100 W was demonstrated by NEC specialists [8]. Later, similar transmitters were created on domestic high-power microwave transistors 12, 9]. In particular, in [9] it is described original research on expanding the scope of use of high-power transistors KT9151A and KT9152A when creating hundred-watt modules of joint amplification in class A mode. .3 times the nominal value in class AB mode. Specialists of the Novosibirsk State Technical University conducted research on the use of domestic high-power microwave transistors in the modules of television power amplifiers with separate amplification. On fig. 2 is a block diagram of an image signal power amplifier for television channels 1 to 5 with an output peak power of 250 watts. The amplifier is made according to the scheme of separate amplification of image and sound signals. For channels 6 - 12, the amplifier is performed in a similar way with the addition of an intermediate stage on a KT9116A transistor operating in class A mode to obtain the required gain.
In the output stage, KT9151A transistors operate in class AB. It is assembled according to a balanced-push-pull scheme. This makes it possible to obtain the rated output power with fairly simple matching circuits in the complete absence of "feeder echo" and the level of even harmonic components of no more than -35 dB. The nonlinearity of the amplitude characteristic of the amplifier is set at a small signal by selecting the offset of the operating point in each stage, as well as by correcting the nonlinearity in the exciter video modulator. The block diagram of the power amplifier for television channels 21 - 60 is shown in fig. 3. The output stage of the amplifier is also made according to a balanced push-pull scheme.
To ensure broadband matching and the transition from asymmetric to symmetrical load in the output stages of the amplifiers of channels 6 - 12, 21 - 60, a two-link low-pass filter is used as a corrective circuit. The inductance of the first link of the matching circuit is implemented in the form of sections of strip microlines on the elements of the general topology of the printed circuit board. The coils of the second link are the outputs of the base of the transistors. The structure of these amplifiers corresponds to Fig. 2 and 3. The separation of power at the input of the amplifying stages and its addition at their output, as well as the matching of the inputs and outputs with a standard load, was performed using XNUMXdB directional couplers. Structurally, each coupler is made in the form of bifilar windings (quarter-wave lines) on a frame placed in a shielding casing. Thus, modern domestic linear microwave transistors make it possible to create powerful - up to 250 W - television amplifier modules. Using the batteries of such modules, it is possible to increase the output power given to the antenna-feeder path up to 2 kW. As part of the transmitters, the developed amplifiers meet all modern requirements for electrical characteristics and reliability. Powerful linear microwave transistors have recently begun to be widely used also in the construction of power amplifiers for base stations of a cellular communication system. According to their technical level, powerful microwave linear transistors developed by NIIET can be used as an element base for creating modern broadcasting, television and other national economic and amateur radio equipment. Literature
Authors: A.Assessors, V.Assessors, V.Kozhevnikov, S.Matveev, Voronezh
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