Video transmitter. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Television, video equipment
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Option 1
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The preliminary stage of the transmitting amplifier) is assembled on a field-effect transistor VT1 with a gain of about 10.
At the input there is a capacitor C1, which has a very small capacitance. The signal is amplified by two stages of a linear power amplifier. The first one, which has the highest gain, uses a VT2 transistor operating in class A. The output stage (its power gain is 5) is made on a VT3 transistor, which has increased electrical strength and good linearity. For the temperature stability of the cascade, a thermistor R9 is used, mounted on a radiator, near the transistor VT3. In addition, the cascades are separated by screens. The transmission coefficient of the whole device is about 40 dB. The amplifier at the same time develops an output power of about 2,5 watts. Its output is designed to connect a load with a resistance of 75 ohms. During installation, the capacitor C1 is directly soldered to the central core of the cable coming from the VCR. Capacitor leads must be as short as possible, as are all other connections. To ensure stable operation, the base circuits of the transistors are separated from the collector circuits. The transistors VT2 and VT3 themselves are placed in recesses on the radiator, and a thermistor R3 is installed next to the transistor VT9.
The rest of the parts are assembled on a printed circuit board made of double-sided foil glass-textolite, mounted on racks on the radiator. The foil on one side is used as a screen. Holes for parts must be countersunk. The screens between the base and collector circuits of the transistors are also made of foil fiberglass. They must be connected to the common screen of the circuit. The following parts are used in the amplifier: thermistor - type KMT-1, trimmer capacitors C6, C9, C11 - type KT4-21V. The Dr1 inductor is wound on a MLT-0,25 resistor with a resistance of 100 kOhm with a PEV-1 0,12 wire in one row until it is filled, about 30 turns. Coils - frameless and wound with silver-plated wire with a diameter of 1 mm, L1 contains 3 turns, the outer diameter of the coil is 10 mm, the length of the winding is 6 mm; L2 contains 4 turns, the outer diameter of the coil is 8 mm, the winding length is 8 mm; L3 contains 3 turns, the outer diameter of the coil is 8 mm, the winding length is 5 mm; L4 contains 4 turns, the outer diameter of the coil is 10 mm, the winding length is 8 mm. You can power the amplifier from any stabilized source that provides a current of about 20 A at a voltage of 0,5 V, and about 12 mA at 80 V.
For the A2 receiving antenna, it is better to use a 5-element "wave channel" type antenna
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