Video Recorder RF Power Amplifier

This RF power amplifier can output 2-3 channel signals, covering an area of ​​about one square kilometer. It is an ideal device for opening a small TV transmitter and is suitable for audio-visual education. However, when using it, it should be reported to the local radio management committee and can only be implemented after approval.
Circuit working principle
The circuit working principle of this device is shown in Figure 8.31. The transistors BG1, BG2, and BG3 form a staggered tuned amplifier. The collector load of BG3 is the input matching network of the lower level, which can avoid self-excitation and maximize the signal transmission gain. BGl, BG2, and BG3 work in Class A state. BG4 focuses on improving high-frequency dynamic characteristics and works in Class C state with high efficiency. Its base adopts a self-supplied negative bias circuit, which is conducive to improving the temperature stability of the transistor. The output of BG4 is a matching network, which enables the signal to be effectively transmitted to the transmitting antenna. The whole machine is powered by a negative power supply. C7-C10 are through-hole capacitors, which are used for anti-interference of the power supply line.
Component selection and production
BG1 and BG2 use ultra-high frequency low-power triodes with high fT and low noise, such as 3DG30C or 9018. BG3 uses medium-power triodes such as 2G711, fT>=500MH. IC=50mA, PCM=500mW. BG4 uses triodes such as 3DA37 or MRF517, requiring fT>=500MHz, PCM>=1w. All coils used in the circuit are air-core coils. L1 and L2 use 0.97mm diameter enameled wire to wind 7 turns on the round pencil rod, and tap it at the place where it is 3 turns to the ground. L3 and L4 use 1.3mm diameter enameled wire to remove paint and tin, then wind 6 turns on the colored pen rod and pull it to 150mm. L6 and L7 are made of 1.3mm diameter enameled wire, which is removed from the paint and tinned, and then wound 3 times on the colored pen holder and then pulled to 7mm. L5 is made of 0.77mm diameter enameled wire, which is wound 8 times on the pencil holder. All RFC coils are made of 0.41mm diameter enameled wire, which is wound 16 times on a 1/2w, 1.5k resistor.
The printed circuit board of this machine adopts double-sided epoxy resin copper-clad board, one side is welded with components, and the other side is connected to the ground for shielding. When welding, the component pins should be short, and the strength and resistance of the welding pieces at both ends of the variable capacitor should be fully utilized. The non-grounded end of the capacitor is used as a support point to connect the component in the air, and the movable piece of the variable capacitor is grounded. All triodes are connected in a basket-like manner to prevent high-frequency self-excitation. After all components are welded, install the isolation board and shielding box, then install the through-core capacitor on the side of the shielding box, and weld the high-frequency choke.
When debugging, first adjust the DC working point of the triode. Disconnect the coupling capacitors between each stage, connect the voltmeter to the emitter resistors of DG1 to BG3, and adjust the resistance values ​​of R1, R2, and R3 respectively, so that the voltmeter indicates: BG1: e is 15V, BG2: e is 1.0V, and BG3: e is 1.3V. Then connect the coupling capacitor and the load, turn on the power supply, if self-excitation occurs, fine-tune the fine-tuning capacitor to eliminate it. Then connect the 2 or 3 channel RF signal output by the video recorder to the input end of this amplifier, while fine-tuning the capacitor, observe the image on the TV screen. First disconnect the third stage. Connect the 75 ohm antenna to the output end of the second stage, adjust C1 and C2 so that the image and accompanying sound of the TV are good, then connect the third and fourth stages, and connect the antenna to the output end. Reduce the power supply voltage, adjust C3 and C4 to maximize the collector current of BG4, and adjust C5 and C6 to minimize the collector current of BG4. Finally, place the TV far away and fine-tune C1~C6 to make the image clear, the sound loud and pleasant, and the color brightest. This amplifier is only suitable for video recorders with RF output of 2~3 channels. For video recorders and video players with other RF channels, the video and audio signals must be modulated on channel 3 before being sent to this amplifier.