Production of TV antenna amplifier

For users who do not have cable TV yet, they still need a TV antenna to watch TV programs. Here we introduce an amplifier suitable for indoor TV antennas. It has a simple circuit but works well.

1. Circuit Principle

The circuit of the indoor TV antenna amplifier is shown in Figure 1. It is composed of only one ultra-high frequency triode.

This amplifier is powered by G1 and G2 dual power supplies. Its circuit features are: for high-frequency AC, this circuit is a common emitter grounding circuit, but for DC channels, it is a common base circuit. Therefore, this amplifier has a high voltage gain, but low noise. Because the power supply is divided into two groups, the b, e junction and b, c junction currents of the transistor do not affect each other, so the input and output ends of the circuit have a good isolation effect, which effectively improves the operating frequency of the circuit and ensures sufficient bandwidth. The gain of its single-stage amplifier can reach 8 dB. If the receiving point is far away from the TV station, a two-stage amplifier can be assembled, and the gain can reach more than 15 dB. Due to the good isolation between the stages, there will be no serious mismatch or self-excitation when assembling the two-stage amplifier. The input and output feed lines of the amplifier are all 75Ω coaxial cables, so it can be well matched with the TV antenna and the TV port.

2. Component selection and production

VT uses 3DG141, 3DG30C, 3DG30D, 3DG80, 2G911 and other silicon NPN ultra-high frequency transistors, requiring fT ≥ 600 MHz, β ≥ 100. In addition to the three lead pins c, b, and e, this type of transistor also has a fourth lead pin, which is the tube shell grounding wire. This 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 ultra-high frequency ceramic capacitors. Inductors L1 and L2 need to be made by themselves, and both use φ0.51mm enameled wire to wind 20 turns on a 3.5 mm drill bit, and then remove it after winding. Power supplies G1 and G2 both use two No. 5 batteries.

Figure 2 is the printed circuit board of this machine. The size of the printed circuit board is 55×25 mm. The printed circuit board should be made of epoxy resin copper-clad board with low high-frequency dielectric loss, and a large-area grounding method should be adopted. During installation, the leads of each component should be cut as short as possible, the cap of the transistor should be embedded in the round hole of the printed circuit board, and the four lead pins should be soldered on the reverse side in sequence to form a flower basket. The printed circuit board should be surrounded by copper or tin-plated iron sheets about 15 mm high, and soldered firmly with tin, and covered on the top and bottom, and sealed into a metal box (see Figure 3) to prevent interference from external clutter signals. The input and output lines are led out of the box with 75Ω coaxial cables. Then lead out the plastic power cord from the small hole on the side, and it can work reliably when connected to the power supply.