Here is an amplifier suitable for indoor TV antennas. It has a simple circuit but works well.
Circuit principle
The circuit of the indoor TV is shown in Figure 1. It consists of only one UHF triode.
The amplifier is powered by G1 and G2 dual power supplies. The circuit features are: for high frequency AC, this circuit is a common emitter ground circuit, but for the DC channel it is a common base circuit. Therefore, this amplifier has a high voltage gain, but the noise is small. Because the power supply is divided into two groups, the b and e junctions of the triode and the b and c junction currents do not affect each other. Therefore, the input and output of the circuit have good isolation, which effectively improves the operating frequency of the circuit and ensures Sufficient bandwidth, with a single-stage amplifier gain of up to 8 dB. If the receiving point is far from the TV station, a two-stage amplifier can be assembled with a gain of more than 15 dB. Because of the better isolation between the stages, when the two-stage amplification is assembled, there will be no serious mismatch or self-excitation. The amplifier's input and output feeders all use a 75Ω coaxial cable, so it fits well with the TV antenna and the port of the TV.
2. Component selection and production
VT adopts silicon NPN ultra-high frequency triodes such as 3DG141, 3DG30C, 3DG30D, 3DG80, 2G911, etc., requiring fT≥600 MHz, β≥100. In addition to the three lead legs c, b, and e, the third type of lead tube has a fourth lead pin, that is, a shell grounding wire, and the lead pin should be grounded during installation. The resistors are all RTX-1/8 w type carbon film resistors, and the capacitors are all CC1 type UHF ceramic capacitors. Inductors L1 and L2 need to be self-made, and they are wound around a 3.5 mm drill bit with a φ0.51mm enameled wire. The power supplies G1 and G2 are both made of two batteries of No. 5 battery.
Figure 2 is a printed circuit board diagram of the machine. The printed circuit board measures 55 x 25 mm. Printed boards should be made of epoxy-coated copper plates with low dielectric loss and high-ground grounding. When installing, the lead wires of each component should be cut as short as possible. The cap of the triode should be embedded in the circular hole of the printed circuit board, and the four lead legs are welded to the opposite side in a blue-like shape. The circumference of the printed board should be surrounded by copper or tinplate with a height of about 15 mm. It should be soldered firmly, capped up and down, sealed into a metal box (see Figure 3) to prevent interference from external clutter signals. The input and output lines are taken out of the box with a 75Ω coaxial cable. Then pull out the plastic power cord from the small hole on the side, and connect it to the power supply to work reliably.
Figure 2 TV printed circuit board
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