Broadband power amplifier from SV-747. Scheme, description

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Broadband power amplifier from SV-747. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Civil radio communications

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The ALAN firm under various trademarks serially produces the power amplifier model "SV-747", intended for use in the CB band. A simple modification allows you to turn it into a broadband amplifier with an operating frequency band of 1,8 ... 30 MHz. With a supply voltage of 13,8 V and an input signal power of 10 W, the output power of the modified amplifier in the specified frequency band will be in the range of 130 ... 70 W, respectively. The maximum current consumed by the amplifier is 14 A. Types of operation - AM, FM, CW and SSB. The amplifier circuit is shown in fig. 1.

Broadband power amplifier from SV-747

The refinement of the amplifier is reduced to a change in the design of its transformers and the introduction of some additional elements that increase its performance.

The "native" transformer of the amplifier T1 is removed from the board, and a new one, made according to the following method, is installed in its place. Two tubes are glued from four ferrite annular magnetic cores of size K8x4,5x5 mm with a magnetic permeability of 600 NN. Then these tubes are put on a shielding braid 30 mm long from a PK50-4-11 or PK75-4-11 coaxial cable (Fig. 2).

Broadband power amplifier from SV-747

Three turns of MGTF 0,35 wire are threaded inside the screen. Gradually pulling the opposite ends of the wire, bring the tubes with their side surfaces to each other. The reduced surfaces are glued with epoxy resin. The result is a traditional "binocular" type transformer.

The transformer is installed on the board. The winding of three turns is soldered in parallel with the capacitor C3, and the shielding braid is in parallel with the resistor R6. At the "native" transformer T2, the secondary winding is removed. Three turns of MGTF 0,35 wire are wound in its place and soldered according to the scheme.

An additional SA3 switch is installed on the front panel of the amplifier. Its contacts are connected with wires of minimum length to the terminals of capacitor C9. When the contacts of the SA3 switch are closed, the amplifier operates in the frequency band of 1,8 ... 21 MHz, and when open - 24 ... 30 MHz. If the capacitor C9 is replaced with a jumper, then there will be a noticeable blockage in the frequency response at high frequencies, which will reduce the output power in the frequency band of 24 ... 30 MHz. For this reason, it was decided to use a switch that either shorts C9 (working on the 1,8-21 MHz amateur bands) or turns it on (when operating on 24-30 MHz).

Since transistor power amplifiers are the most difficult to tolerate without load, it was decided to install the simplest protection at the amplifier output - a 500 ohm resistor with a power of at least 2 watts. In the event of an emergency, the entire power of the amplifier will be dissipated by this resistor, while the SWR will be approximately 10. It is easier for transistors to "survive" this mode than the complete absence of a load at the output.

At the input and output of the amplifier, we install additional isolation capacitors C10 and C11. We also install an additional capacitor C12 in the high-frequency VOX rectifier.

Since the power of the amplifier has increased significantly in the low-frequency part of the operating range, the heat sink of the amplifier hardly provides cooling for the transistors. It is desirable to replace it with a heatsink with a larger surface area or install a small fan for forced cooling.

Establishing an amplifier is reduced to setting the quiescent current of transistors VT1-VT4 within 0,3 ... 0,5 A by selecting resistor R2. In the author's version of the amplifier, the resistor R2 \u17d XNUMX kOhm.

Long-term operation of this amplifier in conjunction with the Rosa transceiver showed its good performance on all KB bands. According to subjective assessments of both near and distant correspondents, the signal quality when working with a power amplifier is good or excellent. The spectrum of the signal is within the normal range.

Unfortunately, due to the lack of measuring instruments, no objective measurements were made.

Author: D.Stenenkov (UAOQNE)

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