Tunable low noise antenna amplifier. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Antenna amplifiers

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By using an antenna amplifier, you can improve the quality of reception of television and radio broadcasting programs at the border of the reliable reception zone. The version of such an amplifier proposed in the article has a significant advantage - not only the supply voltage is supplied through the signal cable, but also the operating frequency of the device is tuned.

To improve the quality of the television image and sound or the sound of broadcasting stations, directional antennas have to be used, as well as antenna amplifiers. When receiving a weak signal from a remote television center and there are strong signals from local television or radio broadcasting stations, cordless telephones, etc., broadband antenna amplifiers often do not have a positive effect due to signal overload from nearby transmitters.

In this case, selective antenna amplifiers help out.

To receive signals from several channels, the amplifier must be tunable. However, when such amplifiers are placed near the antenna, a separate wire is required for tuning, which is not very convenient from a structural point of view. In the proposed antenna amplifier, restructuring occurs when the supply voltage supplied through the drop cable changes.

The circuit diagram of the amplifier is shown in fig. 1. It provides frequency dependent gain from 18(50MHz) to 14(230MHz) dB. It uses a low-noise gallium arsenide field-effect transistor, which makes it possible to obtain high sensitivity. The input circuit, formed by the inductance of the coil L1 and the capacitances of the varicap, diodes and transistor, provides frequency selection of the signal and matching the high input impedance of the transistor with the low output impedance of the antenna. The circuit is rebuilt by changing the capacitance of the varicap when adjusting the voltage applied to it.

The supply voltage of the transistor is stabilized by a microcircuit voltage regulator DA1. The DC transistor mode is set by resistors R2, R3. To match with the drop cable, an L2 coil with a tap is used. Diodes VD1, VD2, VD4, VD5 protect the transistor from breakdown by powerful signals and pickups. The supply voltage to the amplifier is supplied via the drop cable through the L3 inductor.

For tuning, an adjustable voltage from 7 to 15 V is supplied to the amplifier from a stabilized power supply located next to the TV or radio. This voltage is applied to the stabilizer DA1, and through the zener diode VD6 - to the varicap VD3. At a supply voltage of 7 V, a current begins to flow through the zener diode VD3 and a voltage of about 0,2 V is applied to the varicap. In this case, its capacitance is maximum and the circuit is tuned to the lower frequency of the tuning interval. As the supply voltage on the varicap increases, it also increases, the capacitance of the varicap decreases, and the tuning frequency of the input circuit increases.

The frequency overlap factor of the input circuit is slightly less than two. This means that the amplifier can be used to receive television signals in the MB1 subband (48 ... 100 MHz) or in the MB2 subband (174 ... 230 MHz), as well as to receive only radio station programs in the VHF bands (65 ... 108 MHz). To do this, change the parameters of the coil L1.

The power supply of the antenna amplifier is provided by the block, the diagram of which is shown in fig. 2. It is assembled on an integral adjustable stabilizer. The output voltage of the block is changed by resistor R3. Through the L1 choke, it enters the XS1 socket, to which the drop cable from the antenna amplifier is connected. The received signals from the socket XS1 pass through the capacitor C4 along the cable with the XP2 plug. It is connected to the TV input.

In the amplifier, in addition to those indicated in the diagram, transistors AP325A-2, AP331A-2 or similar, varicaps KV109A, KV109V, KV109G, KV122A, KV122B, KV122V, zener diode KS168A, diodes KD512A, KD514A are applicable. It is desirable to use small-sized resistors: P1-4, P1-12 or MLT. It is better to use open-frame capacitors K10-17V or small-sized capacitors with leads of a minimum length.

Coils L2, L3 are wound with PEV-2 0,12 wire on K5x2x1,5 ferrite rings with a permeability of 600 ... 2000. Coil L2 contains 10 turns in two twisted wires (after winding, the beginning of one wire is connected to the end of the other and an average output is obtained), coil L3 - 15-20 turns of a single wire. Coil L1 is wound with wire PEV-2 0,9 on a mandrel with a diameter of 5 mm. If the coil has 11,5 turns (tap from the third turn), the tuning interval is 48 ... 92 MHz, if 6,5 turns (tap from the second turn) - the interval is 65 ... 110 MHz, and if 3,5 turns (tap from 0,3 ... 0,5 of the first turn) - 150 ... 230 MHz. For a slight shift in the tuning interval towards higher frequencies, the coil turns are slightly moved apart.

In the power supply, you can use polar capacitors of the K50 series, non-polar K10-17, KD or KT, a variable resistor - SPO, SP4, constant MLT, S2-33. Inductor L1 is similar to inductor L3 in the amplifier. The transformer must provide an alternating voltage on the secondary winding of about 15 V.

Setting up the amplifier comes down to setting the required tuning interval by selecting the number of turns of the L1 coil and a bandwidth of at least 7 MHz by changing the tap location. In the power supply, by selecting resistors R2 and R3, the required interval for changing the output voltage is set. When the amplifier is self-excited at high frequencies, a ferrite ring (“bead”) must be put on the drain terminal of the transistor or an adhesive composition (based on epoxy glue) with a filler of powdered carbonyl iron should be applied.

All parts of the amplifier are placed on a printed circuit board made of double-sided foil fiberglass. The arrangement of printed conductors is shown in fig. 3. The second side of the board is left metallized, except for the input and output pad cutouts indicated by the dashed line, and is connected by foil to the common wire of the first side along the entire circuit. After installation and adjustment, the field-effect transistor is poured with a drop of epoxy glue, the board is closed from the side of the parts with a metal cover-screen, and the device is covered on all sides with a protective layer of waterproof paint or varnish.

The parts of the power supply are installed on a printed circuit board made of one-sided foil-coated fiberglass, the printed conductors of which are shown in fig. 4.

The amplifier can be made switchable by adding two relays to it, turning them on in accordance with the diagram in fig. 5, then when the supply voltage is turned off, the antenna will be connected directly to the drop cable, bypassing the amplifier. To do this, you can use the RES-34, REC-43 relays with a response voltage of about 6 V. The dimensions of the board will have to be increased, and the wiring of the conductors will have to be slightly changed.

Author: I.Nechaev

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