Uhf Tv-Line Amplifier
Posted on Feb 24, 2013 8917
Under: RF Amplifier Circuits
This circuit offers 10- to 15-dB gain from 400 to 850 MHz and is therefore eminently suitable for situations where th
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e television signal is weak. Moreover, the filters can be adapted to the individual needs of users. Construction is simplicity itself if the ready-made PC board shown on the next page is used. The tracks should be tinned or silvered for optimum performance and long life. The opening at the center of the board is intended to accommodate the transistor. This device has two emitter pins, both of which should be connected to ground. The drawings show that the board is divided into two by a small piece of tin plate, which should have a small cut-out for the transistor. The input and output terminals are made from small cable clamps and M3 nuts and bolts. One side of disc capacitors C4, C5, C8, and C9 is soldered direct to the board. Input and output capacitors, C1/C2 and C6/C7 respectively are surface-mount types. C1/C2/L1 form an input filter and C6/C7/L2 form an output filter. The value of the capacitors might have to be lowered to 3.9 pF to obtain the correct frequency range. The amplifier can be housed in a watertight case and then mounted near the antenna at the top of the mast (if used). The power is obtained from a simple stabilized 12-V supply: a mains adapter with a 78L12 will do nicely. This can be kept indoors, of course. The amplifier can be powered via the coaxial feeder cable, for which purpose a 10- to 100- choke is inserted in the supply line. Calibrating the amplifier is straightforward: set PI to the center of its travel and then adjust it for optimum picture quality. In practice, the collector current of the transistor is then 5 to 15 mA. This may be checked by temporarily replacing jump lead A by one suitable meter. PARTS LIST Rl, R2 = 1 KOhmhm R3 = 2 ki2 R4 = 470 PI = 5 preset potentiometer CI, C2, C6, C7 = 10 pF surface mount C3 = 10 -, 35 V C4, C8 = 1 nF disc C5, C9 Â« 1 nF disc LI, L2 = air core, 2 turns of 3mm diameter enamelled copper wire L3, L4 = 10- choke or 10 turns of 0.2 mm diameter enamelled copper wire on a ferrite bead. T1 = 2SC3358