The power amplifier can extend the power of the 1-2W, 88-108MHZ FM transmitter to 10-15W. It is composed of a single-tube Class C amplifier and a multi-stage low-pass filter. It has a high conversion efficiency and a strong reputation. Wave suppression ability.
The circuit is shown in the figure, using a high-power transmitter tube C1972, its parameters are as follows: 175MHZ, 4A, 25W, power gain ≥8.5db, according to the parameters shown in the figure, the circuit working center frequency is about 98MHZ, when the input of about 2W RF power , The rated output can reach 15W. In order to ensure that the output reaches the rated value at any frequency point within 88~108MHZ, some components can be adjusted appropriately according to the center frequency of the previous stage. If necessary, the number of low-pass wave filters can be reduced to increase the output power. The expanded power signal is filtered by a three-stage low-pass filter to remove high-order wave components and fed into the transmitting antenna.
1. Component selection:
In addition to electrolytic capacitors, other high-frequency ceramic capacitors are used. C11, C12, and C14 use adjustable capacitors with good high-frequency characteristics and stable performance. Choke inductors RFC1 and RFC2 use finished inductors. Pay attention to the current carrying capacity of RFC2. The inductor with a magnetic core with a thicker wire diameter should be selected. L1—L6 can be made of high-strength enameled wire of ø0.8mm, with a diameter of about 5mm, and the number of turns is indicated by "T" in the unit. Q1 uses ordinary Q9 sockets, which are used in conjunction with plugs. Q2 uses a dedicated 50Ω RF output connector, which has a smaller connection resistance, which is more conducive to impedance matching. The power amplifier tube uses the more common transmission dedicated tube C1972. Of course, if you have enough money to buy a high gain tube such as C2538, the power will be greater.
2. Circuit debugging:
When debugging the circuit, you must pay attention to the high power of the circuit, be sure to connect a dummy load (I used 30 1W, 1500Ω high-precision metal film resistors in parallel), and there must be enough heat dissipation devices, and the power of the power supply is not Below 2.5A, the antenna impedance is strictly equal to 50Ω, and short rod antennas cannot be used. Otherwise, the strong RF feedback current will cause the circuit to cause self-interference. Most of the RF energy cannot be radiated into the space and is consumed on the power tube, causing it to overheat and damage ; It must be directed to an outdoor antenna for transmission through a 50Ω transmission dedicated coaxial cable. The key to the normal operation of the circuit lies in the debugging of the circuit, and the whole process must be very careful. When debugging, only input a small excitation power, the power supply voltage drops to 9V, use a high-frequency voltmeter (cannot use ordinary multimeters) to monitor the high-frequency voltage value at both ends of the dummy load, adjust C12, C14, L3, L4, L5, L6 , Make the voltage amplitude reach about 15-20V, and then adjust C11 and L1 to maximize the voltage. Then gradually increase the voltage, and adjust C12, C14, C11, and L1 repeatedly every time the voltage is increased to make the output voltage the highest. Note that the voltage should be increased synchronously with the RF input excitation power to ensure the accuracy of the debugging results. When the rated value is reached, the working current is about 2A at the power supply voltage of 13.8V, the voltage across the 50Ω pure resistance dummy load is ≥40V, and the RF output power reaches 15W.
Use this RF power amplifier with a 50Ω umbrella-shaped omni-directional vertical transmitting antenna (gain is about 2dB), tested with an ordinary FM radio, and the transmission distance coverage is not less than 15Km.
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