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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Broadband amplifier. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Antenna amplifiers

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The amplifier, the circuit of which is shown below, is designed to operate in the frequency band 1-5 and 6-12 television channels. The last stage of the amplifier is an emitter follower that matches the amplifier with a low cable impedance (75 ohms).

Broadband amplifier. Schematic diagram of the amplifier
(click to enlarge)

Since when operating in the VHF range, repeaters are prone to self-excitation, their input circuit includes antiparasitic active resistance (R15), which can be calculated using the formula R = 2 p faLin, where fa is the limiting frequency of the transistor gain (for transistors fa is usually given in reference books );

Lin - inductance of the coil at the input of the repeater. The winding data of the amplifier coils are shown below. All coils are wound on frames with a diameter of 7,5 mm with cores SCR-1 turn to turn with a PEV wire of 0,15 mm.

Coil number L1 L2 L3 L4 L5 L6 L7 L8 L9 L10
Number of turns 2,5 3 2 2,5 2 2 2,5 2 2,5 2

When setting up an amplifier, it should be borne in mind that the rotation of the cores of coils with odd numbers on the circuit will change the shape of the amplitude-frequency characteristic in the frequency range of 175-230 MHz, and the rotation of the cores of coils with even numbers will change the shape of the characteristic in the frequency range of 50-100 MHz.

The amplifier is tuned using any VHF standard signal generator that has a frequency range in which the amplifier should operate. As an output indicator, a voltmeter with a high-resistance input is used, which allows measuring alternating voltage with a frequency of up to 250 MHz. The tuning is carried out in the usual manner, that is, by changing the position of the cores of the coils, they achieve that the shape of the amplitude-frequency characteristic of the amplifier differs as little as possible from that shown in Fig. 2.

Broadband amplifier. Amplifier frequency response

The amplifier is designed to connect to TVs with a sensitivity of 200-500 microvolts.

Author: V. Demyanov; Publication: cxem.net

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